Rainbow Electronics MAX8728 User Manual

General Description

The MAX8728 generates all the supply rails for thin-film
transistor (TFT) liquid-crystal display (LCD) panels in TVs
and monitors. It includes step-down and step-up regula­tors, positive and negative charge pumps, and a dual­mode, logic-controlled high-voltage switch control block.
The MAX8728 can operate from input voltages from 7V to

13.2V and is optimized for LCD TV panel and LCD moni­tor applications running directly from 12V supplies.

The step-up and step-down regulators feature internal
power MOSFETs and high-frequency operation allow­ing the use of small inductors and capacitors, resulting
in a compact solution. Both switching regulators use
fixed-frequency, current-mode control architectures,
providing fast load-transient response and easy com­pensation. The positive and negative charge-pump reg­ulators provide TFT gate-driver supply voltages. Both
output voltages can be adjusted with external resistive
voltage-dividers.

The MAX8728 is available in a small (5mm x 5mm), low­profile (0.8mm), 32-pin TQFN package and operates
over the -40°C to +85°C temperature range.

Applications

LCD Monitors LCD TVs

Features

♦ Optimized for 10.8V to 13.2V Input Supply
♦ 7V to 13.2V Input Supply Range
♦ Selectable Frequency (500kHz/1MHz/1.5MHz)
♦ Current-Mode Step-Down Regulator

14V Internal n-Channel MOSFET

1.5% Accurate Output

♦ Current-Mode Step-Up Regulator

19V Internal n-Channel MOSFET
1% Accurate Output
True Shutdown™ (Output Goes to Zero)

♦ 180° Out-of-Phase Switching
♦ Adjustable Positive/Negative Charge Pumps
♦ Soft-Start and Timer Delay Fault Latch for All

Outputs

♦ Logic-Controlled, High-Voltage Switches
♦ Power-Up and Power-Down Sequences
♦ Thermal-Overload Protection

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

MAX8728

AV

DD

V

GON

GATEINL

V

IN

OUT1

V

GOFF

V

L

V

L

LX1

ON/OFF

OUT1

FBI
FSEL

GND1

GND

FBP

SRC

REF

V

CC

GON

DRN

MODE

THR

COMP

SHDN

LCD ENABLE

EN

FROM T

CON

CTL

DEL

FB2

LX2

GND2

FBN
DRVN

SUPP

DRVP

VL

BST
IN

V

IN

Simplified Operating Circuit

19-3910; Rev 0; 1/06

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

PART

TEMP RANGE

PIN­PACKAGE

PACKAGE

CODE

MAX8728ETJ+

32 TQFN-EP*
5mm x 5mm

T3255-4

Pin Configuration

MAX8728

TQFN

TOP VIEW

29

30

28

27

12

11

13

OUT1

CTL

IN

LX1

BST

14

GND1

FB2

DRN

GON

FBN

SRC

FSEL

12EN4567

2324 22 20 19 18

MODE

DEL

FBP

REF

GND

SHDN

DRVN

THR

3

21

31

10

COMP

V

CC

32

9

FB1

VL

GATE

26

15

GNDP

LX2

25

16

DRVP

INL

SUPP

8

17

GND2

+Denotes lead-free package.
*EP = Exposed pad.

True Shutdown is a trademark of Maxim Integrated Products, Inc.

-40°C to +85°C

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, VIN= V

INL

= V

SUPP

= 12V, V

OUT1

= +3.3V, V

SRC

= 28V, GND1 = GND2 = GNDP = GND = 0, I

REF

= 0, TA= 0°C

to +85°C. Typical values are at T

A

= +25°C, unless otherwise noted.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

IN, INL, SUPP to GND ............................................-0.3V to +14V

SUPP to IN ..........................................................................±0.3V

DRVP to GNDP.........................................-0.3V to V

SUPP

+ 0.3V

CTL, EN,

SHDN, OUT1, VL, VCCto GND ................-0.3V to +6V

COMP, FB1, FB2, FBN, FBP, FSEL, DEL,

THR, MODE, REF to GND ........................-0.3V to V

CC

+ 0.3V

GND1, GND2, GNDP to GND.............................................±0.3V

BST to GND1 ..........................................................-0.3V to +20V

LX1 to BST................................................................-6V to +0.3V

LX2 to GND2 ..........................................................-0.3V to +19V

DRVN, LX1, GATE to GND1 ..........................-0.3V to V

IN

+ 0.3V

GON, SRC to GND .................................................-0.3V to +40V

SRC to GON ...........................................................-0.3V to +40V

SRC to SUPP ..........................................................-0.3V to +30V

SRC to SUPP (momentary)......................................-14V to +30V

GON to SUPP ..........................................................-14V to +30V

SRC to DRN............................................................-0.3V to +40V

DRN to GND ...........................................................-0.3V to +40V

GON to DRN...........................................................-0.3V to +30V

V

L

Short Circuit to GND..............................................Momentary

REF Short Circuit to GND ...........................................Continuous

DRVN RMS Current..........................................................-400mA

DRVP RMS Current.........................................................+100mA

LX2 RMS Current ................................................................+1.6A

GND2 RMS Current ............................................................+1.6A

LX1 RMS Current .................................................................-1.6A

Continuous Power Dissipation (T

A

= +70°C)

32-Pin Thin QFN (derate 34.5mW/°C above +70°C) .....2758mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +160°C

Lead Temperature (soldering, 10s) .................................+300°C

PARAMETER CONDITIONS MIN TYP MAX UNITS

GENERAL

IN, INL Input Voltage Range For VL regulator operation 7.0 12.0 13.2 V

INL Quiescent Current

V

FB2

= V

FBP

= 2.2V, V

FBN

= 0,

LX2 not switching, LX1 switching

7mA

IN Standby Supply Current VIN = 7V to 13.2V, EN = SHDN = GND 0.5 mA

FSEL = GND 1275 1500 1730

FSEL = V

CC

850 1000 1150Switching Frequency

FSEL = REF 425 530 610

kHz

Phase Difference Between
Step-Down/Positive and
Step-Up/Negative Regulators

180

Degrees

VL REGULATOR

VL Output Voltage

7V < V

INL

< 13.2V, V

FB1

= V

FB2

= V

FBP

=

1.9V, V

FBN

= 0.5V, IVL = 25mA

4.8 5.0 5.1 V

VL Undervoltage Lockout
Threshold

VL rising, 2.5% hysteresis 3.8 4.0 4.1 V

REFERENCE

REF Output Voltage No external load 1.98 2.00 2.02 V

REF Load Regulation 0 < I

REF

< 50µA 10 mV

REF Sink Current REF in regulation 0 10 µA

REF Undervoltage Lockout
Threshold

Rising edge, 200mV hysteresis 1.5 V

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, VIN= V

INL

= V

SUPP

= 12V, V

OUT1

= +3.3V, V

SRC

= 28V, GND1 = GND2 = GNDP = GND = 0, I

REF

= 0, TA= 0°C

to +85°C. Typical values are at T

A

= +25°C, unless otherwise noted.)

PARAMETER CONDITIONS MIN TYP MAX UNITS

STEP-DOWN REGULATOR

OUT1 Voltage in Fixed Mode

V

IN

= 7.0V to 13.2V, EN = VCC,

I

LOAD

= 0.5A (Note 1)

3.25 3.30 3.35 V

FB1 Regulation Voltage in
Adjustable Modes

20% to 35% duty cycle, EN = VCC,
I

LOAD

= 0.5A (Note 1)

1.97 2.00 2.03 V

FB1 Adjustable Mode Threshold
Voltage

0.10 0.15 0.20 V

Output Voltage Adjust Range 2.0 3.6 V

Fixed mode, OUT1 falling 2.640

Step-Down Regulator Fault Trip
Level

Adjustable mode, FB1 falling 1.536 1.600 1.664

V

FB1 Input Leakage Current V

FB1

= 2.1V -100 +100 nA

Low-Frequency Operation
OUT1 Threshold

LX1 only 1.3 V

FSEL = GND 250

FSEL = V

CC

167

Low-Frequency Operation
Switching Frequency

LX1 only

FSEL = REF 83

kHz

DC Load Regulation 0 < I

OUT1

< 2A, EN = V

CC

0.5 %

DC Line Regulation 7V <V

IN

< 13.2V, EN = V

CC

0.1 %/V

LX1-to-IN Switch
On-Resistance

200 300 mΩ

LX1-to-GND1 Switch
On-Resistance

10 22 40 Ω

Positive Current Limit 2.5 2.8 3.1 A

Skip Mode I

MAX

Threshold EN = GND 0.50 0.60 0.75 A

Soft-Start Ramp Time 1.7 ms

Maximum Duty Cycle 70 77 85 %

STEP-UP REGULATOR

Output Voltage Range V

IN

17 V

Maximum Duty Cycle 65 75 85 %

Minimum On-Time 65 100 ns

FB2 Regulation Voltage FB2 = COMP, C

COMP

= 1nF 1.98 2.00 2.02 V

FB2 Fault Trip Level Falling edge 1.728 1.800 1.872 V

FB2 Load Regulation 0 < I

AVDD

< full, transient only -1 %

FB2 Line Regulation V

IN

= 10.8V to 13.2V 0.08 0.15 %/V

FB2 Input Bias Current V

FB2

= 2V -150 +150 nA

FB2 Transconductance ΔI = ±2.5µA at COMP, FB2 = COMP 75 160 280 µS

FB2 Voltage Gain FB2 to COMP 700 V/V

LX2 Leakage Current V

FB2

= 2.1V, V

LX2

= 13V 4 40 µA

LX2 Current Limit V

FB2

= 1.8V, duty cycle is 25% 1.2 1.5 1.8 A

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, VIN= V

INL

= V

SUPP

= 12V, V

OUT1

= +3.3V, V

SRC

= 28V, GND1 = GND2 = GNDP = GND = 0, I

REF

= 0, TA= 0°C

to +85°C. Typical values are at T

A

= +25°C, unless otherwise noted.)

PARAMETER CONDITIONS MIN TYP MAX

UNITS

Current-Sense Transresistance

0.6 1.2 1.8 V/A

LX2 On-Resistance 0.5 1.0 Ω

Soft-Start Period 3ms

POSITIVE CHARGE-PUMP REGULATOR

FBP Regulation Voltage 1.98 2.00 2.02 V

FBP Line Regulation Error VIN = V

SUPP

= 10.8V to 13.2V 6 mV

FBP Input Bias Current V

FBP

= 2.1V -50 +50 nA

DRVP p-Channel MOSFET
On-Resistance

4 Ω

DRVP n-Channel MOSFET
On-Resistance

1 Ω

FBP Fault Trip Level Falling edge 1.536

1.664 V

Positive Charge-Pump Soft-Start

Period

3ms

NEGATIVE CHARGE-PUMP REGULATOR

FBN Regulation Voltage V

REF

- V

FBN

1.727

1.773 V

FBN Input Bias Current V

FBN

= 250mV -50 +50 nA

FBN Line Regulation V

IN

= 10.8V to 13.2V 6 mV

DRVN p-Channel MOSFET
On-Resistance

4 Ω

DRVN n-Channel MOSFET

On-Resistance

1 Ω

FBN Fault Trip Level Rising edge 600 mV

Negative Charge-Pump

Soft-Start Period

3ms

SEQUENCE CONTROL

SHDN Input Low Voltage 0.4 V
SHDN Input High Voltage 2 V
SHDN Input Current 1µA

EN Charge Current During startup, VEN = 1.0V 456µA

EN Turn-On Threshold 0.95 1.00 1.05 V

DEL Capacitor Charge Current During startup, V

DEL

= 1.0V 456µA

DEL Turn-On Threshold 0.95 1.00 1.05 V

GATE Output Sink Current EN = high, GATE = IN 8 11 14 µA

GATE On Voltage EN = high V

IN

- 6

V

IN

- 4 V

GATE Done Threshold EN = high, V

GATE_DONE

- V

GATE_ ON

01 V

GATE Pullup Resistance EN = low, V

GATE

= V

IN

- 5V 1 kΩ

1.600

1.750

V

- 5

IN

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

_______________________________________________________________________________________ 5

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, VIN= V

INL

= V

SUPP

= 12V, V

OUT1

= +3.3V, V

SRC

= 28V, GND1 = GND2 = GNDP = GND = 0, I

REF

= 0, TA= 0°C

to +85°C. Typical values are at T

A

= +25°C, unless otherwise noted.)

PARAMETER CONDITIONS MIN TYP MAX

UNITS

DEL, EN Discharge Switch
On-Resistance

SHDN = low or fault tripped 20 Ω

FBN Discharge Switch
On-Resistance

EN = low or fault tripped 5 kΩ

POSITIVE GATE-DRIVER TIMING AND CONTROL SWITCHES

CTL Input Low Voltage 0.6 V

CTL Input High Voltage 2.0 V

CTL Input Leakage Current -1 +1 µA

CTL-to-GON Rising Propagation
Delay

1kΩ from DRN to GND, 1.5nF from GON to
GND

100 ns

CTL-to-GON Falling Propagation
Delay

1kΩ from DRN to GND, 1.5nF from GON to
GND

250 ns

SRC Input Voltage Range 38 V

V

MODE

= V

REF

, V

DEL

= V

CTL

= 3V 1.5 2.0

SRC Input Current

V

MODE

= V

REF

, V

DEL

= 3V, V

CTL

= 0 0.14 0.20

mA

DRN Input Current

V

MODE

= V

REF

, V

DRN

= 8V, V

DEL

= 3V,

V

GON

> V

DRN

, V

CTL

= 0

01µA

SRC Switch On-Resistance V

MODE

= V

REF

, V

DEL

= V

CTL

= 3V 15 30 Ω

SRC Switch Saturation Current

V

MODE

= V

REF

, V

DEL

= V

CTL

= 3V,

V

SRC

- V

GON

> 5V

260 mA

DRN Switch
On-Resistance

V

MODE

= V

REF

, V

DEL

= 3V, V

CTL

= 0, V

GON

= 28V, V

THR

= 1.4V

25 50 Ω

DRN Switch Saturation Current

V

MODE

= V

REF

, V

DEL

= 3V, V

CTL

= 0, V

GON

= 28V, V

THR

= 1.4V, V

GON

- V

DRN

> 5V

100 mA

MODE Switch On-Resistance SHDN = GND 1 kΩ

MODE Current-Source Stop
Voltage Threshold

MODE rising 1.2 1.4 1.6 V

MODE Charge Current Operating mode 2, V

MODE

= 0.7V 40 50 60 µA

MODE Voltage Threshold Enabling DRN switch control in mode 2 0.8 1.0 1.2 V

THR to GON Voltage Gain 9.4 10.0 10.6 V/V

FAULT DETECTION

Duration to Trigger Fault 50 ms

Thermal Shutdown Threshold 15°C typical hysteresis +160 °C

SWITCHING-FREQUENCY SELECTION

FSEL = VCC (1MHz)

FSEL = REF (0.5MHz) 1.65 2.35FSEL Input Levels

FSEL = GND (1.5MHz) 0.5

V

FSEL Input Current Forced to V

CC

10 µA

VCC - 0.4

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

6 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, VIN= V

INL

= V

SUPP

= 12V, V

OUT1

= +3.3V, V

SRC

= 28V, GND1 = GND2 = GNDP = GND = 0, I

REF

= 0, TA=

-40°C to +85°C.) (Note 2)

PARAMETER CONDITIONS MIN TYP MAX

GENERAL

IN, INL Input Voltage Range For VL regulator operation 7.0 13.2 V

IN Standby Supply Current VIN = 7V to 13.2V, EN = SHDN = GND 0.5 mA

FSEL = GND 1175 1800

FSEL = V

CC

780 1150Switching Frequency

FSEL = REF 400 610

kHz

VL REGULATOR

VL Output Voltage

7V < V

INL

< 13.2V, V

FB1

= V

FB2

= V

FBP

= 1.9V,

V

FBN

= 0.5V, IVL = 25mA

4.8 5.1 V

VL Undervoltage Lockout
Threshold

VL rising, 2.5% hysteresis 3.8 4.1 V

REFERENCE

REF Output Voltage No external load 1.97 2.02 V

REF Load Regulation 0 < I

RFI

< 50µA 10 mV

STEP-DOWN REGULATOR

OUT1 Voltage in Fixed Mode

V

IN

= 6.0V to 13.2V, EN = VCC,

I

LOAD

= 0.5A (Note 1)

3.23 3.35 V

FB1 Regulation Voltage in
Adjustable Mode

20% to 35% duty cycle, EN = VCC,
I

OUT1

= 0.5A (Note 1)

1.97 2.03 V

FB1 Adjustable-Mode
Threshold Voltage

0.10 0.20 V

Output Voltage Adjust Range 2.0 3.6 V

Step-Down Regulator Fault Trip
Level

Adjustable mode, FB1 falling

1.664 V

LX1-to-IN Switch
On-Resistance

550 mΩ

LX1-to-GND1 Switch
On-Resistance

840Ω

Positive Current Limit 2.3 3.1 A

Skip Mode I

MAX

Threshold EN = GND 0.45 0.75 A

Maximum Duty Cycle 70 85 %

STEP-UP REGULATOR

Output Voltage Range V

IN

17 V

Maximum Duty Cycle 65 85 %

FB2 Regulation Voltage FB2 = COMP, C

COMP

= 1nF 1.97 2.02 V

LX2 Current Limit V

FB2

= 1.8V, duty cycle is 25% 1.2 1.8 A

LX2 On-Resistance 1 Ω

1.536

UNITS

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

_______________________________________________________________________________________ 7

Note 1: When the inductor is in continuous conduction (EN = VCCor heavy load), the output voltage has a DC regulation level lower

than the error comparator threshold by 50% of the output voltage ripple. In discontinuous conduction (EN = GND with light
load), the output voltage has a DC regulation level higher than the error comparator threshold by up to 50% of the output
voltage ripple.

Note 2: Specifications to -40°C are guaranteed by design, not production tested.

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, VIN= V

INL

= V

SUPP

= 12V, V

OUT1

= +3.3V, V

SRC

= 28V, GND1 = GND2 = GNDP = GND = 0, I

REF

= 0, TA=

-40°C to +85°C.) (Note 2)

PARAMETER CONDITIONS MIN TYP MAX

UNITS

CHARGE-PUMP REGULATORS

FBP Regulation Voltage 1.97 2.02 V

FBN Regulation Voltage V

REF

- V

FBN

1.71 1.78 V

SEQUENCE CONTROL

SHDN Input Low Voltage 0.4 V
SHDN Input High Voltage 2 V

EN Turn-On Threshold 0.95 1.10 V
DEL Turn-On Threshold 0.95 1.10 V

GATE On Voltage EN = high

V

IN

- 4 V

GATE Done Threshold EN = high, V

GATE_DONE

- V

GATE_ON

0V

POSITIVE GATE-DRIVER TIMING AND CONTROL SWITCHES

CTL Input Low Voltage 0.6 V

CTL Input High Voltage 2.1 V

SRC Input Voltage Range 38 V

V

MODE

= V

REF

, V

DEL

= V

CTL

= 3V 2.3

SRC Input Current

V

MODE

= V

REF

, V

DEL

= 3V, V

CTL

= 0 0.2

mA

SRC Switch On-Resistance V

MODE

= V

REF

, V

DEL

= V

CTL

= 3V 30 Ω

DRN Switch On-Resistance

V

MODE

= V

REF

, V

DEL

= 3V, V

CTL

= 0,

V

GON

= 28V, V

THR

= 1.4V

50 Ω

MODE Current-Source Stop­Voltage Threshold

MODE rising 1.2 1.6 V

MODE Voltage Threshold Enabling DRN switch control in mode 2 0.8 1.2 V

THR-to-GON Voltage Gain 9.4 10.6 V/V

V

- 6

IN

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

8 _______________________________________________________________________________________

Typical Operating Characteristics

(Circuit of Figure 1. VIN= V

INL

= V

SUPP

= 12V, AVDD= 13.5V, V

GON

= 28V, V

GOFF

= -6V, V

OUT1

= 3.3V, FSEL = GND, TA= +25°C,

unless otherwise noted.)

STEP-DOWN REGULATOR EFFICIENCY

vs. LOAD CURRENT

LOAD CURRENT (A)

EFFICIENCY (%)

MAX8728 toc01

50

55

60

65

70

75

80

85

90

0.01 0.1 1 10

EN = GND

EN = VL

NORMALIZED STEP-DOWN REGULATOR

OUTPUT VOLTAGE vs. LOAD CURRENT

LOAD CURRENT (A)

OUTPUT VOLTAGE (V)

MAX8728 toc02

3.10

3.15

3.20

3.25

3.30

3.35

3.40

0.01 0.1 1 10

EN = GND

EN = VL

STEP-DOWN REGULATOR

LOAD TRANSIENT RESPONSE

MAX8728toc03

0V

0V

0V

LOAD CONTROL, 5V/div

LX1, 20V/div

OUT1, AC, 100mV/div

INDUCTOR CURRENT, 1A/div

4μs/div

STEP-DOWN REGULATOR

SOFT-START (HEAVY LOAD)

MAX8728toc04

0V

0V

0V

V

IN

, 5V/div

OUT1, 2V/div

INDUCTOR CURRENT, 1A/div

400μs/div

STEP-UP REGULATOR EFFICIENCY

vs. LOAD CURRENT (MEASURED AT L1/C3 JUNCTION)

LOAD CURRENT (A)

EFFICIENCY (%)

MAX8728 toc05

60

65

70

75

80

85

90

95

100

0.001 0.01 0.1 1 10

NORMALIZED STEP-UP REGULATOR

OUTPUT VOLTAGE vs. LOAD CURRENT

LOAD CURRENT (A)

OUTPUT VOLTAGE (V)

MAX8728 toc06

13.30

13.35

13.40

13.45

13.50

13.55

13.60

0.01 0.1 1 10

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

_______________________________________________________________________________________ 9

Typical Operating Characteristics (continued)

(Circuit of Figure 1. VIN= V

INL

= V

SUPP

= 12V, AVDD= 13.5V, V

GON

= 28V, V

GOFF

= -6V, V

OUT1

= 3.3V, FSEL = GND, TA= +25°C,

unless otherwise noted.)

STEP-UP REGULATOR

SOFT-START (HEAVY LOAD)

MAX8728toc07

0V

0V

0V

EN, 2V/div

INDUCTOR CURRENT,
500mA/div

1ms/div

AVDD, 5V/div

GATE, 5V/div

STEP-UP REGULATOR LOAD TRANSIENT

RESPONSE (100mA TO 600mA)

MAX8728toc08

0V

0V

LOAD CONTROL,
5V/div

INDUCTOR CURRENT,
500mA/div

AV

DD

, AC,

200mV/div

10μs/div

STEP-UP REGULATOR PULSED-LOAD

TRANSIENT RESPONSE (100mA TO 1A)

MAX8728toc09

0V

0V

LOAD CONTROL,
5V/div

INDUCTOR CURRENT,
500mA/div

AV

DD

, AC,

100mV/div

4μs/div

TIMER-DELAY

OVERCURRENT PROTECTION

MAX8728toc10

0V

VGON, 20V/div,
10kΩ LOAD

INDUCTOR CURRENT,
1A/div

10ms/div

V

GOFF

, 5V/div, AVDD, 10kΩ LOAD

OUT1, 2V/div, 2.5A OVERLOAD

AVDD, 5V/div, 150Ω LOAD

SWITCHING FREQUENCY

vs. INPUT VOLTAGE

INPUT VOLTAGE (V)

SWITCHING FREQUENCY (MHz)

MAX8728 toc11

7 8 9 10 11 12 13

1.500

1.505

1.510

1.515

1.520
FSEL = GND

NORMALIZED VL OUTPUT VOLTAGE

vs. VL CURRENT

VL CURRENT (mA)

VL VOLTAGE (V)

MAX8728 toc12

0 10203040506070

4.80

4.85

4.90

4.95

5.00

5.05

EN = GND

EN = VL

Typical Operating Characteristics (continued)

(Circuit of Figure 1. VIN= V

INL

= V

SUPP

= 12V, AVDD= 13.5V, V

GON

= 28V, V

GOFF

= -6V, V

OUT1

= 3.3V, FSEL = GND, TA= +25°C,

unless otherwise noted.)

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

10 ______________________________________________________________________________________

POSITIVE CHARGE-PUMP

LOAD TRANSIENT RESPONSE

MAX8728toc16

SRC, AC,
200mV/div

I

SRC

,

20mA/div

40μs/div

NEGATIVE CHARGE-PUMP OUTPUT VOLTAGE

ERROR vs. INPUT VOLTAGE (V

OUT

= -6V)

INPUT VOLTAGE (V)

OUTPUT VOLTAGE ERROR (%)

MAX8728 toc17

7 8 9 10111213

-5

-4

-3

-2

-1

0

1

2

I

OUT

= 10mA

I

OUT

= 100mA

NEGATIVE CHARGE-PUMP

OUTPUT VOLTAGE ERROR vs. LOAD CURRENT

LOAD CURRENT (A)

OUTPUT VOLTAGE ERROR (%)

MAX8728 toc18

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

0.001 0.01 0.1 1

REF VOLTAGE vs. REF CURRENT

REF CURRENT (μA)

REF VOLTAGE (V)

MAX8728 toc13

0 20406080100

1.994

1.995

1.996

1.997

1.998

1.999

2.000

POSITIVE CHARGE-PUMP OUTPUT VOLTAGE

ERROR vs. INPUT VOLTAGE (V

OUT

= 28V)

INPUT VOLTAGE (V)

OUTPUT VOLTAGE ERROR (%)

MAX8728 toc14

10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

I

OUT

= 10mA

I

OUT

= 50mA

POSITIVE CHARGE-PUMP OUTPUT

VOLTAGE ERROR vs. LOAD CURRENT

LOAD CURRENT (A)

OUTPUT VOLTAGE ERROR (%)

MAX???? toc15

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

0.001 0.01 0.1

Typical Operating Characteristics (continued)

(Circuit of Figure 1. VIN= V

INL

= V

SUPP

= 12V, AVDD= 13.5V, V

GON

= 28V, V

GOFF

= -6V, V

OUT1

= 3.3V, FSEL = GND, TA= +25°C,

unless otherwise noted.)

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 11

NEGATIVE CHARGE-PUMP

LOAD TRANSIENT RESPONSE

MAX8728toc19

V

GOFF

, AC,

50mV/div

I

VGOFF

,

50mA/div

40μs/div

POWER-UP SEQUENCE

MAX8728toc20

2ms/div

DEL,
C

DEL

= 0.01μF

2V/div

V

GON

, 20V/div, 10kΩ LOAD

V

GOFF

, 5V/div,

10kΩ LOAD

EN, C

EN

= 0.01μF,

2V/div

AV

DD

, 5V/div, 150Ω LOAD

OUT1, 5V/div,

100Ω LOAD

INL SUPPLY CURRENT vs. INL VOLTAGE

INL VOLTAGE (V)

INL SUPPLY CURRENT (mA)

MAX8728 toc21

7 8 9 10111213

0

2

4

6

8

10

EN = GND

EN = GND

INL SUPPLY CURRENT vs. TEMPERATURE

TEMPERATURE (°C)

INL SUPPLY CURRENT (mA)

MAX87228 toc22

-40 -15 10 35 60 85

0

1

2

3

4

5

6

7

EN = VL

EN = GND

HIGH-VOLTAGE SWITCH

CONTROL (MODE 1)

MAX8728toc23

2μs/div

V

CTL

, 5V/div

0V

0V

0V

V

MODE

, 2V/div

V

GON

, 10V/div

HIGH-VOLTAGE SWITCH

CONTROL (MODE 2)

MAX8728toc23

2μs/div

V

CTL

, 5V/div

0V

0V

0V

V

MODE

, 2V/div

V

GON

, 10V/div

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

12 ______________________________________________________________________________________

Pin Description

PIN

FUNCTION

1

Step-Down Regulator and Negative Charge-Pump Power Ground

2

Step-Down Regulator Output Sense Input. OUT1 is the inverting input to the internal current-sense amplifier.
Connect OUT1 directly to the step-down regulator output.

3

Negative Charge-Pump Regulator Driver Output. See the Negative Charge-Pump Regulator section for details.

4 CTL

High-Voltage Switch-Control Block Timing Control Input. See the High-Voltage Switch Control section for

5 IN Step-Down Regulator and Negative Charge-Pump Regulator Supply Input

6 LX1

Step-Down Regulator Switching Node. LX1 is the source of the internal high-side MOSFET. Connect the
inductor and Schottky catch diode to LX1 and minimize the trace area for low EMI.

7 BST

Step-Down Regulator Bootstrap Pin. BST is the supply for the high-side MOSFET gate driver. Connect a 0.1µF
ceramic capacitor from BST to LX1.

8 INL

5V Internal Linear Regulator and Startup Circuitry Supply Input. The input voltage range of INL is between
+7.0V and +13.2V. Connect a 0.22µF ceramic capacitor between INL and GND. Place the capacitor close to
the IC.

9VL

5V Internal Linear Regulator Output. VL powers the internal MOSFET gate drivers and the control circuitry.
Bypass VL

to GND with a 1µF ceramic capacitor. VL can provide up to 25mA external load current.

10 V

CC

Internal Reference Supply Input. Connect VCC directly to VL.

11

Active-Low Shutdown Control Input. All outputs (except for REF and VL) are disabled and the GATE pin goes
high when SHDN is low.

12

Analog Ground

13 REF

Reference Output. Connect a 0.22µF ceramic capacitor between REF and GND. All regulator outputs are
disabled until REF exceeds its UVLO threshold.

14 FBP

Positive Charge-Pump Regulator Feedback Input. Connect FBP to the center of a resistive voltage-divider
between the positive output and GND to set the positive charge-pump regulator output voltage. Place the
resistive voltage-divider close to FBP.

15

Positive Charge-Pump Power Ground

16

Positive Charge-Pump Regulator Driver Output. See the Positive Charge-Pump Regulator section for details.

17

Positive Charge-Pump Regulator Supply Input. Connect SUPP directly to IN and bypass SUPP to GNDP with a
minimum 0.1µF ceramic capacitor.

18

Frequency Select Pin. Connect FSEL to REF for 500kHz operation. Connect FSEL to VCC for 1MHz operation.
Connect to GND for 1.5MHz operation.

19 SRC High-Voltage Switch Control Block Input. SRC is the source of the internal, high-voltage, p-channel MOSFET.

20

High-Voltage Switch Control Block Output. GON is the common junction of the internal high-voltage MOSFETs.
GON is internally pulled to GND through a 4mA internal current source when the switch control block is
disabled.

NAME

GND1

OUT1

DRVN

SHDN

GND

GNDP

DRVP

SUPP

FSEL

GON

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 13

Pin Description (continued)

PIN

NAME

FUNCTION

21 DRN

High-Voltage Switch Control Input. DRN is the drain of the internal high-voltage p-channel MOSFET connected
to GON. See the High-Voltage Switch Control section for details.

22 THR

GON Falling Regulation Adjustment Input. Connect THR to the center of a resistive voltage-divider between a
reference supply and GND to adjust the GON falling regulation set point. CTL and MODE allow GON to
disconnect from SRC and be discharged through DRN; discharge stops when GON reaches 10 x V

THR

. See

the High-Voltage Switch Control section for details.

23 FB2

Step-Up Regulator Feedback Input. Connect FB2 to the center of a resistive voltage-divider between the step­up regulator output and GND to set the step-up regulator output voltage. Place the resistive voltage-divider
close to FB2.

24 FBN

Negative Charge-Pump Regulator Feedback Input. Connect FBN to the center of a resistive voltage-divider
between the negative output and REF to set the negative charge-pump regulator output voltage. Place the
resistive voltage-divider close to FBN.

25

Step-Down Regulator Power Ground

26 LX2

Step-Up Regulator Switching Node. Connect the inductor and the Schottky diode to LX2 and minimize the
trace area for low EMI.

27

Input MOSFET Gate-Driver Output. GATE controls an external p-channel MOSFET between the input voltage
and the step-up regulator’s inductor. The switch is off when the step-up regulator is turned off, so that the
regulator’s output discharges to ground. During startup, the step-up regulator’s soft-start begins when V

GATE

falls below the GATE done threshold.

28 EN

Enable Input. Pulling EN high or leaving EN unconnected enables the step-up regulator and the negative
charge pump. Connecting EN to GND disables the above blocks and puts the step-down regulator in skip
mode. EN sources 5µA to allow a capacitor-controlled startup delay.

29

High-Voltage Switch-Control Block Mode Selection Input and Timing-Adjustment Input. See the High-Voltage
Switch Control section for details.

30 DEL

Positive Charge-Pump Regulator and High-Voltage Switch-Control Delay Input. Connect a capacitor between
DEL and GND to set the delay time. A 5µA current source charges C

DEL

. DEL is internally pulled to GND

through a 20Ω internal resistor in shutdown.

31

Step-Up Regulator Error Amplifier Compensation Pin. See the Loop Compensation section for details.

32 FB1

Step-Down Regulator Feedback Input. Connect FB1 to the center of a resistive voltage-divider between the
step-down regulator output and GND to set the step-down regulator output voltage.

—EP

Exposed Pad. Connect the exposed backside pad to GND and provide adequate thermal path to cool the IC.
See the PC Board Layout and Grounding section.

GND2

GATE

MODE

COMP

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

14 ______________________________________________________________________________________

MAX8728

AV

DD

13.5V/500mA

GATE

P1

D1

D6

INL

V

IN

10.8V TO 13.2V

OUT1

3.3V/2A

V

GOFF

-6V/150mA

D5

D4

IN

26

25

23

31

4

22

29

21

20

19

14

15

LX1

OUT1

FB1

EN

SHDN

GND1

GND

FBP

GNDP

SRC

REF

GON

DRN

MODE

THR

COMP

C1

10μF

16V

C9

22μF

6.3V

C2

0.22μF

L1

6.4μH

C3
10μF
16V

C24
1000pF

C4
10μF
16V

C7

1μF

C10

0.1μF

R9

44.2kΩ

1%

R10

158kΩ

1%

R3

160kΩ

R4

127kΩ

1%

R5

22.1kΩ
1%

R12

10.0kΩ,
1%

R11

6.49kΩ,1%

R6

2kΩ

R1
115kΩ
1%

R2
20kΩ
1%

R7
287kΩ
1%

R8

22.1kΩ
1%

FROM
TCON

CTL

FB2

LX2

5

8

VLV

CC

L2

2.6μH

910 27

7

6

1

2

32

11

28

DEL

C11

0.1μF

C12

0.22μF

C25
47pF

C14
220pF

C15
1μF

C28
10pF

C13
100pF

C16

0.1μF

C17
1μF

C21

0.1μF

C20

0.1μF

C18

0.1μF

C19

0.1μF

30

13

12

24

3

18

17 16

D2

C8

0.1μF

C5
10μF
16V

C6
10μF
16V

GND2

FBN

DRVN

SUPPFSEL

DRVP

BST

V

GON

28V/50mA

ON/OFF

D3

VIN

Figure 1. Typical Operating Circuit

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 15

Typical Operating Circuit

The typical operating circuit (Figure 1) of the MAX8728 is
a complete power-supply system for TFT LCD panels in
monitors and TVs. The circuit generates a +3.3V logic
supply, a +13.5V source driver supply, a +28V positive
gate driver supply, and a -6V negative gate driver supply
from a 12V ±10% input supply and operates at 1.5MHz.
Table 1 lists some selected components and Table 2
lists the contact information for component suppliers.

Detailed Description

The MAX8728 is a multiple-output power supply
designed primarily for TFT LCD panels used in monitors
and TVs. It contains a step-down switching regulator to
generate the logic supply rail, a step-up switching regu­lator to generate the source driver supply, and two
charge-pump regulators to generate the gate-driver
supplies. Each regulator features adjustable output volt­age, digital soft-start, and timer-delayed fault protection.
Both the step-down and step-up regulators use fixed­frequency current-mode control architectures. The two
switching regulators are 180° out of phase to minimize
the input ripple. The internal oscillator offers three pin­selectable frequency options (500kHz/1MHz/1.5MHz)
allowing users to optimize their designs based on the
specific application requirements. In addition, the
MAX8728 features a high-voltage switch-control block,
an internal 5V linear regulator, a 2V reference output,
well-defined power-up and power-down sequences,
and thermal-overload protection. Figure 2 shows the
MAX8728 functional diagram.

Step-Down Regulator

The step-down regulator consists of an internal n-chan­nel MOSFET with gate driver, a lossless current-sense
network, a current-limit comparator, and a PWM con­troller block. The external power stage consists of a
Schottky diode rectifier, an inductor, and output capac­itors. The output voltage is regulated by changing the
duty cycle of the high-side MOSFET. A bootstrap circuit
that uses a 0.1µF flying capacitor between LX1 and
BST provides the supply voltage for the high-side gate
driver. Although the MAX8728 also includes a 25Ω (typ)
low-side MOSFET, this switch is used to charge the
bootstrap capacitor during startup and maintains fixed­frequency operation at light load and cannot be used
as a synchronous rectifier. An external Schottky diode
(D2 in Figure 1) is always required.

PWM Controller Block

The heart of the PWM control block is a multi-input,
open-loop comparator that sums three signals: the out-

put voltage signal with respect to the reference voltage,
the current-sense signal, and the slope compensation.
The PWM controller is a direct-summing type, lacking a
traditional error amplifier and the phase shift associated
with it. This direct-summing configuration approaches
ideal cycle-by-cycle control over the output voltage.

When EN is high or floating, the controller always oper­ates in fixed-frequency PWM mode. Each pulse from
the oscillator sets the main PWM latch that turns on the
high-side switch until the PWM comparator changes
state. As the high-side switch turns off, the low-side
switch turns on. The low-side switch stays on until the
beginning of the next clock cycle.

When EN is low, the controller operates in skip mode.
The skip mode dramatically improves light-load effi­ciency by reducing the effective frequency, which
reduces switching losses. It keeps the actual peak
inductor current at about 0.8A in an active cycle, allow­ing subsequent cycles to be skipped. Skip mode tran­sitions seamlessly to fixed-frequency PWM operation as
load current increases.

Table 1. Component List (1.5MHz)

DESIGNATION DESCRIPTION

C1, C3, C4, C5, C6

10µF ±20%, 16V X5R ceramic
capacitors (1206)
TDK C3216X5R1C106M

D1, D2

3A, 30V Schottky diode (M-flat)
Toshiba CMS02 (top mark S2)

D3, D4, D5

220mA, 100V dual diode (SOT23)
Fairchild MMBD4148SE (top mark D4)

L1

6.4µH, 1.5ADC inductor
Sumida CDRH6D12-6R4

L2

2.6µH, 2.6ADC inductor
Sumida CDRH6D12-2R6

P1

2.4A, -20V p-channel MOSFET
(3-pin SuperSOT)
Fairchild FDN304P (top mark 304)

Table 2. Component Suppliers

SUPPLIER PHONE

Fairchild Semiconductor 408-822-2000

Sumida 847-545-6700

TDK 847-803-6100

Toshiba 949-455-2000

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

16 ______________________________________________________________________________________

V

L

IN

BST

GATEINL

LX2

GND2

FB2

COMP

LX1

GND1

OUT1

FB1

SHDN

EN

DEL

REF

GND

FBN

FSEL

DRVN

SUPP

CTL

THR

MODE

DRN

GON

SRC

FBP

DRVP

V

IN

OUT1

V

CC

V

L

V

L

V

GOFF

AV

DD

V

GON

FROM TCON

ON/OFF

LCD ENABLE

IN

STEP-DOWN

REGULATOR

SEQUENCE

CONTROL

V

L

REGULATOR

STEP-UP

REGULATOR

THERMAL

SHUTDOWN

OSCILLATOR

FAULT

LOGIC AND

TIMER

REFERENCE

NEGATIVE

REGULATOR

SWITCH

CONTROL

BLOCK

POSITIVE

REGULATOR

MAX8728

VIN

Figure 2. Functional Diagram

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 17

Current Limiting and Lossless Current Sensing

The current-limit circuit turns off the high-side MOSFET
switch whenever the voltage across the high-side
MOSFET exceeds an internal threshold corresponding to
the actual current limit of 2.8A ±10%.

For current-mode control, an internal lossless sense
network derives a current-sense signal from the induc­tor DC resistance. The time constant of the current­sense network is not required to match the time
constant of the inductor and has been chosen to pro­vide sufficient current-ramp signal for stable operation
at each operating frequency. The current-sense signal
is AC-coupled into the PWM comparator, eliminating
most DC output voltage variation with load current.

Low-Frequency Operation

The step-down regulator of the MAX8728 enters into
low-frequency operating mode if the voltage on OUT1
is below 1.3V. In the low-frequency mode, the switching
frequency of the step-down regulator is 1/6 the oscilla­tor frequency. This feature prevents potentially uncon­trolled inductor current if OUT1 is overloaded or
shorted to ground.

Soft-Start and Fault Protection

The step-down regulator includes a 7-bit soft-start DAC
that steps the internal reference voltage from zero to 2V
in 128 steps. The soft-start period is 3ms (typ) and FB1
fault detection is disabled during this period. The soft­start feature effectively limits the inrush current during
startup (see the Step-Down Regulator Soft-Start
Waveforms in the Typical Operating Characteristics).
The MAX8728 monitors OUT1 (fixed-output mode) or
FB1 (adjustable-output mode) for undervoltage condi­tions. If the voltage is continuously below 80% (typ) of
the nominal regulation point for approximately 50ms,
the MAX8728 sets a fault latch, shutting down all out­puts except VL and REF.

Step-Up Regulator

The step-up regulator employs a current-mode, fixed­frequency PWM architecture to maximize loop band­width and provide fast transient response to pulsed
loads typical of TFT LCD panel source drivers. The inte­grated MOSFET and the built-in digital soft-start function
reduce the number of external components required
while controlling inrush currents. The output voltage can
be set from VINto 28V with an external resistive voltage­divider. The regulator controls the output voltage and
the power delivered to the output by modulating the
duty cycle of the internal power MOSFET in each
switching cycle.

PWM Controller Block

An error amplifier compares the signal at FB2 to 2.0V
and changes the COMP output. The voltage at COMP
sets the peak inductor current. As the load varies, the
error amplifier sources or sinks current to the COMP
output accordingly to produce the inductor peak cur­rent necessary to service the load. To maintain stability
at high duty cycles, a slope-compensation signal is
summed with the current-sense signal.

On the rising edge of the internal clock, the controller
sets a flip-flop, turning on the n-channel MOSFET and
applying the input voltage across the inductor. The cur­rent through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the cur­rent-feedback signal and the slope compensation
exceed the COMP voltage, the controller resets the flip­flop and turns off the MOSFET. Since the inductor cur­rent is continuous, a transverse potential develops
across the inductor that turns on the diode (D1). The
voltage across the inductor then becomes the differ­ence between the output voltage and the input voltage.
This discharge condition forces the current through the
inductor to ramp back down, transferring the energy
stored in the magnetic field to the output capacitor and
the load. The MOSFET remains off for the rest of the
clock cycle.

Input Switch Control

The GATE pin of the MAX8728 controls an optional
external p-channel MOSFET between the input supply
and the inductor of the step-up regulator. This function
disconnects the step-up regulator from the input supply
and allows the regulator output to discharge to ground
when the step-up regulator is disabled. When EN is low,
GATE is internally pulled up to the input supply through
a 1kΩ resistor. Once EN and SHDN are high and the
negative charge-pump regulator is in regulation, the
MAX8728 starts pulling down GATE with an 11µA inter­nal current source. The external p-channel MOSFET
turns on and connects the input supply to the step-regu­lator when V

GATE

falls below the turn-on threshold of the

MOSFET. When V

GATE

reaches VIN- 4V, the step-up
regulator is enabled and initiates a soft-start routine.
V

GATE

continues to fall until it reaches VIN- 5V.

Soft-Start and Fault Protection

The step-up regulator achieves soft-start by linearly
ramping up its internal current limit. The soft-start termi­nates when the output reaches regulation or the full
current limit has been reached. The current limit rises
from zero to the full current limit in approximately 3ms.

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

18 ______________________________________________________________________________________

The soft-start feature effectively limits the inrush current
during startup (see the Step-Up Regulator Soft-Start
Waveforms in the Typical Operating Characteristics).
The MAX8728 monitors FB2 for undervoltage condi­tions. If the voltage is continuously below 90% of the
nominal regulation point for approximately 50ms, the
MAX8728 sets a fault latch, shutting down all outputs
except VL, REF, and the step-down regulator.

Positive Charge-Pump Regulator

The positive charge-pump regulator is typically used to
generate the positive supply rail for the TFT LCD gate­driver ICs. The output voltage is set with an external
resistive voltage-divider from its output to GND with the
midpoint connected to FBP. The number of charge­pump stages and the setting of the feedback divider
determine the output voltage of the positive charge­pump regulator. The charge pump includes a high­side, p-channel MOSFET (P1) and a low-side,
n-channel MOSFET (N1) to control the power transfer
as shown in Figure 3.

The error comparator compares the feedback signal
(FBP) with a 2.0V internal reference. If the feedback
signal is below the reference, the charge-pump regula­tor turns on P1 and turns off N1 when the rising edge of
the oscillator clock arrives, level shifting the flying
capacitors (C18 and C19) by V

SUPP

volts. If the result-

ing voltage on C18 and C19 is greater than their asso-

ciated reservoir capacitors (C20 and C15), charge
flows until the diode connecting each flying capacitor
to its reservoir capacitor turns off. The falling edge of the
oscillator clock turns off P1 and turns on N1, charging
the flying capacitors (C18 and C19) through the diodes
that connect them to the reservoir capacitors (C21 and
C20). If the feedback signal is above the reference
when the rising edge of the oscillator comes, the regula­tor ignores this clock edge and keeps N1 on and P1 off.

The positive charge-pump regulator’s startup can be
delayed by connecting an external capacitor from DEL to
GND. An internal constant-current source begins charg­ing the DEL capacitor when EN and SHDN are logic high,
the negative charge pump reaches regulation, and GATE
has gone low. When the DEL voltage exceeds V

REF

/2,
the positive charge-pump regulator is enabled. Each time
it is enabled, the positive charge-pump regulator goes
through a soft-start routine by ramping up its internal ref­erence voltage from 0 to 2V in 128 steps. The soft-start
period is 3ms (typ) and FBP fault detection is disabled
during this period. The soft-start feature effectively limits
the inrush current during startup. The MAX8728 also
monitors the FBP voltage for undervoltage conditions. If
VFBP is continuously below 80% of its nominal regulation
point for approximately 50ms, the MAX8728 sets a fault
latch, shutting down all outputs except VL, REF, and the
step-down regulator.

LEVEL
SHIFT

REF

Q

CLK

D

OSC

FF

ERROR

COMPARATOR

P1

N1

DRVP

C21

C20

C15

C28

C19

C18

GNDP

SUPP

FBP

POSITIVE CHARGE-PUMP REGULATOR

INPUT

SUPPLY

OUTPUT

MAX8728

Figure 3. Positive Charge-Pump Regulator Block Diagram

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 19

Negative Charge-Pump Regulator

The negative charge-pump regulator is typically used to
generate the negative supply rail for the TFT LCD gate­driver ICs. The output voltage is set with an external
resistive voltage-divider from its output to REF with the
midpoint connected to FBN. The number of charge­pump stages and the setting of the feedback-divider
determine the output of the negative charge-pump regu­lator. The charge-pump controller includes a high-side,
p-channel MOSFET (P2) and a low-side, n-channel
MOSFET (N2) to control the power transfer as shown in
Figure 4.

The error comparator compares the feedback signal
(FBN) with a 250mV internal reference. If the feedback
signal is above the reference, the charge-pump regula­tor turns on N2 and turns off P2 when the rising edge of
the oscillator clock arrives, level shifting the flying
capacitor (C16). The falling edge of the oscillator clock
turns off N2 and turns on P2, charging the flying capac­itor (C16) through the diode that connects it to the
reservoir capacitor (C1). If the feedback signal is below
the reference (output is in regulation) when the rising
edge of the oscillator comes, the regulator ignores this
clock edge and keeps P2 on and N2 off.

The negative charge-pump regulator is enabled when
SHDN and EN are logic high and the step-down regula­tor reaches regulation. Each time it is enabled, the neg­ative charge-pump regulator goes through a soft-start
routine by ramping down its internal reference voltage
from 2V to 250mV in 128 steps. The soft-start period is
3ms (typ) and FBN fault detection is disabled during
this period. The soft-start feature effectively limits the
inrush current during startup. The MAX8728 also moni­tors the FBN voltage for undervoltage conditions. If
V

FBN

is continuously above 600mV for approximately
50ms, the MAX8728 sets a fault latch, shutting down all
outputs except VL, REF, and the step-down regulator.

High-Voltage Switch Control

The MAX8728’s high-voltage switch control block
(Figure 5) consists of two high-voltage, p-channel
MOSFETs: Q1, between SRC and GON and Q2,
between GON and DRN. The switch control block is
enabled when V

DEL

goes above V

REF

/ 2. Q1 and Q2
are controlled by CTL and MODE. There are two differ­ent modes of operation (see the Typical Operating
Characteristics section).

Activate the first mode by connecting MODE to REF.
When CTL is logic high, Q1 turns on and Q2 turns off,
connecting GON to SRC. When CTL is logic low, Q1

0.25V

Q

CLK

D

OSC

FF

ERROR

COMPARATOR

P2

N2

DRVN

GND1

IN

FBN

NEGATIVE CHARGE-PUMP REGULATOR

INPUT

SUPPLY

OUTPUT

REF

MAX8728

C17

C16

C1

Figure 4. Negative Charge-Pump Regulator Block Diagram

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

20 ______________________________________________________________________________________

turns off and Q2 turns on, connecting GON to DRN.
GON can then be discharged through a resistor con­nected between DRN and GND or AVDD. Q2 turns off
and stops discharging GON when V

GON

reaches 10

times the voltage on THR.

When V

MODE

is less than 0.9 x V

REF

, the switch control
block works in the second mode. The rising edge of
V

CTL

turns on Q1 and turns off Q2, connecting GON to
SRC. An internal n-channel MOSFET Q3 between
MODE and GND is also turned on to discharge an
external capacitor between MODE and GND. The
falling edge of VCTL turns off Q3, and an internal 50µA
current source starts charging the MODE capacitor.
Once V

MODE

exceeds 0.5 x V

REF

, the switch control
block turns off Q1 and turns on Q2, connecting GON to
DRN. GON can then be discharged through a resistor
connected between DRN and GND or AVDD. Q2 turns
off and stops discharging GON when V

GON

reaches 10

times the voltage on THR.

When the LCD is shut down or in a fault state, the
switch control block is disabled, DZL is held low, and
GON is discharged to GND through an internal 4mA
current source. If the DRN resistor connects DRN to
AVDDor another voltage above ground, the Q2 body
diode conducts. To prevent the body diode conduc­tion, an external diode must be added in series with the
DRN resistor (D6 in Figure 1). During startup, the 4mA
current source and Q4 are released when GATE reach­es the GATE DONE threshold.

Linear Regulator (VL)

The MAX8728 includes an internal linear regulator. INL
is the input of the linear regulator. The input voltage
range is between 7V and 13.2V. The output voltage is
set to 5V. The regulator powers the internal MOSFET
drivers, PWM controllers, charge-pump regulators, and
logic circuitry. The total external load capability is
25mA. Bypass VL to GND with a minimum 1µF ceramic
capacitor.

Reference Voltage (REF)

The reference output is nominally 2V, and can
source at least 50µA (see the Typical Operating
Characteristics section). VCCis the input of the internal
reference block. Bypass REF with a 0.22µF ceramic
capacitor connected between REF and GND.

Frequency Selection (FSEL)

The step-down regulator and step-up regulator use the
same internal oscillator. The FSEL input selects the
switching frequency. Table 3 shows the switching fre­quency based on the FSEL connection. High-frequency
(1.5MHz) operation optimizes the application for the

smallest component size, trading off efficiency due to
higher switching losses. Low-frequency (500kHz) oper­ation offers the best overall efficiency at the expense of
component size and board space.

To reduce the input RMS current, the step-down regu­lator and the step-up regulator operate 180° out of
phase from each other. The feature allows the use of
less input capacitance.

Power-Up Sequence

The step-down regulator starts up when the MAX8728’s
internal reference voltage (REF) is above its undervolt­age lockout (UVLO) threshold and SHDN is logic high.
The FB1 fault-detection circuit is enabled after the step­down regulator reaches regulation. The negative
charge-pump regulator starts up when both EN and
SHDN are logic high and REF is above its UVLO thresh­old. Once the negative charge-pump regulator output is
in regulation, the MAX8728 enables the FBN fault-detec­tion circuit and the input-switch control block, which starts
pulling down GATE with a 11µA internal current source.
The external p-channel MOSFET turns on and connects
the input supply to the step-up regulator when V

GATE

falls below the turn-on threshold of the MOSFET.

When V

GATE

reaches the GATE DONE threshold, the
MAX8728 enables the step-up regulator and the posi­tive charge-pump adjustable delay block. The FB2
fault-detection circuit is enabled after the step-up regu­lator reaches regulation. The delay block charges the
DEL capacitor with an internal 5µA current source and
V

DEL

rises linearly. When V

DEL

exceeds 1V (typ), the
MAX8728 enables the positive charge-pump regulator
and the high-voltage switch control block. The FBP fault
detection is enabled after the positive charge-pump
regulator reaches regulation.

Power-Down Control

The MAX8728 disables the step-up regulator, positive
charge-pump regulator, negative charge-pump regula­tor, input switch control block, delay block, and high­voltage switch control block when EN or SHDN is logic
low, or when any fault latch is set. The step-down regu­lator is disabled only when SHDN is logic low, the step­down fault latch is set, or during thermal overload.

Table 3. Frequency Selection

FSEL

SWITCHING FREQUENCY (kHz)

GND 1500

VCC 1000

REF 500

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 21

MODE

CTL

DEL

SRC

GON

DRN

0.5 x V

REF

GATE DONE

5μA

REF

Q1

4mA

Q4

Q3

R

4R

5R

50μA

SWITCH CONTROL

THR

9R

1kΩ

R

REF

1kΩ

SHDN

FAULT

EN

MAX8728

Q2

Figure 5. Switch-Control Functional Diagram

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

22 ______________________________________________________________________________________

Fault Protection

During steady-state operation, if any output of the four
regulators (step-down regulator, step-up regulator,
positive charge-pump regulator, and negative charge­pump regulator) does not exceed its respective fault­detection threshold, the MAX8728 activates an internal
fault timer. If any condition or the combination of condi­tions indicates a continuous fault for the fault-timer
duration (50ms typ), the MAX8728 sets a fault latch. If
the fault is caused by the step-up regulator or one of
the charge pumps (LCD fault), the MAX8728 shuts
down all the outputs except VL, REF, and the step­down regulator. Once the fault condition is removed,
toggle EN or SHDN, or cycle the input voltage to clear
the LCD fault latch and restart the LCD supplies. If the
fault is caused by the step-down regulator, the
MAX8728 shuts down all the outputs except VL and
REF. Once the fault condition is removed, toggle SHDN
or cycle the input voltage to clear the step-down fault
latch and restart the supplies.

Thermal-Overload Protection

The thermal-overload protection prevents excessive
power dissipation from overheating the MAX8728.
When the junction temperature exceeds TJ= +160°C, a
thermal sensor immediately activates the fault protec­tion, which shuts down all the outputs except the refer­ence, allowing the device to cool down. Once the
device cools down by approximately 15°C, the
MAX8728 automatically restarts all the supplies.

The thermal-overload protection protects the controller
in the event of fault conditions. For continuous opera­tion, do not exceed the absolute maximum junction
temperature rating of TJ= +150°C.

Design Procedure

Step-Down Regulator Design

Inductor Selection

Three key inductor parameters must be specified:
inductance value (L), peak current (I

PEAK

), and DC
resistance (RDC). The following equation includes a
constant, LIR, which is the ratio of peak-to-peak induc­tor ripple current to DC load current. A higher LIR value
allows smaller inductance, but results in higher losses
and higher ripple. A good compromise between size
and losses is typically found at a 30% ripple-current to
load-current ratio (LIR = 0.3), which corresponds to a
peak inductor current 1.15 times the DC load current:

where I

OUT1(MAX)

is the maximum DC load current, and
the switching frequency fSWis 1.5MHz when FSEL is
tied to GND, 1MHz when FSEL is tied to VCC, and
500kHz when FSEL is tied to REF. The exact inductor
value is not critical and can be adjusted to make trade­offs among size, cost, and efficiency. Lower inductor
values minimize size and cost, but they also increase
the output ripple and reduce the efficiency due to high­er peak currents. On the other hand, higher inductor
values increase efficiency, but at some point resistive
losses due to extra turns of wire will exceed the benefit
gained from lower AC current levels.

The inductor’s saturation current must exceed the peak
inductor current. The peak current can be calculated by:

The inductor’s DC resistance should be low for good
efficiency. Find a low-loss inductor having the lowest
possible DC resistance that fits in the allotted dimen­sions. Ferrite cores are usually the best choice, espe­cially at the higher frequency settings. Shielded-core
geometries help keep noise, EMI, and switching wave­form jitter low.

Input Capacitors

The input filter capacitors reduce peak currents drawn
from the power source and reduce noise and voltage
ripple on the input caused by the regulator’s switching.
They are usually selected according to input ripple cur­rent requirements and voltage rating, rather than
capacitance value. The input voltage and load current
determine the RMS input ripple current (I

RMS

):

The worst case is I

RMS

= 0.5 x I

OUT1

, which occurs at

VIN= 2 x V

OUT1

.

For most applications, ceramic capacitors are used
because of their high ripple current and surge-current
capabilities. For optimal circuit long-term reliability,
choose an input capacitor that exhibits less than +10°C
temperature rise at the RMS input current correspond­ing to the maximum load current.

II

VVV

V

RMS OUT

OUT IN OUT

IN

=×

×−

()

1

11

I

VVV

fL V

II

I

OUT RIPPLE

OUT IN OUT

SW OUT IN

OUT PEAK OUT MAX

OUT RIPPLE

1

11

1

11

1

2

_

_()

_

=

×−

()

××

=+

L

VVV

V f I LIR

OUT

OUT IN OUT

IN SW OUT MAX

1

11

1 ()

=

×−

()

×× ×

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 23

Output-Capacitor Selection

Since the MAX8728’s step-down regulator is internally
compensated, it is stable with any reasonable amount
of output capacitance. However, the actual capaci­tance and equivalent series resistance (ESR) affect the
regulator’s output ripple voltage and transient
response. The rest of this section deals with how to
determine the output capacitance and ESR needs
according to the ripple voltage and load-transient
requirements.

The output voltage ripple has two components: varia­tions in the charge stored in the output capacitor, and
the voltage drop across the capacitor’s ESR caused by
the current into and out of the capacitor:

where I

OUT1_RIPPLE

is defined in the Step-Down

Regulator, Inductor Selection section, C

OUT1

is output

capacitance, and R

ESR_OUT1

is the ESR of output

capacitor C

OUT1

. In Figure 1’s circuit, the inductor rip­ple current is 0.6A. If the voltage ripple requirement of
Figure 1’s circuit is ±1% of the 3.3V output, then the
total peak-to-peak ripple voltage should be less than
66mV. Assuming that the ESR ripple and the capacitive
ripple each should be less than 50% of the total peak­to-peak ripple, then the ESR should be less than 55mΩ
and the output capacitance should be more than 1.5µF
to meet the total ripple requirement. A 22µF capacitor
with ESR (including PC board trace resistance) of 10mΩ
is selected for the standard application circuit in Figure 1,
which easily meets the voltage-ripple requirement.

The step-down regulator’s output capacitor and ESR
also affect the voltage undershoot and overshoot when
the load steps up and down abruptly. The undershoot
and overshoot also have two components: the voltage
steps caused by ESR and voltage sag and soar due to
the finite capacitance and inductor slew rate. Use the
following formulae to check if the ESR is low enough
and the output capacitance is large enough to prevent
excessive soar and sag.

The amplitude of the ESR step is a function of the load
step and the ESR of the output capacitor:

V

OUT1_ESR_STEP

= ΔI

OUT1

x R

ESR_OUT1

The amplitude of the capacitive sag is a function of the
load step, the output capacitor value, the inductor

value, the input-to-output voltage differential, and the
maximum duty cycle:

The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor
value and the output voltage:

Given the component values in the circuit of Figure 1,
during a 2A step-load transient, the voltage step due to
capacitor ESR is negligible. The voltage sag and soar
are 40.2mV and 71.6mV, respectively.

Rectifier Diode

The MAX8728’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. In general, a 2A Schottky
diode works well in the MAX8728’s step-down regulator.

Output-Voltage Selection

Connect a resistive voltage-divider between OUT1 and
GND with the center tap connected to FB1 to adjust the
output voltage. Choose R12 (resistance from FB1 to
GND) to be between 5kΩ and 50kΩ, and solve for R11
(resistance from OUT1 to FB1) using the equation:

where V

FB1

= 2V, and V

OUT1

may vary from 2V to 3.6V.
Connecting a small capacitor (e.g., 47pF) between FB1
and GND reduces FB1 noise sensitivity.

Step-Up Regulator Design

Inductor Selection

The inductance value, peak-current rating, and series
resistance are factors to consider when selecting the
step-up inductor. These factors influence the convert­er’s efficiency, maximum output load capability, tran­sient response time, and output voltage ripple. Physical
size and cost are also important factors to be consid­ered.

The maximum output current, input voltage, output volt­age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-

RR

V

V

OUT

FB

11 12 1

1

1

=× −

⎛

⎝

⎜

⎞

⎠

⎟

V

LI

CV

OUT SOAR

OUT OUT

OUT OUT

1

11

2

11

2

_

()

=

×

××

Δ

V

LI

CV DV

OUT SAG

OUT OUT

OUT IN MIN MAX OUT

1

11

2

11

2

_

()

()

=

×

×× ×−

()

Δ

VV V

VIR

V

I

Cf

OUT RIPPLE OUT RIPPLE ESR OUT RIPPLE C

OUT RIPPLE ESR OUT RIPPLE ESR OUT

OUT RIPPLE C

OUT RIPPLE

OUT SW

11 1

11 1

1

1

1

8

__()_()

_() _ _

_()

_

=+

=×

=

××

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

24 ______________________________________________________________________________________

rent ripple and therefore reduce the peak current,
which decreases core losses in the inductor and I2R
losses in the entire power path. However, large induc­tor values also require more energy storage and more
turns of wire, which increase physical size and can
increase I

2

R losses in the inductor. Low inductance val­ues decrease the physical size but increase the current
ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size, and cost.

The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current
to the average DC inductor current at the full load cur­rent. The best trade-off between inductor size and cir­cuit efficiency for step-up regulators generally has an
LIR between 0.2 and 0.5. However, depending on the
AC characteristics of the inductor core material and
ratio of inductor resistance to other power-path resis­tances, the best LIR can shift up or down. If the induc­tor resistance is relatively high, more ripple can be
accepted to reduce the number of turns required and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can decrease losses throughout the power
path. If extremely thin, high-resistance inductors are
used, as is common for LCD panel applications, the
best LIR can increase to between 0.5 and 1.0.

Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficien­cy improvements in typical operating regions.

Calculate the approximate inductor value using the typ­ical input voltage (V

IN

), the maximum output current

(I

AVDD(MAX)

), the expected efficiency (η

TYP

) taken from
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:

Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur­rent at the minimum input voltage V

IN(MIN)

using con-

servation of energy and the expected efficiency at that
operating point (η

MIN

) taken from an appropriate curve

in the Typical Operating Characteristics:

Calculate the ripple current at that operating point and
the peak current required for the inductor:

The inductor’s saturation current rating and the
MAX8728’s LX2 current limit should exceed I

AVDD_PEAK

and the inductor’s DC current rating should exceed
I

IN(DC,MAX)

. For good efficiency, choose an inductor

with less than 0.1Ω series resistance.

Considering the Typical Operating Circuit in Figure 1,
the maximum load current (I

AVDD(MAX)

) is 500mA with
a 13.5V output and a typical input voltage of 12V.
Choosing an LIR of 0.3 and estimating efficiency of
95% at this operating point:

Using the circuit’s minimum input voltage (10.8V) and
estimating efficiency of 90% at that operating point:

The ripple current and the peak current are:

Output-Capacitor Selection

The total output voltage ripple has two components: the
capacitive ripple caused by the charging and dis­charging of the output capacitance, and the ohmic rip­ple due to the capacitor’s ESR:

where I

AVDD_PEAK

is the peak-inductor current (see the

Inductor Selection section). For ceramic capacitors, the

VV V

V

I

C

VV

Vxf

and

VIxR

AVDD RIPPLE AVDD RIPPLE C AVDD RIPPLE ESR

AVDD RIPPLE C

AVDD

AVDD

AVDD IN

AVDD SW

AVDD RIPPLE ESR AVDD PEAK ESR

__()_()

_()

_() _ _

,

=+

≈

−

⎛

⎝

⎜

⎞

⎠

⎟

≈

I

VVV

H V MHz

A

IAAA

RIPPLE

PEAK

. . .

. . .

.

. . .

=

×−

()

××

≈

=+ ≈

10 8 13 5 10 8

6 4 13 5 1 5

023

069

023

2

081

μ

I

AV

V

A

IN DC MAX(, )

. .

. .

.=

×

×

≈

0 5 13 5

10 8 0 9

069

L

V

V

VV

A MHz

H

AVDD

.

.

..

.

.

.=

⎛
⎝

⎜

⎞
⎠

⎟

−

×

⎛
⎝

⎜

⎞
⎠

⎟

⎛
⎝

⎜

⎞
⎠

⎟

≈

12

13 5

13 5 12

05 15

095

05

64

2

μ

I

VVV

LVf

II

I

AVDD RIPPLE

IN MIN AVDD IN MIN

AVDD AVDD SW

AVDD PEAK IN DC MAX

AVDD RIPPLE

_

() ()

_(,)

_

=

×−

()

××

=+

2

I

IV

V

IN DC MAX

AVDD MAX AVDD

IN MIN MIN

(, )

()

()

=

×

×η

L

V

V

VV

I f LIR

AVDD

IN

AVDD

AVDD IN

AVDD MAX SW

TYP

()

=

⎛

⎝

⎜

⎞

⎠

⎟

−
×

⎛

⎝

⎜

⎞

⎠

⎟

⎛
⎝

⎜

⎞
⎠

⎟

2

η

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 25

output voltage ripple is typically dominated by
V

AVDD_RIPPLE(C)

. The voltage rating and temperature
characteristics of the output capacitor must also be
considered.

Input-Capacitor Selection

The input capacitor reduces the current peaks drawn
from the input supply and reduces noise injection into
the IC. Two 10µF ceramic capacitors are used in the
Typical Applications Circuit (Figure 1) because of the
high-source impedance seen in typical lab setups.
Actual applications usually have much lower source
impedance since the step-up regulator often runs
directly from the output of another regulated supply.
Typically, the input capacitance can be reduced below
the values used in the Typical Operating Circuit.

Rectifier Diode

The MAX8728’s high-switching frequency demands a
high-speed rectifier. Schottky diodes are recommend­ed for most applications because of their fast recovery
time and low forward voltage. In general, a 1A to 2A
Schottky diode complements the internal MOSFET well.

Output-Voltage Selection

The output voltage of the step-up regulator is adjusted
by connecting a resistive voltage-divider from the out­put (V

AVDD

) to GND with the center tap connected to

FB2 (see Figure 1). Select R2 in the 10kΩ to 50kΩ
range. Calculate R1 with the following equation:

where V

FB2

, the step-up regulator’s feedback set point,

is 2.0V. Place R1 and R2 close to the IC.

Loop Compensation

Choose R

COMP

(R3 in Figure 1) to set the high-frequen­cy integrator gain for fast-transient response. Choose
C

COMP

(C13 in Figure 1) to set the integrator zero to

maintain loop stability.

For low-ESR output capacitors, use the following equa­tions to obtain stable performance and good transient
response:

To further optimize transient response, vary R

COMP

in

20% steps and C

COMP

in 50% steps while observing

transient response waveforms.

Charge-Pump Regulators

Selecting the Number of Charge-Pump Stages

For highest efficiency, always choose the lowest num­ber of charge-pump stages that meet the output
requirement.

The number of positive charge-pump stages is given by:

where n

POS

is the number of positive charge-pump

stages, V

GON

is the output of the positive charge-pump

regulator, I

GON

is the positive charge-pump output cur-

rent, V

SUPP

is the supply voltage of the charge-pump

regulators, V

D

is the forward voltage drop of the

charge-pump diode, and R

EFF

is the effective output

resistance of the charge-pump switches (10Ω typ.)

The number of negative charge-pump stages is given by:

where n

NEG

is the number of negative charge-pump

stages, V

GOFF

is the output of the negative charge-

pump regulator, and I

GOFF

is the negative charge-

pump output current.

The above equations assume that the flying capacitors
are large enough to not further limit the output current.

Flying Capacitors

Increasing the flying capacitor (CX) value lowers the
effective source impedance and increases the output
current capability. Increasing the capacitance indefi­nitely has a negligible effect on output current capabili­ty because the internal switch resistance and the diode
impedance place a lower limit on the source imped­ance. A 0.1µF ceramic capacitor works well, except in
cases of low frequency, low headroom, and high cur­rent. The flying capacitor’s voltage rating must exceed
the following:

VCX> n x V

SUPP

where n is the stage number in which the flying capaci­tor appears.

n

V

VxVIxR

NEG

GOFF

SUPP D GOFF EFF

()( )

=

−

−−2

n

VV

VVIxR

POS

GON SUPP

SUPP D GON EFF

( ) ( )

=

−

−× −2

R

VV C

LI

C

VC

IR

COMP

IN AVDD AVDD

AVDD AVDD MAX

COMP

AVDD AVDD

AVDD MAX COMP

()

()

≈

×× ×

×

≈

×

××

250

20

RR

V

V

AVDD

FB

12 1

2

=× −

⎛

⎝

⎜

⎞

⎠

⎟

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

26 ______________________________________________________________________________________

Charge-Pump Output Capacitor

Decreasing the flying capacitance reduces the output
ripple. Increasing the output capacitance reduces the
output ripple and improves the transient response. Use
the following equations to approximate the output ripple:

where V

OUT_POS

is the positive charge-pump output

voltage, C

X_POS

is the flying capacitor of the positive

charge pump, C

OUT_POS

is the output capacitor of the

positive charge-pump, V

OUT_NEG

is the negative

charge-pump output voltage,

CX_NEG

is the flying

capacitor of the negative charge pump, and C

OUT_NEG

is the output capacitor of the negative charge pump.

Output-Voltage Selection

Adjust the positive charge-pump regulator’s output volt­age by connecting a resistive voltage-divider from SRC
to GND with the center tap connected to FBP (Figure 1).
Select the lower resistor of divider R7 in the 10kΩ to
30kΩ range. Calculate upper resistor R8 with the follow­ing equation:

where V

FBP

= 2V (typ). Adding a small capacitor
(e.g., 10pF) across R7 reduces pulse grouping and
output noise.

Adjust the negative charge-pump regulator’s output
voltage by connecting a resistive voltage-divider from
V

GOFF

to REF with the center tap connected to FBN

(Figure 1). Select R9 in the 35kΩ to 68kΩ range.
Calculate R10 with the following equation:

where V

FBN

= 250mV, V

REF

= 12V. Note that REF can

only source up to 50µA; using a resistor less than 35kΩ
for R9 results in higher bias current than REF can supply.

PC Board Layout and Grounding

Careful PC board layout is important for proper operation.
Use the following guidelines for good PC board layout:

1) Minimize the area of respective high-current loops
by placing each DC-DC converter’s inductor, diode,
and output capacitors near its input capacitors and
its LX_ and GND_ pins. For the step-down regulator,
the high-current input loop goes from the positive
terminal of the input capacitor to the IC’s IN pin, out
of LX1, to the inductor, to the positive terminals of
the output capacitors, reconnecting the output
capacitor and input capacitor ground terminals. The
high-current output loop is from the inductor to the
positive terminals of the output capacitors, to the
negative terminals of the output capacitors, and to
the Schottky diode (D2). For the step-up regulator,
the high-current input loop goes from the positive
terminal of the input capacitor to the inductor, to the
IC’s LX2 pin, out of GND2, and to the input capaci­tor’s negative terminal. The high-current output loop
is from the positive terminal of the input capacitor to
the inductor, to the output diode (D1), to the positive
terminal of the output capacitors, reconnecting
between the output capacitor and input capacitor
ground terminals. Connect these loop components
with short, wide connections. Avoid using vias in the
high-current paths. If vias are unavoidable, use
many vias in parallel to reduce resistance and
inductance.

2) Create a power ground island (GND1) for the step­down regulator, consisting of the input and output
capacitor grounds and the GND1 pin. Connect all
these together with short, wide traces or a small
ground plane. Similarly, create a power ground
island (GND2) for the step-up regulator, consisting
of the input and output capacitor grounds and the
GND2 pin. Maximizing the width of the power
ground traces improves efficiency and reduces out­put voltage ripple and noise spikes. Create an ana­log ground plane (GND) consisting of the GND pin,
all the feedback-divider ground connections, the
COMP and DEL capacitor ground connections, and
the device’s exposed backside pad, with a large
area of in-the-layer or solder-side copper with large
or multiple vias to the backside pad to cool the IC.
Connect the GND1, GND2, and GND islands by
connecting the three ground pins directly to the
exposed backside pad. Make no other connections
between these separate ground planes.

RR

VV

VV

FBN GOFF

REF FBN

10 9

=×

−

−

RR

V

V

GON

FBP

78 1 =× −

⎛

⎝

⎜

⎞

⎠

⎟

V

nxV xnxVV

n

X

C

C

V

nxV xnxVV

n

X

C

C

RIPPLE POS

POS SUPP POS D OUT POS

POS

X POS

OUT POS

RIPPLE NEG

NEG SUPP NEG D OUT NEG

NEG

X NEG

OUT NEG

_

_

_

_

_

_

_

_

()

=

+− −

⎡

⎣

⎢

⎤

⎦

⎥

=

−+

⎡

⎣

⎢

⎤

⎦

⎥

12

2

MAX8728

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

______________________________________________________________________________________ 27

3) Place all feedback voltage-divider resistors as close
to their respective feedback pins as possible. The
divider’s center trace should be kept short. Placing
the resistors far away causes their FB traces to
become antennas that can pick up switching noise.
Care should be taken to avoid running any feedback
trace near LX1, LX2, DRVP, or DRVN.

4) Place the INL pin and REF pin bypass capacitors as
close to the device as possible. The ground connec­tion of the INL bypass capacitor should be connect­ed directly to the GND pin with a wide trace.

5) Minimize the length and maximize the width of the
traces between the output capacitors and the load
for best transient responses.

6) Minimize the size of the LX1 and LX2 nodes while
keeping them wide and short. Keep the LX1 and LX2
nodes away from feedback nodes (FB1, FB2, FBP,
and FBN) and analog ground. Use DC traces as
shield if necessary.

Refer to the MAX8728 evaluation kit for an example of
proper board layout.

Chip Information

TRANSISTOR COUNT: 6752

PROCESS: BiCMOS

MAX8728

Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs

28 ______________________________________________________________________________________

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages

.)

QFN THIN.EPS

D2

(ND-1) X e

e

D

C

PIN # 1

I.D.

(NE-1) X e

E/2

E

0.08 C

0.10 C

A

A1

A3

DETAIL A

E2/2

E2

0.10 M C A B

PIN # 1 I.D.

b

0.35x45°

D/2

D2/2

L

C

L

C

e e

L

CC

L

k

L

L

DETAIL B

L

L1

e

AAAAA

MARKING

I

1

2

21-0140

PACKAGE OUTLINE,
16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm

-DRAWING NOT TO SCALE-

L

e/2

Low-Cost, Multiple-Output

Power Supply for LCD Monitors/TVs

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 29

© 2006 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

Package Information (continued)

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages

.)

COMMON DIMENSIONS

MAX.

EXPOSED PAD VARIATIONS

D2

NOM.MIN.

MIN.

E2

NOM. MAX.

NE

ND

PKG.
CODES

1. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.5M-1994.

2. ALL DIMENSIONS ARE IN MILLIMETERS. ANGLES ARE IN DEGREES.

3. N IS THE TOTAL NUMBER OF TERMINALS.

4. THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION SHALL
CONFORM TO JESD 95-1 SPP-012. DETAILS OF TERMINAL #1 IDENTIFIER ARE
OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE TERMINAL #1
IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE.

5. DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN

0.25 mm AND 0.30 mm FROM TERMINAL TIP.

6. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY.

7. DEPOPULATION IS POSSIBLE IN A SYMMETRICAL FASHION.

8. COPLANARITY APPLIES TO THE EXPOSED HEAT SINK SLUG AS WELL AS THE TERMINALS.

9. DRAWING CONFORMS TO JEDEC MO220, EXCEPT EXPOSED PAD DIMENSION FOR
T2855-3 AND T2855-6.

NOTES:

SYMBOL

PKG.

N

L1

e

E

D

b

A3

A

A1

k

10. WARPAGE SHALL NOT EXCEED 0.10 mm.

JEDEC

0.70 0.800.75

4.90

4.90

0.25

0.250--

4

WHHB

4

16

0.350.30

5.10

5.105.00

0.80 BSC.

5.00

0.05

0.20 REF.

0.02

MIN. MAX.NOM.

16L 5x5

L

0.30 0.500.40

---

---

WHHC

20

5

5

5.00

5.00

0.30

0.55

0.65 BSC.

0.45

0.25

4.90

4.90

0.25

0.65

--

5.10

5.10

0.35

20L 5x5

0.20 REF.

0.75

0.02

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

---

WHHD-1

28

7

7

5.00

5.00

0.25

0.55

0.50 BSC.

0.45

0.25

4.90

4.90

0.20

0.65

--

5.10

5.10

0.30

28L 5x5

0.20 REF.

0.75

0.02

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

---

WHHD-2

32

8

8

5.00

5.00

0.40

0.50 BSC.

0.30

0.25

4.90

4.90

0.50

--

5.10

5.10

32L 5x5

0.20 REF.

0.75

0.02

NOM.

0

0.70

MIN.

0.05

0.80

MAX.

0.20 0.25 0.30

DOWN
BONDS
ALLOWED

YES3.103.00 3.203.103.00 3.20T2055-3

3.103.00 3.203.103.00 3.20

T2055-4

T2855-3 3.15 3.25 3.35 3.15 3.25 3.35

T2855-6

3.15 3.25 3.35 3.15 3.25 3.35

T2855-4 2.60 2.70 2.80 2.60 2.70 2.80
T2855-5 2.60 2.70 2.80 2.60 2.70 2.80

T2855-7 2.60 2.70

2.80

2.60 2.70 2.80

3.20

3.00 3.10T3255-3 3 3.203.00 3.10

3.203.00 3.10T3255-4 3 3.203.00 3.10

NO

NO
NO

NO

YES
YES

YES

YES

3.203.00T1655-3 3.10 3.00 3.10 3.20 NO
NO3.203.103.003.10T1655N-1 3.00 3.20

3.353.15T2055-5 3.25 3.15 3.25 3.35

YES

3.35

3.15

T2855N-1

3.25 3.15 3.25 3.35

NO

3.353.15T2855-8 3.25 3.15 3.25 3.35

YES

3.203.10T3255N-1 3.00

NO

3.203.103.00

L

0.40

0.40

**
**
**

**

**
**
**
**
**

**
**

**

**

**

SEE COMMON DIMENSIONS TABLE

±0.15

11. MARKING IS FOR PACKAGE ORIENTATION REFERENCE ONLY.

I

2

2

21-0140

PACKAGE OUTLINE,
16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm

-DRAWING NOT TO SCALE-

12. NUMBER OF LEADS SHOWN ARE FOR REFERENCE ONLY.

3.30T4055-1 3.20 3.40 3.20 3.30 3.40

**

YES

0.050 0.02

0.600.40 0.50

10

-----

0.30
40

10

0.40 0.50

5.10

4.90 5.00

0.25 0.35 0.45

0.40 BSC.

0.15

4.90

0.250.20

5.00 5.10

0.20 REF.

0.70

MIN.

0.75 0.80

NOM.

40L 5x5

MAX.

13. LEAD CENTERLINES TO BE AT TRUE POSITION AS DEFINED BY BASIC DIMENSION "e", ±0.05.

T1655-2

**

YES3.203.103.003.103.00 3.20

T3255-5 YES3.003.103.00

3.20

3.203.10

**

exceptions