Rainbow Electronics MAX1579 User Manual

General Description

The MAX1578/MAX1579 provide four regulated outputs
to meet all the voltage requirements for small active­matrix TFT-LCD displays in handheld devices where
minimum external components and high efficiency are
required. Each device consists of three advanced
charge pumps for LCD bias power and a step-up con­verter for driving up to 8 series white LEDs for back­lighting. The input voltage range is from 2.7V to 5.5V.

The charge pumps provide fixed +5V, +15V, and -10V
for the LCD bias circuits. No external diodes are need­ed. A high-efficiency, fractional (1.5x/2x) charge pump
followed by a low-dropout linear regulator provides +5V
to power the source driver. Automatic mode changing
achieves the highest conversion efficiency. Two multi­stage, high-voltage charge pumps generate +15V and

-10V to provide VONand V

OFF

, respectively. Utilizing a
unique clocking scheme and internal drivers, these
charge pumps eliminate parasitic charge-current glitch­es and reduce maximum input current, resulting in low
electromagnetic emissions. The outputs are sequenced
during startup and shutdown. In shutdown, the outputs
are discharged to zero.

The high-efficiency inductor step-up converter drives
up to 8 white LEDs in series with a constant current to
provide backlighting. The series connection allows the
LED currents to be identical for uniform brightness and
minimizes the number of traces to the LEDs. The
MAX1578 regulates constant LED current over the
entire temperature range. The MAX1579 features a tem­perature derating function to avoid overdriving the
white LEDs during high ambient temperatures, enabling
higher drive current below +42°C.

The MAX1578/MAX1579 are available in space-saving
24-lead 4mm x 4mm thin QFN packages.

Applications

PDAs, Palmtops
Smart Phones
Internet Appliances
LCD Displays with White LED Backlight

Features

♦ Four Regulators in One Package
♦ Bias Power Using Charge Pumps

+5V at 25mA for Source Driver
+15V at 100µA for V

ON

-10V at 100µA for V

OFF

No External Diodes Required
Output Sequencing
POS, NEG, and MAIN Are Autodischarged

During Shutdown

♦ LED Backlight Power Using Boost Converter

Series LED Connection for Uniform Illumination
Supports Up to 8 LEDs at 25mA (max)
900mW (max) Power
PWM or Analog Dimming Control
Overvoltage Protection
Low Input/Output Ripple
Soft-Start with Zero Inrush Current
Fast 1MHz PWM Operation for Small

Component Size

Temperature Derating Function (MAX1579)

♦ High Efficiency

Bias: 83% (5.0V at 25mA, 15V/-10V at 100µA)
LED: 84% (6 LEDs at 20mA)

♦ Uses Only Ceramic Capacitors and Only One

Inductor

♦ Independent Enable Inputs for LED and Bias

Power

♦ Thermal-Shutdown Protection
♦ 1µA Shutdown Current
♦ Tiny 4mm x 4mm Thin QFN Package

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

________________________________________________________________ Maxim Integrated Products 1

19-3359; Rev 0; 8/04

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.

Ordering Information

EVALUATION KIT

AVAILABLE

PART

PIN-PACKAGE

MAX1578ETG

24 Thin QFN 4mm x 4mm
(T2444-4)

MAX1579ETG

24 Thin QFN 4mm x 4mm
(T2444-4)

MAX1578/MAX1579

See Figure 3 for Typical Application Circuit.

ONBIAS

CD2

GND

NEG

PMPB

CD1

LX

CTRL

PGND

OUT

COMP

CS

CU1

CU3

MAIN

CU2

PMP

POS

19
20
21
22
23
24

1 2 3 4 5 6

18 17 16 15 14 13

12
11
10

9
8
7

MAX1578

MAX1579

THIN QFN

4mm x 4mm

C2N

C2P

C1N

IN

V

DD

C1P

Pin Configuration

TEMP RANGE

-40°C to +85°C

-40°C to +85°C

MAX1578/MAX1579

Complete Bias and White LED Power Supplies
for Small TFT Displays

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 3, VIN= 3V, CTRL = ONBIAS = IN, TA= -40°C to +85°C, typical values are at TA= +25°C, unless otherwise noted. Note 1)

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, CS, C1N, C2N, MAIN, ONBIAS, VDDto GND.....-0.3V to +6V

CTRL to GND..................-0.3V to the lesser of +6V or (V

IN

+ 2V)

LX, OUT to GND.....................................................-0.3V to +37V

COMP to GND .............................................-0.3V to (V

IN

+ 0.3V)

CU1 to MAIN ............................................................-0.3V to +6V

CU2 to CU1..............................................................-0.3V to +6V

CU3 to CU2..............................................................-0.3V to +6V

CU3 to POS............................................................-0.3V to +18V

CU3 to GND ...........................................................-0.3V to +18V

POS to GND ...........................................................-0.3V to +18V

CD1 to MAIN ..........................................................+0.3V to -12V

CD1 to GND .............................................................+0.3V to -6V

CD2 to CD1..............................................................+0.3V to -6V

NEG to CD2..............................................................+0.3V to -6V

NEG, CD2 to GND..................................................+0.3V to -12V

C1P, C2P to GND ...........................................-0.3V to (V

IN

+ 6V)

PMP, PMPB to GND................................-0.3V to (V

MAIN

+ 0.3V)

GND to PGND .......................................................-0.3V to +0.3V

I

LX

...................................................................................1.0A

RMS

Continuous Power Dissipation (TA= +70°C)

24-Pin 4mm x 4mm Thin QFN

(derate 20.8mW/°C above +70°C).............................1667mW

Short-Circuit Duration (MAIN, POS, NEG)..................Continuous

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

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

Storage Temperature Range.............................-65°C to +150°C

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

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

IN Operating Supply Range 2.7 5.5 V
IN Undervoltage-Lockout (UVLO)

Threshold

Rising edge, 30mV hysteresis 2.1

2.6 V

IN Quiescent Current Switching 3 5 mA

TA = +25°C 0.4 1

IN Shutdown Current V

CTRL

= V

ONBIAS

= 0V

T

A

= +85°C 1

µA

Thermal Shutdown Rising temperature, 20°C hysteresis (typ)

°C

MAIN CHARGE PUMP WITH LINEAR REGULATOR

Main Pump Efficiency I

LOAD

= 25mA, VIN = 3.9V 83 %

VIN ≥ 3.8V in 1.5x mode 9 20

VDD Charge-Pump Open-Loop
Output Impedance

V

IN

≥ 3.0V in 2.0x mode 7.5 20

Ω

Operating Frequency

kHz

VDD Output Voltage Charge-pump pause threshold 5.2 5.5 5.7 V

V

IN

Fal l i ng S w i tchover to 2.0x M od e

V

V

IN

Ri si ng S w i tchover to 1.5x M od e

3.8 3.9 4.0 V

Quiescent Current
(Charge Pumps Only)

V

CTRL

= 0V, ONBIAS = IN

mA

V

MAIN

Regulation Voltage 0.1mA < I

LOAD

< 25mA 4.9 5.0 5.1 V

Discharge Switch Resistance
at V

MAIN

V

ONBIAS

= 0V 1 3 kΩ

POS, NEG CHARGE PUMPS

Operating Frequency

kHz
Duty Cycle 50 %
POS Pump Efficiency I

LOAD

= 100µA 97 %

POS Output Voltage I

LOAD

= 0 to 100µA

V

P OS D i schar g e S w i tch Resi stance

V

ONBIAS

= 0V 3 6 kΩ

NEG Pump Efficiency I

LOAD

= -100µA 97 %

2.35

200 250 300

3.75 3.85 3.95

12.0 15.6 19.5

13.9 14.7 15.3

+160

0.87 1.30

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

_______________________________________________________________________________________ 3

PARAMETER CONDITIONS

UNITS

NEG Output Voltage I

LOAD

= 0 to -100µA

V

N E G D i schar g e S w i tch Resi stance

V

ONBIAS

= 0V 1.5 3 kΩ

LOGIC INPUT (ONBIAS)

Logic Input Low Voltage

V

Logic Input High Voltage 1.6 V

TA = +25°C

1

Input Current

T

A

= +85°C 1

µA

LED BACKLIGHTING

Efficiency LOAD = 6 LEDs in series at 20mA 84 %
OUT Voltage Range (Note 2)

(V

IN

­32 V

Overvoltage-Lockout (OVLO)
Threshold

V

OUT

rising, 2V hysteresis 32 34 36 V

V

OUT

= 32V, V

CTRL

> 0.24V 10 20 32

1

OUT Input Bias Current

V

OUT

= VIN, V

CTRL

= 0

0.1

µA

ERROR AMPLIFIER

TA = +25°C

CTRL to CS Regulation

V

CTRL

= 1.5V, VIN = 2.7V to

5.5V

T

A

= -40°C to +85°C

V

TA = +25°C

1

CS Input Bias Current VCS = V

CTRL

/ 5

T

A

= +85°C

µA

MAX1578

CTRL Input Resistance V

CTRL

< 1.0V

kΩ

CTRL Dual Mode™ Threshold 5mV hysteresis

mV

CTRL Shutdown Delay (Note 3) 6.5 8.2

ms

CS to COMP Transconductance V

COMP

= 1.0V 32 60 90 µS

CS Regulation Derating Function
Start Temperature

V

CTRL

= 3V, MAX1579 only

°C

CS Regulation Derating Function
Slope

V

CTRL

= 3V, TA = +65°C, MAX1579 only -6

mV/°C

MAX1578, V

CTRL

= 3V

CS Maximum Brightness Clamp
Voltage

MAX1579, V

CTRL

= 3V, TA = +25°C

mV

MAX1578

CS Maximum Brightness Voltage
at CTRL

MAX1579

V

OSCILLATOR

Operating Frequency f

BOOST

0.8 1.0 1.2 MHz

PWM mode 12

Minimum Duty Cycle

Pulse skipping 0

%

Maximum Duty Cycle CTRL = IN, CS = GND 92 95 %

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 3, VIN= 3V, CTRL = ONBIAS = IN, TA= -40°C to +85°C, typical values are at TA= +25°C, unless otherwise noted. Note 1)

Dual Mode is a trademark of Maxim Integrated Products, Inc.

TA = +25°C 0.01
TA = +85°C

TA = +25°C 250 500 780

= +85°C 185

T

A

MAX1579

MIN TYP MAX

-10.2 -9.8 -9.3

0.01

VD1)

0.295 0.300 0.305

0.292 0.300 0.308

0.01

0.03

250 500 780

100 170 240

+42

0.72

10.5

310 327 345
322 340 358

1.635

1.70

MAX1578/MAX1579

Complete Bias and White LED Power Supplies
for Small TFT Displays

4 _______________________________________________________________________________________

Note 1: All devices are 100% production tested at TA= +25°C. Limits over the operating temperature range are guaranteed by design.
Note 2: V

D1

is the forward-voltage drop of diode D1 in Figure 3.

Note 3: Time from CTRL going below the Dual-Mode threshold to IC shutdown.

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 3, VIN= 3V, CTRL = ONBIAS = IN, TA= -40°C to +85°C, typical values are at TA= +25°C, unless otherwise noted. Note 1)

PARAMETER CONDITIONS

MIN

TYP

MAX

UNITS

N-CHANNEL SWITCH

LX On-Resistance ILX = 190mA

1.5 Ω

TA = +25°C

5

LX Leakage Current V

LX

= 28V, CTRL = GND

T

A

= +85°C 1

µA

LX Current Limit Duty cycle = 90%

mA

Typical Operating Characteristics

(Circuit of Figure 3, VIN= 3.6V, I

LED

= 20mA, 4 LEDs, CTRL = IN, TA= +25°C, unless otherwise noted.)

50

55

60

65

70

75

80

85

90

2.5 3.0 3.5 4.0 4.5 5.0 5.5

LED EFFICIENCY vs. V

IN

MAX1578toc02

INPUT VOLTAGE (V)

EFFICIENCY (%)

I

LED

= 2mA

I

LED

= 10mA

I

LED

= 20mA

0

10

5

20

15

25

30

-40 85

LED CURRENT vs. TEMPERATURE

MAX1578toc03

TEMPERATURE (°C)

I

LED

(mA)

10-15 35 60

MAX1579

R

CS

= 13

Ω

MAX1578

R

CS

= 22

Ω

0

5

15

10

20

25

01.00.5 1.5 2.0 2.5

LED CURRENT vs. V

CTRL

MAX1578toc04

CTRL VOLTAGE (V)

I

LED

(mA)

0.82

0.01

500 700 900

LED EFFICIENCY vs. I

90
85
80
75
70
65

EFFICIENCY (%)

60
55
50

0 5 10 15 20 25

4 LEDs

6 LEDs

2 LEDs

LED CURRENT (mA)

LED

8 LEDs

MAX1578toc01

Typical Operating Characteristics (continued)

(Circuit of Figure 3, VIN= 3.6V, I

LED

= 20mA, 4 LEDs, CTRL = IN, TA= +25°C, unless otherwise noted.)

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

_______________________________________________________________________________________ 5

PWM-DIMMING WAVEFORMS

MAX1578toc08

V

CTRL

I

IN

V

OUT

AC-COUPLED

V

IN

AC-COUPLED

2V/div

50mV/div

20mA/div

20mV/div

40µs/div

0

400

200

800

600

1200

1000

1400

023145

LCD BIAS SUPPLY CURRENT

vs. INPUT VOLTAGE

MAX1578toc09

INPUT VOLTAGE (V)

I

IN

(µA)

V

CTRL

= 0V

LCD BIAS STARTUP SEQUENCE

MAX1578toc10

V

ONBIAS

V

POS

V

NEG

V

MAIN

5V/div

5V/div

10V/div

5V/div

10ms/div

BOOST STARTUP AND

SHUTDOWN WAVEFORMS

MAX1578toc07

V

CTRL

I

IN

V

OUT

V

IN

AC-COUPLED

2V/div

5V/div

0V
100mA/div

20mV/div

4ms/div

0

5

10

15

20

25

0405020 3010 60 70 80 90 100

LED CURRENT vs.

DIRECT-PWM DUTY CYCLE

MAX1578toc05

PWM DUTY CYCLE (%)

I

LED

(mA)

f

PWM

= 200Hz TO 200kHz

BOOST-SWITCHING WAVEFORMS

MAX1578toc06

V

IN

AC-COUPLED

V

LX

V

OUT

AC-COUPLED

20mV/div

10V/div

200mV/div

400ns/div

MAX1578/MAX1579

Complete Bias and White LED Power Supplies
for Small TFT Displays

6 _______________________________________________________________________________________

-5.0

-3.5

-4.0

-4.5

-3.0

-2.5

-2.0

-1.5

-1.0

POS

NEG

-0.5

0

04020 60 80 100

POS AND NEG LOAD REGULATION

MAX1578toc12

LOAD CURRENT (µA)

VOLTAGE DROOP (%)

-0.6

-0.4

-0.5

-0.2

-0.3

-0.1

0

025

MAIN OUTPUT LOAD REGULATION

MAX1578toc11

LOAD CURRENT (mA)

VOLTAGE DROOP (%)

1051520

PIN NAME FUNCTION

1 MAIN

LDO Output and the POS and NEG Charge-Pump Inputs. V

MAIN

is regulated to 5V. Bypass to GND

with a 1µF capacitor. Output is internally discharged with a 1kΩ resistor when V

ONBIAS

= 0V.
2 CU1 POS Charge-Pump Capacitor Connection 1. Connect a 1µF capacitor between CU1 and PMP.
3 CU2 POS Charge-Pump Capacitor Connection 2. Connect a 1µF capacitor between CU2 and PMPB.

4 CU3 POS Charge-Pump Capacitor Connection 3. Connect a 1µF capacitor between CU3 and GND.
5 POS

Output of Positive (3x) Charge Pump. Bypass POS to GND with a 1µF capacitor. POS is internally
discharged with a 3kΩ resistor when V

ONBIAS

= 0V.

6PMP

Charge-Pump Capacitor Connection. Connect a 1µF capacitor between PMP and CU1 and another
1µF capacitor between PMP and CD1.

7 PMPB

Charge-Pump Capacitor Connection. Connect a 1µF capacitor between PMPB and CU2 and another
1µF capacitor between PMPB and CD2. PMPB is 180° out of phase with PMP.

8 CD1

NEG Charge-Pump Capacitor Connection 1. Connect a 1µF capacitor and a 200Ω ±5% resistor in
series between CD1 and PMP.

9 CD2

NEG Charge-Pump Capacitor Connection 2. Connect a 1µF capacitor and a 200Ω ±5% resistor in
series between CD2 and PMPB.

10 NEG

Output of Inverting (-2x) Charge Pump. Bypass NEG to GND with a 1µF capacitor. Output is internally
discharged with a 1.5kΩ resistor when V

ONBIAS

= 0V.

Pin Description

Typical Operating Characteristics (continued)

(Circuit of Figure 3, VIN= 3.6V, I

LED

= 20mA, 4 LEDs, CTRL = IN, TA= +25°C, unless otherwise noted.)

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

_______________________________________________________________________________________ 7

PIN NAME FUNCTION

11 ONBIAS

Logic Input to Enable V

DD

, MAIN, POS, and NEG Charge Pumps. Drive ONBIAS high to enable all the

charge pumps. Connect to GND to disable the charge pumps.

12 GND Ground. Connect to PGND and the exposed pad directly under the IC.
13 COMP

LED Driver Compensation. Connect a 0.1µF from COMP to GND. C

COMP

stabilizes the driver and

sets the soft-start time.

14 CS

Current-Sense Feedback Input. Connect a resistor from CS to GND to set the LED current. For the
MAX1578, CS regulates to V

CTRL

/ 5 or 0.327V, whichever is lower. For the MAX1579, CS regulates to

V

CTRL

/ 5 or 0.340V, whichever is lower.

15 CTRL

LE D Br i g htness C ontr ol Inp ut. C onnect C TRL to a 0.24V to 1.65V i np ut to set the b r i g htness of the
exter nal LE D s. H ol d C TRL b el ow 100m V for m or e than 10.5m s, to shut d ow n the LE D d r i ver . D r i ve C TRL
w i th a 200H z t o 200kH z unfi l ter ed P W M d i m m i ng si g nal for D C LE D cur r ent that i s p r op or ti onal to the
si g nal ’ s d uty cy cl e.

16 OUT

Over vol tag e S ense Inp ut. The M AX 1578/M AX 1579 tur n off the n- channel M OS FE T w hen V

OU T

exceed s 34V .

Once V

OU T

d r op s b el ow 32V , the IC r e- enter s soft- star t. B yp ass OU T to GN D w i th a 0.1µF cap aci tor .

17 LX

Inductor Connection. Connect to the switched side of the external inductor as well as the anode of the
external diode. LX is high impedance during shutdown.

18 PGND Power Ground. Connect to GND and the exposed pad directly under the IC.
19 C1N

Main Charge-Pump Transfer Capacitor Negative Connection 1. Connect a 2.2µF capacitor between
C1N and C1P.

20 C2N

Main Charge-Pump Transfer Capacitor Negative Connection 2. Connect a 2.2µF capacitor between
C2N and C2P.

21 IN P ow er - S up p l y Inp ut. C onnect to a 2.7V to 5.5V i np ut sup p l y. Byp ass IN to GN D w i th a 4.7µF cap aci tor .
22 C2P

Main Charge-Pump Transfer Capacitor Positive Connection 2. Connect a 2.2µF capacitor between
C2P and C2N.

23 C1P

Main Charge-Pump Transfer Capacitor Positive Connection 1. Connect a 2.2µF capacitor between
C1P and C1N.

24 V

DD

Regulated Main Charge-Pump Output. VDD is regulated to 5.5V. Bypass VDD to GND with a 4.7µF
capacitor. V

DD

is connected to IN when ONBIAS is pulled low.

— EP Exposed Paddle. Connect directly to a ground plane, GND, and PGND directly under the IC.

Pin Description (continued)

MAX1578/MAX1579

Complete Bias and White LED Power Supplies
for Small TFT Displays

8 _______________________________________________________________________________________

BIAS

REF

PWM

CONTROL

1MHz

OSCILLATOR

OSC

170mV

TIMER

CTRL

COMP

IN

OVERVOLTAGE

PROTECTION

OUT

PGND

LX

CS

CHARGE-PUMP

CONTROL

ONBIAS

SHUTDOWN
SEQUENCING

DIVIDE BY FOUR

OSC

MULTIMODE

CHARGE PUMP

1.5X/2X

C1P
C1N
C2P

V

DD

5V LDO

C2N

GND

MAIN

SHUTDOWN

SHUTDOWN

3X POSITIVE

CHARGE PUMP

-2X INVERTING

CHARGE PUMP

DIVIDE BY 64

OSC

PMP

PMPB

CD1

CD2

CU1
CU2
CU3

NEG

POS

N

SHUTDOWN

N

N

N

MAX1578

MAX1579

Block Diagram

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

_______________________________________________________________________________________ 9

Detailed Description

Bias Power and UVLO

The MAX1578/MAX1579 contain an LED driver boost
converter and three charge pumps for LCD bias. The
undervoltage-lockout (UVLO) feature disables the LED
boost converter and the charge pumps when the input
voltage is below 2.35V (typ). Once V

IN

rises above

2.35V, and V

CTRL

and V

ONBIAS

are high, the boost

converter and charge pumps are enabled, respectively.

Charge-Pump Output Sequencing

The outputs of the MAX1578/MAX1579 charge pumps
are sequenced to turn on and off in a predictable fash­ion. The turn-on sequence is as follows (Figure 1):

1) When ONBIAS is high, the MAIN regulator (5V) is
enabled.

2) When V

MAIN

exceeds 4.6V, the NEG charge pump

(-10V) is enabled.

3) When V

NEG

reaches -8V, the POS charge pump

(+15V) is enabled.

The turn-off sequence is as follows (Figure 2):

1) When ONBIAS is driven low, the NEG charge pump
(-10V) is disabled.

2) Once V

NEG

is discharged to -0.87V, the POS charge

pump (+15V) is disabled.

3) Once V

POS

falls to 0.87V, the MAIN regulator (+5V)

is disabled and discharged.

MAIN Charge Pump

The MAX1578/MAX1579 include a charge pump that
uses two external capacitors to provide +5.5V output
(VDD) that is used to power the regulated LDO +5V out­put (MAIN). The control logic configures the pump to
switch automatically between 1.5x and 2x modes to
maximize efficiency. If VDDexceeds 5.5V, the charge
pump stops switching. When ONBIAS is driven low,
VDDis connected to IN.

A low-dropout linear regulator regulates the output of
the main charge pump to +5V at MAIN. The MAIN out­put is capable of sourcing as much as 25mA to an
external load and also supplies the POS and NEG
charge pumps. Drive ONBIAS low to disable the MAIN,
POS, and NEG outputs. During shutdown, MAIN is dis­charged to GND with an internal 1kΩ resistor.

POS/NEG Charge Pumps

The MAX1578/MAX1579 include a positive and nega­tive charge pump for LCD bias. The POS and NEG
charge pumps are powered from V

MAIN

. The POS and
NEG charge pumps operate at 15.6kHz with a 50%
duty cycle.

NEG Charge Pump (-10V Supply)

The NEG charge pump uses capacitors at CD1 and
CD2 to generate -10V (-2 x V

MAIN

). Connect 1µF ceram­ic capacitors and 200Ω ±5% resistors in series between
CD1 and PMP and between CD2 and PMPB. Drive
ONBIAS high to enable MAIN, NEG, and POS. During
shutdown, the NEG output is discharged to GND with an
internal 1.5kΩ resistor.

POS Charge Pump (+15V Supply)

The POS charge pump uses capacitors at CU1, CU2,
and CU3 to generate +15V (3 x V

MAIN

). Connect 1µF
ceramic capacitors between CU1 and PMP, between
CU2 and PMPB, and between CU3 and GND. Drive
ONBIAS high to enable MAIN, NEG, and POS. During
shutdown, POS is discharged to GND with an internal
3kΩ resistor.

LED Backlighting Power

LED power is supplied by an internal MOSFET, 1MHz
boost converter. The boost converter is capable of dri­ving up to 8 series LEDs at 25mA.

The output of the boost converter is regulated to main­tain a constant voltage at CS, and therefore a constant
current through the LEDs. Once VINis increased above
the UVLO voltage (2.35V) and V

CTRL

is above 0.17V,
the boost converter enters soft-start and charges the
output to its regulation voltage. An overvoltage-protec­tion circuit shuts down the boost converter if V

OUT

exceeds 34V.

V

MAIN

(5V)

V

POS

(+15V)

V

NEG

(-10V)

ONBIAS

Figure 2. Charge-Pump Turn-Off Sequence

V

MAIN

(5V)

V

POS

(+15V)

V

NEG

(-10V)

ONBIAS

Figure 1. Charge-Pump Turn-On Sequence

MAX1578/MAX1579

Complete Bias and White LED Power Supplies
for Small TFT Displays

10 ______________________________________________________________________________________

Soft-Start

The LED boost converter utilizes a soft-start function to
eliminate inrush current during startup. Once the boost
converter is enabled, LX begins switching at the mini­mum duty cycle until C

COMP

is charged to 1.25V. Once
this occurs, the duty cycle increases to further charge
the output until VCSreaches 20% of V

CTRL

. The soft­start time is adjustable using the capacitor from COMP
to GND. Calculate the required COMP capacitor as:

where tSSis the desired soft-start time in seconds.

Overvoltage Protection

The output of the LED boost converter is protected from
overvoltage conditions by internal overvoltage circuitry.
If V

OUT

exceeds 34V, the LX switching terminates.

Once V

OUT

falls below 32V, LX switches normally and

soft-start is re-initiated.

Ambient Temperature Derating Function

(MAX1579)

The MAX1579 limits the maximum LED current depend­ing on the die temperature. VCSis limited to 340mV up
to +42°C. Once the temperature reaches +42°C, the
maximum VCSdeclines by 6mV/°C until the minimum
40mV threshold is reached at +100°C. Due to the pack­age’s exposed paddle, the die temperature is always
very close to the PC board temperature.

The temperature derating function allows the LED cur­rent to be safely set higher at normal operating temper­atures, thereby allowing either a brighter display or
fewer LEDs to be used for normal display brightness.

Shutdown

The MAX1578/MAX1579 include a low-quiescent-current
shutdown mode. To enter shutdown, drive CTRL below

0.1V for longer than 10.5ms and drive ONBIAS low. The
quiescent current is reduced to less than 1µA when the
boost converter and charge pumps are disabled.

To disable the LED boost converter, drive CTRL below

0.1V for longer than 10.5ms. During shutdown, the
internal boost switch from LX to PGND is high imped­ance; however, a DC path exists from IN to OUT
through the external inductor and Schottky diode. Drive
CTRL with an analog voltage between 0.24V and 1.65V
or a 200Hz to 200kHz digital PWM dimming signal for
normal operation. The quiescent current is reduced to
870µA when the boost converter is shut down and the
charge pumps are enabled.

Drive ONBIAS low to shut down the internal POS and
NEG charge pumps and disable the MAIN LDO output.
On-chip pulldown resistors discharge these outputs
during shutdown. Drive ONBIAS high for normal opera­tion. V

DD

is connected to IN when ONBIAS is low. The
quiescent current is reduced to 430µA when the charge
pumps are shut down and the boost converter is
enabled.

Applications Information

Adjusting LED Current

Set the maximum LED current using a resistor from CS
to GND. Calculate the resistance as follows:

where I

LED

is the desired maximum current through the

LEDs in Amps when V

CTRL

is 1.65V.

LED Dimming Control Using a DAC

V

CTRL

controls the LED drive current. The voltage at CS

regulates to 20% of V

CTRL

to control the current
through the LEDs and, therefore, the brightness. Drive
CTRL using a DAC with an output voltage between

0.24V and 1.65V to control the brightness of the LEDs.
Increasing V

CTRL

beyond 1.65V results in no further
brightness increase. Hold CTRL below 100mV for
longer than 10.5ms to shut down the boost converter.

LED Dimming Using Direct PWM into CTRL

Another useful technique for LED dimming control is the
application of a logic-level PWM signal applied directly
to CTRL. LED current may be varied from zero to full
scale. The frequency range of the PWM signal is from
200Hz to 200kHz, while 0% duty cycle corresponds to
zero current and 100% duty cycle corresponds to full
current. The error amplifier and compensation capaci­tor form a lowpass filter so PWM dimming results in DC
current to the LEDs without the need for any additional
RC filters. See the Typical Operating Characteristics.

Input/Output Ripple

For LED drivers, input and output ripple may be impor­tant. Input ripple depends on the source supply’s output
impedance. Adding a lowpass filter to the input further
reduces input ripple. Alternately, increasing CINto 10µF
cuts input ripple in half. Likewise, an output filter or high­er output capacitance value reduces output ripple.

R

mV

I

for the MAX

R

mV

I

for the MAX

CS

LED

CS

LED

=

=

330

1578

340

1579

C

At

V

COMP

SS

=

×12

125µ.

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

______________________________________________________________________________________ 11

Component Selection

Use only ceramic capacitors with an X5R, X7R, or bet­ter dielectric. See Table 1 for a list of recommended
components.

Capacitor Selection

Use low-ESR ceramic capacitors. Recommended val­ues for the capacitors are shown in Table 1. To ensure
stability over a wide temperature range, ceramic
capacitors with an X5R or X7R dielectric are recom­mended. Place these capacitors as close to the IC as
possible.

Inductor Selection

Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
applications while maintaining low 15mV

P-P

input ripple.
With input voltages near 5V, a larger value of inductance
can be more efficient. To prevent core saturation, ensure
that the inductor-saturation current rating exceeds the
peak inductor current for the application. Calculate the
peak inductor current with the following formula:

Schottky Diode Selection

The MAX1578/MAX1579 require a high-speed rectifica­tion diode (D1) for optimum performance. A Schottky
diode is recommended due to its fast recovery time
and low forward-voltage drop. Ensure that the diode’s
average and peak current ratings exceed the average
output current and the peak inductor current, respec­tively. In addition, the diode’s reverse breakdown volt­age must exceed V

OUT

. The RMS diode current is

calculated as:

PC Board Layout and Routing

Due to fast switching waveforms, careful PC board lay­out is required. An evaluation kit (MAX1578EVKIT) is
available to speed design. When laying out a board,
minimize trace lengths between the IC and R1, the

inductor, the diode, the input capacitor, and the output
capacitor. Keep traces short, direct, and wide. Keep
noisy traces, such as the LX node trace, away from CS.
The IN bypass capacitor (C

IN

) should be placed as
close to the IC as possible. The transfer capacitors for
the charge pumps should be located as close as possi­ble to the IC. PGND and GND should be connected
directly to the exposed paddle underneath the IC. The
ground connections of CINand C

OUT

should be as
close together as possible. The traces from IN to the
inductor and from the Schottky diode to the LEDs may
be longer. The MAX1579 evaluation kit contains a sam­ple layout to speed designs.

Chip Information

TRANSISTOR COUNT: 3801
PROCESS: BiCMOS

III

DIODE RMS OUT PEAK()

=×

I

VIVVs

L

PEAK

OUT MAX LED MAX

IN MIN

IN MIN

=

×

×

+

×

×

() ()

()

()

..08

08

2

µ

MAX1578/MAX1579

Complete Bias and White LED Power Supplies
for Small TFT Displays

12 ______________________________________________________________________________________

DESIGNATION

DESCRIPTION

C1, C8

4.7µF, 6.3V X5R ceramic capacitors (0603)

Murata GRM188R60J475KE19

C2

0.1µF, 6.3V X5R ceramic capacitor (0402)

TDK C1005X5R1A104K

C3

0.1µF, 50V X7R ceramic capacitor (0603)
TDK C1608X7R1H104K

C4, C5, C12

1µF, 16V X7R ceramic capacitors (0805)
TDK C2012X7R1C105K

C6, C7

2.2µF, 6.3V X5R ceramic capacitors (0603)

Taiyo Yuden JMK107BJ225KA

C9, C10, C11,

C13, C14

1µF, 6.3V X5R ceramic capacitors (0402)
Murata GRM155R60J105KE19

D1

40V, 0.5A Schottky diode
International Rectifier MBRX0540

D2–D7

White LEDs
Nichia NSCW215T

L1

22µH, 250mA inductor (1210)
Murata LQH32CN220K53

R1 22.1Ω ±1% resistor (0402)

R2, R3 200Ω ±5% resistors (0402)

Table 1. Recommended Components for
the Typical Application Circuit

MAX1578
MAX1579

IN

MAIN

ONBIAS
C1P

C1N
C2P

C2N

GND

CTRL
COMP

V

DD

OUT

PGND

LX

INPUT

2.7V TO 5.5V

L1

22µH

MAIN OUTPUT

+5V, 25mA

PWM OR
ANALOG
DIMMING

OUTPUT UP TO
8 LEDS IN SERIES

CS

ON

OFF

D2–D7

D1

C2

0.1µF

C9

1µF

C6

2.2µF

C7

2.2µF

C8

4.7µF

C1

4.7µF
C3

0.1µF

POSITIVE OUTPUT
+15V, 100µA

POS

CU3

NEG

NEGATIVE OUTPUT

-10V, 100µA

C4
1µF

C5
1µF

C12
1µF

R1

22.1Ω

CU1
PMP
CD1

C10
1µF

C13
1µF

R2

200Ω

CU2

PMPB

CD2

C11
1µF

C14
1µF

R3

200Ω

Figure 3. Typical Application Circuit

MAX1578/MAX1579

Complete Bias and White LED Power Supplies

for Small TFT Displays

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 ____________________ 13

© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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

.)

24L QFN THIN.EPS

C

1

2

21-0139

PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm

C

2

2

21-0139

PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm