Rainbow Electronics MAX1748 User Manual

General Description

The MAX1748 triple-output DC-DC converter in a low­profile TSSOP package provides the regulated voltages
required by active matrix, thin-film transistor (TFT) liquid
crystal displays (LCDs). One high-power DC-DC con­verter and two low-power charge pumps convert the
+3.3V to +5V input supply voltage into three independent
output voltages.

The primary high-power DC-DC converter generates a
boosted output voltage (V

MAIN

) up to 13V that is regu­lated within ±1%. The low-power BiCMOS control cir­cuitry and the low on-resistance (0.35Ω) of the
integrated power MOSFET allows efficiency up to 93%.
The 1MHz current-mode PWM architecture provides
fast transient response and allows the use of ultra-small
inductors and ceramic capacitors.

The dual charge pumps independently regulate one
positive output (V

POS

) and one negative output (V

NEG

).
These low-power outputs use external diode and
capacitor stages (as many stages as required) to regu­late output voltages up to +40V and down to -40V. A
proprietary regulation algorithm minimizes output rip­ple, as well as capacitor sizes for both charge pumps.

The MAX1748 is available in the ultra-thin TSSOP pack­age (1.1mm max height).

________________________Applications

TFT Active Matrix LCD Displays

Passive Matrix LCD Displays

PDAs

Digital Still Cameras

Camcorders

Features

♦ Three Integrated DC-DC Converters
♦ 1MHz Current-Mode PWM Boost Regulator

Up to +13V Main High-Power Output
±1% Accuracy
High Efficiency (93%)

♦ Dual Charge-Pump Outputs

Up to +40V Positive Charge-Pump Output
Down to -40V Negative Charge-Pump Output

♦ Internal Supply Sequencing
♦ Internal Power MOSFETs
♦ +2.7V to +5.5V Input Supply
♦ 0.1µA Shutdown Current
♦ 0.6mA Quiescent Current
♦ Internal Soft-Start
♦ Power-Ready Output
♦ Ultra-Small External Components
♦ Thin TSSOP Package (1.1mm max)

MAX1748

Triple-Output TFT LCD DC-DC Converter

________________________________________________________________ Maxim Integrated Products 1

Pin Configuration

19-1740; Rev 0; 6/00

For free samples and the latest literature, visit www.maxim-ic.com or phone [phone].
For small orders, phone [phone].

Ordering Information

Typical Operating Circuit appears at end of data sheet.

16 TSSOP

PIN-PACKAGETEMP. RANGE

-40°C to +85°CMAX1748EUE

PART

TOP VIEW

1

RDY TGND

FB

2

INTG

3

MAX1748

4

IN

GND

5

REF

6

FBP

7

FBN

8

TSSOP

16

15

LX

14

PGND

13

SUPP

12

DRVP

SUPN

11

10

DRVN

9

SHDN

MAX1748

Triple-Output TFT LCD DC-DC Converter

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VIN= +3.0V, SHDN = IN, V

SUPP

= V

SUPN

= 10V, TGND = PGND = GND, C

REF

= 0.22µF, C

INTG

= 470pF, TA= 0°C to +85°C, unless

otherwise noted. Typical values are at T

A

= +25°C.)

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, SHDN, TGND to GND .........................................-0.3V to +6V

DRVN to GND .........................................-0.3V to (V

SUPN

+ 0.3V)

DRVP to GND..........................................-0.3V to (V

SUPP

+ 0.3V)

PGND to GND.....................................................................±0.3V

RDY to GND ...........................................................-0.3V to +14V

LX, SUPP, SUPN to PGND .....................................-0.3V to +14V

INTG, REF, FB, FBN, FBP to GND...............-0.3V to (V

IN

+ 0.3V)

Continuous Power Dissipation (T

A

= +70°C)

16-Pin TSSOP (derate 9.4mW/°C above +70°C) ..........755mW

Operating Temperature Range

MAX1748EUE .................................................-40°C to +85°C

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

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

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

)

)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Input Supply Range V

Input Undervoltage Threshold V

IN Quiescent Supply Current I

SUPP Quiescent Current I

SUPN Quiescent Current I

IN Shutdown Current V

SUPP Shutdown Current V

SUPN Shutdown Current V

MAIN BOOST CONVERTER

Output Voltage Range V

FB Regulation Voltage V

FB Input Bias Current I

Operating Frequency f

Oscillator Maximum Duty Cycle 78 85 90 %

Load Regulation I

Line Regulation 0.1 % / V

Integrator Gm 320 µmho

LX Switch On-Resistance R

LX Leakage Current I

LX Current Limit I

Maximum RMS LX Current 1A

Soft-Start Period t

FB Fault Trip Level 1.07 1.1 1.14 V

POSITIVE CHARGE PUMP

V

Input Supply Range V

SUPP

IN

UVLO

IN

SUPP

SUPN

MAIN

FB

FB

OSC

LX(ON

LX

VIN rising, 40mV hysteresis (typ) 2.2 2.4 2.6 V

VFB = V

V

FBP

V

FBN

SHDN

SHDN

SHDN

= 1.5V, V

FBP

= 1.5V 0.4 0.8 mA

= -0.1V 0.4 0.8 mA

= 0, V

IN

= 0, V

SUPP

= 0, V

SUPN

TA = 0°C to +85°C 1.235 1.248 1.261 V

VFB = 1.25V, INTG = GND -50 50 nA

= 0 to 200mA, V

MAIN

ILX = 100mA 0.35 0.7 Ω

VLX = 13V 0.01 20 µA

Phase I = soft-start (1.0ms) 0.275 0.380 0.500

Phase II = soft-start (1.0ms) 0.75

LX(MAX

Phase III = soft-start (1.0ms) 1.12

Phase IV = fully on (after 3.0ms) 1.1 1.5 2.0

SS

SUPP

Power-up to the end of Phase III

= -0.2V 0.6 1 mA

FBN

= 5V 0.1 10 µA

= 13V 0.1 10 µA

= 13V 0.1 10 µA

= 10V 0.2 %

MAIN

2.7 5.5 V

V

IN

13 V

0.85 1 1.15 MHz

A

3072 /

f

OSC

s

2.7 13 V

MAX1748

Triple-Output TFT LCD DC-DC Converter

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VIN= +3.0V, SHDN = IN, V

SUPP

= V

SUPN

= 10V, TGND = PGND = GND, C

REF

= 0.22µF, C

INTG

= 470pF, TA= 0°C to +85°C, unless

otherwise noted. Typical values are at T

A

= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Operating Frequency

FBP Regulation Voltage V

FBP Input Bias Current I

DRVP PCH On-Resistance

DRVP NCH On-Resistance

FBP Power-Ready Trip Level Rising edge 1.091 1.125 1.159 V

FBP Fault Trip Level Falling edge 1.11 V

Maximum RMS DRVP Current 0.1 A

NEGATIVE CHARGE PUMP

V

Input Supply Range V

SUPN

Operating Frequency

FBN Regulation Voltage V
FBN Input Bias Current I

DRVN PCH On-Resistance

DRVN NCH On-Resistance

FBN Power-Ready Trip Level Rising edge 80 110 165 mV

FBN Fault Trip Level Falling edge 130 mV

Maximum RMS DRVN Current 0.1 A

REFERENCE

Reference Voltage V

Reference Undervoltage
Threshold

LOGIC SIGNALS

SHDN Input Low Voltage 0.4V hysteresis (typ) 0.9 V
SHDN Input High Voltage 2.1 V
SHDN Input Current I

RDY Output Low Voltage I
RDY Output High Voltage V

FBP

V

FBP

SUPN

FBN

FBN

REF

SHDN

= 1.5V -50 50 nA

FBP

V

= 1.213V 1.5 4 Ω

FBP

V

= 1.275V 20 kΩ

FBP

V

= -0.05V -50 50 nA

FBN

V

= 0.035V 1.5 4 Ω

FBN

= -0.025V 20 kΩ

V

FBN

-2µA < I

V

REF

SINK

RDY

< 50µA 1.231 1.25 1.269 V

REF

rising 0.9 1.05 1.2 V

= 2mA 0.25 0.5 V

= 13V 0.01 1 µA

0.5 ×
f

OSC

1.20 1.25 1.30 V

310Ω

2.7 13 V

0.5 ×
f

OSC

-50 0 50 mV

310Ω

0.01 1 µA

Hz

Hz

MAX1748

Triple-Output TFT LCD DC-DC Converter

4 _______________________________________________________________________________________

)

)

ELECTRICAL CHARACTERISTICS (continued)

(VIN= +3.0V, SHDN = IN, V

SUPP

= V

SUPN

= 10V, TGND = PGND = GND, C

REF

= 0.22µF, C

INTG

= 470pF, TA= -40°C to +85°C,

unless otherwise noted.) (Note 1)

PARAMETER SYMBOL CONDITIONS MIN MAX UNITS

Input Supply Range V

Input Undervoltage Threshold V

IN Quiescent Supply Current I

SUPP Quiescent Current I

SUPN Quiescent Current I

IN Shutdown Current V

SUPP Shutdown Current V

SUPN Shutdown Current V

MAIN BOOST CONVERTER

Output Voltage Range V

FB Regulation Voltage V

FB Input Bias Current I

Operating Frequency F

Oscillator Maximum Duty Cycle 78 90 %

LX Switch On-Resistance R

LX Leakage Current

LX Current Limit I

FB Fault Trip Level 1.07 1.14 V

POSITIVE CHARGE PUMP

SUPP Input Supply Range V

FBP Regulation Voltage V

FBP Input Bias Current I

DRVP PCH On-Resistance 10 Ω

DRVP NCH On-Resistance

FBP Power-Ready Trip Level Rising edge 1.091 1.159 V

NEGATIVE CHARGE PUMP

SUPN Input Supply Range V

FBN Regulation Voltage V

FBN Input Bias Current

DRVN PCH On-Resistance

DRVN NCH On-Resistance

FBN Power-Ready Trip Level Rising edge 80 165 mV

REFERENCE

Reference Voltage V

Reference Undervoltage

LX(MAX

IN

UVLO

IN

SUPP

SUPN

MAIN

FB

OSC

LX(ON

I

LX

VIN rising, 40mV hysteresis (typ) 2.2 2.6 V

FB

VFB = V

V

FBP

V

FBN

SHDN

SHDN

SHDN

= 1.5V, V

FBP

= 1.5V 0.8 mA

= -0.1V 0.8 mA

= 0, VIN = 5V 10 µA

= 0, V

SUPP

= 0, V

SUPN

VFB = 1.25V, INTG = GND -50 50 nA

I

= 100mA 0.7 Ω

LX

VLX = 13V 20 µA

Phase I = soft-start (1.0ms) 0.275 0.500

Phase IV = fully on (after 3.0ms) 1.1 2.0

SUPP

FBP

V

FBP

SUPN

FBN

I

FBN

REF

= 1.5V -50 50 nA

FBP

V

= 1.213V 4 Ω

FBP

= 1.275V 20 kΩ

V

FBP

V

= -0.05V -50 50 nA

FBN

V

= 0.035V 4 Ω

FBN

= -0.025V 20 kΩ

V

FBN

-2µA < I

V

REF

< 50µA 1.223 1.269 V

REF

rising 0.9 1.2 V

2.7 5.5 V

= -0.2V 1 mA

FBN

= 13V 10 µA

= 13V 10 µA

V

IN

1.225 1.271 V

0.75 1.25 MHz

2.7 13 V

1.20 1.30 V

2.7 13 V

-50 50 mV

13 V

A

10 Ω

MAX1748

Triple-Output TFT LCD DC-DC Converter

_______________________________________________________________________________________ 5

V

(V)

Typical Operating Characteristics

(Circuit of Figure 5, VIN= 3.3V, TA= +25°C, unless otherwise noted.)

ELECTRICAL CHARACTERISTICS (continued)

(VIN= +3.0V, SHDN = IN, V

SUPP

= V

SUPN

= 10V, TGND = PGND = GND, C

REF

= 0.22µF, C

INTG

= 470pF, TA= -40°C to +85°C,

unless otherwise noted.) (Note 1)

Note 1: Specifications from 0°C to -40°C are guaranteed by design, not production tested.

PARAMETER SYMBOL CONDITIONS MIN MAX UNITS

LOGIC SIGNALS

SHDN Input Low Voltage 0.45V hysteresis (typ) 0.9 V
SHDN Input High Voltage 2.1 V

Input Current I

SHDN
RDY Output Low Voltage I
RDY Output High Leakage V

SHDN

= 2mA 0.5 V

SINK

= 13V 1 µA

RDY

1 µA

MAIN OUTPUT VOLTAGE

vs. LOAD CURRENT

10.04

10.02

10.00

9.98

9.96

(V)

9.94

MAIN

V

9.92

9.90

9.88

9.86

9.84
0 200100 300 400 500 600

VIN = 3.3V

I

(mA)

MAIN

VIN = 5.0V

100

MAX1748 toc01

EFFICIENCY (%)

MAIN STEP-UP CONVERTER

EFFICIENCY vs. LOAD CURRENT

(BOOST ONLY)

V

= 10V

MAIN

95

90

85

80

75

70

65

60

0 200100 300 400 500 600

VIN = 3.3V

I

MAIN

(mA)

VIN = 5.0V

100

MAX1748toc02

EFFICIENCY (%)

MAIN STEP-UP CONVERTER

EFFICIENCY vs. LOAD CURRENT

(BOOST ONLY)

V

= 8V

MAIN

95

VIN = 3.3V

I

MAIN

VIN = 5.0V

(mA)

90

85

80

75

70

65

60

0 200100 300 400 500 800600 700

MAX1748toc03

EFFICIENCY vs. LOAD CURRENT

(BOOST CONVERTER AND CHARGE PUMPS)

90

V

= 8V

= 10V

= -5V WITH I

NEG

= 15V WITH I

POS

I

(mA)

MAIN

MAIN

NEG

POS

= 10mA
= 5mA

MAX1748toc04

85

80

75

70

65

EFFICIENCY (%)

60

55

50

0 10050 150 200 250 400300 350

V

MAIN

VIN = 3.3V
V
V

NEGATIVE CHARGE-PUMP OUTPUT

VOLTAGE vs. LOAD CURRENT

-4.60
V

= -5V

NEG

-4.65

-4.70

V

= 6V

-4.75

-4.80

NEG

-4.85

-4.90

-4.95

-5.00

-5.05

015205 10 25303540

SUPN

V

= 8V

SUPN

V

= 10V

SUPN

I

(mA)

NEG

MAX1748toc05

NEGATIVE CHARGE-PUMP EFFICIENCY

vs. LOAD CURRENT

80

70

60

50

EFFICIENCY (%)

40

30

20

015205 10 25303540

I

NEG

(mA)

V

V

SUPN

SUPN

V

= 8V

= 10V

SUPN

V

NEG

= 6V

MAX1748toc06

= -5V

MAX1748

Triple-Output TFT LCD DC-DC Converter

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 5, VIN= 3.3V, TA= +25°C, unless otherwise noted.)

8

10

18

20

12

16

14

22

24

58796101112

MAXIMUM POSITIVE CHARGE-PUMP OUTPUT

VOLTAGE vs. SUPPLY VOLTAGE

MAX1748toc10

V

SUPP

(V)

V

POS

(V)

I

POS

= 10mA

V

POS

= 22V

I

POS

= 1mA

I

POS

= 20mA

0.80

0.85

0.90

1.05

1.00

0.95

1.10

1.15

1.20

2.5 3.53.0 4.0 4.5 5.0 5.5

SWITCHING FREQUENCY

vs. INPUT VOLTAGE

MAX1748toc11

INPUT VOLTAGE (V)

SWITCHING FREQUENCY (MHz)

MEASURED FROM THE
FALLING EDGE OF LX
V

MAIN

= 10V

I

MAIN

= 100mA

1.244

1.246

1.250

1.248

1.252

1.254

0201510 3025535404550

REFERENCE VOLTAGE

vs. REFERENCE LOAD CURRENT

MAX1748toc12

I

REF

(µA)

V

REF

(V)

VIN = 3.3V

V

MAIN

10mV/div

V

NEG

10mV/div

V

POS

10mV/div

RIPPLE WAVEFORMS

MAX1748toc13

1µs/div

V

MAIN

= 10V, I

MAIN

= 200mA,

V

NEG

= -5V, I

NEG

= 10mA,

V

POS

= 15V, I

POS

= 10mA

I

MAIN

200mA/div

I

LX

500mA/div

V

MAIN

200mV/div

LOAD-TRANSIENT RESPONSE

MAX1748 toc14

100µs/div

V

IN

= 3.3V, V

MAIN

= 10V,

R

MAIN

= 500Ω TO 50Ω (20mA TO 200mA)

I

MAIN

200mA/div

I

LX

500mA/div

V

MAIN

200mV/div

LOAD-TRANSIENT RESPONSE

WITHOUT INTEGRATOR

MAX1748toc15

100µs/div

V

IN

= 3.3V, V

MAIN

= 10V, INTG = REF,

R

MAIN

= 500Ω TO 50Ω (20mA TO 200mA)

-11

-9

-10

-7

-8

-4

-5

-6

-3

5867 9101112

MAXIMUM NEGATIVE CHARGE-PUMP

OUTPUT VOLTAGE vs. SUPPLY VOLTAGE

MAX1748toc07

V

SUPN

(V)

V

NEG

(V)

I

NEG

= 20mA

I

NEG

= 1mA

I

NEG

= 10mA

V

NEG

= -10mA

14.4

14.5

14.6

14.9

15.0

14.7

14.8

15.2

15.1

15.3

086102 4 12 14 16 18 20

POSITIVE CHARGE-PUMP OUTPUT

VOLTAGE vs. LOAD CURRENT

MAX1748toc08

I

POS

(mA)

V

POS

(V)

V

SUPN

= 12V

V

SUPN

= 8V

V

SUPN

= 10V

40

50

80

60

70

90

100

086102 4 12 14 16 18 20

POSITIVE CHARGE-PUMP EFFICIENCY

vs. LOAD CURRENT

MAX1748toc09

I

POS

(mA)

EFFICIENCY (%)

V

SUPP

= 12V

V

SUPP

= 8V

V

SUPP

= 10V

MAX1748

Triple-Output TFT LCD DC-DC Converter

_______________________________________________________________________________________ 7

Typical Operating Characteristics (continued)

(Circuit of Figure 5, VIN= 3.3V, TA= +25°C, unless otherwise noted.)

Pin Description

V

MAIN

5V/div

2V

10V

0

I

LX

500mA/div

MAIN BOOST STARTUP WAVEFORM

MAX1748toc16

1ms/div

R

MAIN

= 1kΩ, V

MAIN

= 10V

V

SHDN

2V/div

0

0

V

MAIN

5V/div

2V

10V

0

I

LX

500mA/div

MAIN BOOST STARTUP

WAVEFORM WITH LOAD

MAX1748toc17

1ms/div

V

MAIN

= 10V, R

MAIN

= 50Ω (200mA)

V

SHDN

2V/div

0

0

V

MAIN

5V/div

V

NEG

5V/div
V

POS

10V/div

POWER-UP SEQUENCING

MAX1748toc18

2ms/div

V

MAIN

= 10V, V

NEG

= -5V, V

POS

= 15V

V

SHDN

2V/div

PIN NAME FUNCTION

1 RDY Active-Low Open-Drain Output. Indicates all outputs are ready. The on-resistance is 125Ω (typ).

2FB

3 INTG

4IN

5 GND Analog Ground. Connect to power ground (PGND) underneath the IC.

6 REF

7 FBP

8 FBN Negative Charge-Pump Regulator Feedback Input. Regulates to 0V nominal.

9

10 DRVN Negative Charge-Pump Driver Output. Output high level is V

11 SUPN Negative Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1µF capacitor.

12 DRVP Positive Charge-Pump Driver Output. Output high level is V

Main Boost Regulator Feedback Input. Regulates to 1.248V nominal. Connect feedback resistive
divider to analog ground (GND).

Main Boost Integrator Output. If used, connect 470pF to analog ground (GND). To disable
integrator, connect to REF.

Supply Input. +2.7V to +5.5V input range. Bypass with a 0.1µF capacitor between IN and GND, as
close to the pins as possible.

Internal Reference Bypass Terminal. Connect a 0.22µF capacitor from this terminal to analog
ground (GND). External load capability to 50µA.

Positive Charge-Pump Regulator Feedback Input. Regulates to 1.25V nominal. Connect feedback
resistive divider to analog ground (GND).

SHDN

Active-Low Logic-Level Shutdown Input. Connect SHDN to IN for normal operation.

, and low level is PGND.

SUPN

, and low level is PGND.

SUPP

MAX1748

Triple-Output TFT LCD DC-DC Converter

8 _______________________________________________________________________________________

Detailed Description

The MAX1748 is a highly efficient triple-output power
supply for TFT LCD applications. The device contains
one high-power step-up converter and two low-power
charge pumps. The primary boost converter uses an
internal N-channel MOSFET to provide maximum effi­ciency and to minimize the number of external compo­nents. The output voltage of the main boost converter
(V

MAIN

) can be set from VINto 13V with external resistors.

The dual charge pumps independently regulate a posi­tive output (V

POS

) and a negative output (V

NEG

). These
low-power outputs use external diode and capacitor
stages (as many stages as required) to regulate output
voltages up to +40V and down to -40V. A proprietary
regulation algorithm minimizes output ripple as well as
capacitor sizes for both charge pumps.

Also included in the MAX1748 are a precision 1.25V
reference that sources up to 50µA, logic shutdown,
soft-start, power-up sequencing, fault detection, and an
active-low open-drain ready output.

Main Boost Converter

The MAX1748 main step-up converter switches at a
constant 1MHz internal oscillator frequency to allow the
use of small inductors and output capacitors. The
MOSFET switch pulse width is modulated to control the
power transferred on each switching cycle and to regu­late the output voltage.

During PWM operation, the internal clock’s rising edge
sets a flip-flop, which turns on the N-channel MOSFET
(Figure 1). The switch turns off when the sum of the
voltage-error, slope-compensation, and current-feed­back signals trips the multi-input comparator and
resets the flip-flop. The switch remains off for the rest of
the clock cycle. Changes in the output voltage error
signal shift the switch current trip level, consequently
modulating the MOSFET duty cycle.

Dual Charge-Pump Regulator

The MAX1748 contains two individual low-power charge
pumps. One charge pump inverts the supply voltage
(SUPN) and provides a regulated negative output voltage.
The second charge pump doubles the supply voltage
(SUPP) and provides a regulated positive output voltage.
The MAX1748 contains internal P-channel and N-channel
MOSFETs to control the power transfer. The internal
MOSFETs switch at a constant 500kHz (0.5 ✕f

OSC

).

Negative Charge Pump

During the first half-cycle, the P-channel MOSFET turns
on and the flying capacitor C5 charges to V

SUPN

minus
a diode drop (Figure 2). During the second half-cycle,
the P-channel MOSFET turns off, and the N-channel
MOSFET turns on, level shifting C5. This connects C5 in
parallel with the reservoir capacitor C6. If the voltage
across C6 minus a diode drop is lower than the voltage
across C5, charge flows from C5 to C6 until the diode
(D5) turns off. The amount of charge transferred to the
output is controlled by the variable N-channel on-resis­tance.

Positive Charge Pump

During the first half-cycle, the N-channel MOSFET turns
on and charges the flying capacitor C3 (Figure 3). This
initial charge is controlled by the variable N-channel
on-resistance. During the second half-cycle, the N­channel MOSFET turns off and the P-channel MOSFET
turns on, level shifting C3 by V

SUPP

volts. This connects
C3 in parallel with the reservoir capacitor C4. If the volt­age across C4 plus a diode drop (V

POS

+ V

DIODE

) is
smaller than the level-shifted flying capacitor voltage
(VC3+ V

SUPP

), charge flows from C3 to C4 until the

diode (D3) turns off.

Soft-Start

For the main boost regulator, soft-start allows a gradual
increase of the internal current-limit level during startup
to reduce input surge currents. The MAX1748 divides

Pin Description (continued)

PIN NAME FUNCTION

13 SUPP Positive Charge-Pump Driver Supply Voltage. Bypass to PGND with a 0.1µF capacitor.

14 PGND Power Ground. Connect to GND underneath the IC.

15 LX

16 TGND Must be connected to ground.

Main Boost Regulator Power MOSFET N-Channel Drain. Connect output diode and output capacitor
as close to PGND as possible.

MAX1748

Triple-Output TFT LCD DC-DC Converter

_______________________________________________________________________________________ 9

the soft-start period into four phases. During phase 1,
the MAX1748 limits the current limit to only 0.38A (see
Electrical Characteristics), approximately a quarter of
the maximum current limit (I

LX(MAX)

). If the output does
not reach regulation within 1ms, soft-start enters phase
II and the current limit is increased by another 25%.
This process is repeated for phase III. The maximum

1.5A (typ) current limit is reached within 3.0ms or when
the output reaches regulation, whichever occurs first
(see the Startup Waveforms in the Typical Operating
Characteristics).

For the charge pumps, soft-start is achieved by control­ling the rise rate of the output voltage. The output volt­age regulates within 4ms, regardless of output
capacitance and load, limited only by the regulator’s
output impedance.

Shutdown

A logic-low level on SHDN disables all three MAX1748
converters and the reference. When shut down, supply
current drops to 0.1µA to maximize battery life and the
reference is pulled to ground. The output capacitance
and load current determine the rate at which each out­put voltage will decay. A logic-level high on SHDN
power activates the MAX1748 (see Power-Up
Sequencing). Do not leave SHDN floating. If unused,
connect SHDN to IN.

Power-Up Sequencing

Upon power-up or exiting shutdown, the MAX1748
starts a power-up sequence. First, the reference pow­ers up. Then the main DC-DC step-up converter pow­ers up with soft-start enabled. Once the main boost

Figure 1. PWM Boost Converter Block Diagram

OSC

= 2.7V TO 5.5V

V

IN

IN

S

R

­+

Q

I

LIM

L1

LX

PGND

D1

V

[1 + (R1 / R2)] x V

OUT =

V

= 1.25V

REF

R1

V

MAIN

(UP TO 13V)

C1

REF

+

R

+

-

INTG

C

INTG

MAX1748

Gm

­+

+

-

-

1.25V

FB

REF

GND

C2

COMP

R2

C

COMP

MAX1748

Triple-Output TFT LCD DC-DC Converter

10 ______________________________________________________________________________________

Figure 2. Negative Charge-Pump Block Diagram

Figure 3. Positive Charge-Pump Block Diagram

= 2.7V TO 13V

V

SUPN

D4

D5

OSC

SUPN

DRVN

C5

+

-

MAX1748

GND

OSC

R6

C

REF

0.22µF

R5

V

= (R5 / R6) x V

POS

V

= 1.25V

REF

V

SUPP

D2

D3

FBN

V

-

REF

+

1.25V

PGND

REF

SUPP

DRVP

C3

V

C6

= 2.7V TO 13V

NEG

REF

-

FBP

+

+

V

REF

-

1.25V

R3

R4

V

POS

C4

MAX1748

GND

PGND

V

= [1 + (R3 / R4)] x V

POS

V

= 1.25V

REF

REF

MAX1748

Triple-Output TFT LCD DC-DC Converter

______________________________________________________________________________________ 11

converter reaches regulation, the negative charge
pump turns on. When the negative output voltage
reaches approximately 88% of its nominal value (V

FBN

< 110mV), the positive charge pump starts up. Finally,
when the positive output voltage reaches 90% of its
nominal value (V

FBP

> 1.125V), the active-low ready

signal (RDY) goes low (see Power Ready section).

Power Ready

Power ready is an open-drain output. When the power­up sequence is properly completed, the MOSFET turns
on and pulls RDY low with a typical 125Ω on-resis­tance. If a fault is detected, the internal open-drain
MOSFET appears as a high impedance. Connect a
100kΩ pull-up resistor between RDY and IN for a logic­level output.

Fault Detection

Once RDY is low and if any output falls below its fault­detection threshold, RDY goes high impedance.

For the reference, the fault threshold is 1.05V. For the
main boost converter, the fault threshold is 88% of its
nominal value (VFB< 1.1V). For the negative charge
pump, the fault threshold is approximately 90% of its
nominal value (V

FBN

< 130mV). For the positive charge
pump, the fault threshold is 88% of its nominal value
(V

FBP

< 1.11V).

Once an output faults, all outputs later in the power
sequence shut down until the faulted output rises
above its power-up threshold. For example, if the nega­tive charge-pump output voltage falls below the fault
detection threshold, the main boost converter remains
active while the positive charge pump stops switching
and its output voltage decays, depending on output
capacitance and load. The positive charge-pump out­put will not power up until the negative charge-pump
output voltage rises above its power-up threshold (see
the Power-Up Sequencing section).

Voltage Reference

The voltage at REF is nominally 1.25V. The reference
can source up to 50µA with good load regulation (see
Typical Operating Characteristics). Connect a 0.22µF
bypass capacitor between REF and GND.

Design Procedure

Main Boost Converter

Output Voltage Selection

Adjust the output voltage by connecting a voltage
divider from the output (V

MAIN

) to FB to GND (see
Typical Operating Circuit). Select R2 in the 10kΩ to
20kΩ range. Higher resistor values improve efficiency
at low output current but increase output voltage error

due to the feedback input bias current. Calculate R1
with the following equations:

R1 = R2 [(V

MAIN

/ V

REF

) - 1]

where V

REF

= 1.25V. V

MAIN

may range from VINto 13V.

Feedback Compensation

For stability, add a pole-zero pair from FB to GND in the
form of a series resistor (R

COMP

) and capacitor

(C

COMP

). The resistor should be half the value of the

R2 feedback resistor.

Inductor Selection

Inductor selection depends on input voltage, output
voltage, maximum current, switching frequency, size,
and availability of inductor values. Other factors can
include efficiency and ripple voltage. Inductors are
specified by their inductance (L), peak current (I

PEAK

),
and resistance (RL). The following boost-circuit equa­tions are useful in choosing inductor values based on
the application. They allow the trading of peak current
and inductor value while allowing for consideration of
component availability and cost.

The following equation includes a constant LIR, which
is the ratio of the inductor peak-to-peak AC current to
maximum average DC inductor current. A good com­promise between the size of the inductor, loss, and out­put ripple is to choose an LIR of 0.3 to 0.5. The peak
inductor current is then given by:

The inductance value is then given by:

Considering the typical application circuit, the maxi­mum DC load current (I

MAIN(MAX)

) is 200mA with a 10V
output. A 6.8µH inductance value is then chosen,
based on the above equations and using 85% efficien­cy and a 1MHz operating frequency. Smaller induc­tance values typically offer a smaller physical size for a
given series resistance and current rating, allowing the
smallest overall circuit dimensions. However, due to
higher peak inductor currents, the output voltage ripple
(I

PEAK

✕

output filter capacitor ESR) will be higher.

Use inductors with a ferrite core or equivalent; powder
iron cores are not recommended for use with the
MAX1748’s high switching frequencies. The inductor’s
maximum current rating should exceed I

PEAK

. Under

fault conditions, inductor current may reach up to 2.0A.

I

PEAK

L

=

IV

MAIN(MAX) MAIN

=

Efficiency V

2

V Efficiency (V V )

IN(MIN)

××−

V LIR I f

(MAIN)

×

×

IN(MIN)

2

×× ×

MAIN(MAX) OSC

1 (LIR/2)

×+

[]

MAIN IN(MIN)

MAX1748

Triple-Output TFT LCD DC-DC Converter

The MAX1748’s fast current-limit circuitry allows the use
of soft-saturation inductors while still protecting the IC.

The inductor’s DC resistance significantly affects effi­ciency. For best performance, select inductors with
resistance less than the internal N-channel FET resis­tance. To minimize radiated noise in sensitive applica­tions, use a shielded inductor.

The inductor should have as low a series resistance as
possible. For continuous inductor current, the power
loss in the inductor resistance, PLR, is approximated by:

P

LR

≅ (I

MAIN

✕

V

MAIN

/ VIN)

2

✕

R

L

where RLis the inductor series resistance.

Output Capacitor

A 10µF capacitor works well in most applications. The
equivalent series resistance (ESR) of the output filter
capacitor affects efficiency and output ripple. Output
voltage ripple is largely the product of the peak induc­tor current and the output capacitor ESR. Use low-ESR
ceramic capacitors for best performance. Low-ESR,
surface-mount tantalum capacitors with higher capacity
may be used for load transients with high peak cur­rents. Voltage ratings and temperature characteristics
should be considered.

Input Capacitor

The input capacitor (CIN) in boost designs reduces the
current peaks drawn from the input supply and reduces
noise injection. The value of CINis largely determined
by the source impedance of the input supply. High
source impedance requires high input capacitance,
particularly as the input voltage falls. Since step-up DC­DC converters act as “constant-power” loads to their
input supply, input current rises as input voltage falls. A
good starting point is to use the same capacitance
value for CINas for C

OUT

. Table 1 lists suggested com-

ponent suppliers.

Integrator Capacitor

The MAX1748 contains an internal current integrator
that improves the DC load regulation but increases the
peak-to-peak transient voltage (see the load-transient
waveforms in the Typical Operating Characteristics).
For highly accurate DC load regulation, enable the cur­rent integrator by connecting a 470pF capacitor to
INTG. To minimize the peak-to-peak transient voltage at
the expense of DC regulation, disable the integrator by
connecting INTG to REF and adding a 100kΩ resistor
to GND.

Rectifier Diode

Use a Schottky diode with an average current rating
equal to or greater than the peak inductor current, and
a voltage rating at least 1.5 times the main output volt­age (V

MAIN

).

Charge Pump

Efficiency Considerations

The efficiency characteristics of the MAX1748 regulated
charge pumps are similar to a linear regulator. They are
dominated by quiescent current at low output currents
and by the input voltage at higher output currents (see
Typical Operating Characteristics). So the maximum
efficiency may be approximated by:

Efficiency ≅ V

NEG

/ [V

IN

✕

N];

for the negative charge pump

Efficiency ≅ V

POS

/ [V

IN

✕

(N + 1)];

for the positive charge pump

where N is the number of charge-pump stages.

Output Voltage Selection

Adjust the positive output voltage by connecting a volt­age-divider from the output (V

POS

) to FBP to GND (see
Typical Operating Circuit). Adjust the negative output

Table 1. Component Suppliers

12 ______________________________________________________________________________________

SUPPLIER PHONE FAX

INDUCTORS

Coilcraft 847-639-6400 847-639-1469

Coiltronics 561-241-7876 561-241-9339

Sumida USA 847-956-0666 847-956-0702

Toko 847-297-0070 847-699-1194

CAPACITORS

AVX 803-946-0690 803-626-3123

Kemet 408-986-0424 408-986-1442

Sanyo 619-661-6835 619-661-1055

Taiyo Yuden 408-573-4150 408-573-4159

DIODES

Central
Semiconductor

International
Rectifier

Motorola 602-303-5454 602-994-6430

Nihon 847-843-7500 847-843-2798

Zetex 516-543-7100 516-864-7630

516-435-1110 516-435-1824

310-322-3331 310-322-3332

MAX1748

Triple-Output TFT LCD DC-DC Converter

______________________________________________________________________________________ 13

voltage by connecting a voltage-divider from the output
(V

NEG

) to FBN to REF. Select R4 and R6 in the 50kΩ to
100kΩ range. Higher resistor values improve efficiency
at low output current but increase output voltage error
due to the feedback input bias current. Calculate the
remaining resistors with the following equations:

R3 = R4 [(V

POS

/ V

REF

) - 1]

R5 = R6 (V

NEG

/ V

REF

)

where V

REF

= 1.25V. V

POS

may range from V

SUPP

to

40V, and V

NEG

may range from 0 to -40V.

Flying Capacitor

Increasing the flying capacitor’s value reduces the out­put current capability. Above a certain point, increasing
the capacitance has a negligible effect because the
output current capability becomes dominated by the
internal switch resistance and the diode impedance.
Start with 0.1µF ceramic capacitors. Smaller values
may be used for low-current applications.

Charge-Pump Output Capacitor

Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to­peak transient voltage. Use the following equation to
approximate the required capacitor value:

C

OUT

≥ [I

OUT

/ (500kHz ✕V

RIPPLE

)]

Charge-Pump Input Capacitor

Use a bypass capacitor with a value equal to or greater
than the flying capacitor. Place the capacitor as close
to the IC as possible. Connect directly to PGND.

Rectifier Diode

Use Schottky diodes with a current rating equal to or
greater than 4 times the average output current, and a
voltage rating at least 1.5 times V

SUPP

for the positive

charge pump and V

SUPN

for the negative charge pump.

PC Board Layout and Grounding

Careful printed circuit layout is extremely important to
minimize ground bounce and noise. First, place the
main boost converter output diode and output capacitor
less than 0.2in (5mm) from the LX and PGND pins with
wide traces and no vias. Then place 0.1µF ceramic
bypass capacitors near the charge-pump input pins
(SUPP and SUPN) to the PGND pin. Keep the charge­pump circuitry as close to the IC as possible, using
wide traces and avoiding vias when possible. Locate all
feedback resistive dividers as close to their respective
feedback pins as possible. The PC board should fea­ture separate GND and PGND areas connected at only
one point under the IC. To maximize output power and
efficiency and to minimize output power ripple voltage,
use extra wide power ground traces and solder the IC’s
power ground pin directly to it. Avoid having sensitive
traces near the switching nodes and high-current lines.

Refer to the MAX1748 evaluation kit for an example of
proper board layout.

Applications Information

Boost Converter Using a

Cascoded MOSFET

For applications that require output voltages greater
than 13V, cascode an external N-channel MOSFET
(Figure 4). Place the MOSFET as close to the LX pin as
possible. Connect the gate to the input voltage (V

IN

)

and the source to LX.

MOSFET Selection

Choose a MOSFET with an on-resistance (R

DS(ON)

)
lower than the internal N-channel MOSFET. Lower
R

DS(ON)

will improve efficiency. The external N-channel
MOSFET must have a drain-voltage rating higher than
the main output voltage (V

MAIN

).

Chip Information

TRANSISTOR COUNT: 2846

MAX1748

Triple-Output TFT LCD DC-DC Converter

14 ______________________________________________________________________________________

Figure 4. Power Supply Using Cascoded MOSFET

+25V, 5mA
=

V

POS

1.0µF

OUT

= +18V, 140mA

C

MAIN

V

0.47µF

10µF

COMP

C

68nF

6.8µH

R1

100k

130k

COMP

R

R2

5k

IN SUPP

10k

SUPN

R3

1M

R4

49.9k

INTG

C

470pF

0.22µF

LX

FB

DRVP

0.1µF

FBP

INTG

TGND

GND

MAX1748

SHDN

RDY

DRVN

0.22µF

0.1µF

FBN

REF

PGND

R6

49.9k

R5

319k

REF

C

0.22µF

3.3µF

1.0µF

= 5.0V

IN

V

0.47µF

= -8V, 20mA

NEG

V

MAX1748

Triple-Output TFT LCD DC-DC Converter

______________________________________________________________________________________ 15

Typical Operating Circuit

= +10V, 200mA

C

MAIN

V

OUT

+15V, 10mA
=

POS

V

10µF

R1

70k

COMP

C

6.8nF

5k

COMP

R

R2

10k

FB

SUPN

0.1µF

0.1µF

SUPP

0.1µF

DRVP

FBP

1.0µF

R3

670k

R4

49.9k

PGND

6.8µH

= 3.0V

IN

V

MAX1748

IN LX

3.3µF

0.1µF

100k

SHDN

RDY

DRVN

FBN

0.1µF

R5

1.0µF

= -5V, 20mA

NEG

V

REF

REF

49.9k

C

0.22µF

200k R6

INTG

GND

INTG

C

TGND

470pF

MAX1748

Triple-Output TFT LCD DC-DC Converter

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.

16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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.

16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Package Information

TSSOP.EPS