Rainbow Electronics MAX8727 User Manual

General Description

The MAX8727 is a high-performance step-up DC-DC
converter that provides a regulated supply voltage for
active-matrix thin-film transistor (TFT) liquid-crystal dis­plays (LCDs). The MAX8727 incorporates current­mode, fixed-frequency, pulse-width modulation (PWM)
circuitry with a built-in n-channel power MOSFET to
achieve high efficiency and fast transient response.

Users can select 640kHz or 1.2MHz operation using a
logic input pin (FREQ). The high switching frequencies
allow the use of ultra-small inductors and low-ESR
ceramic capacitors. The current-mode architecture pro­vides fast transient response to pulsed loads. A com­pensation pin (COMP) gives users flexibility in adjusting
loop dynamics. The 30V internal MOSFET can generate
output voltages up to 24V from an input voltage
between 2.6V and 5.5V. Soft-start slowly ramps the input
current and is programmed with an external capacitor.

The MAX8727 is available in a 10-pin thin DFN package.

Applications

Notebook Computer Displays

LCD Monitor Panels

Features

♦ 90% Efficiency

♦ Adjustable Output from V

IN

to 24V

♦ 2.6V to 5.5V Input Supply Range

♦ Input Supply Undervoltage Lockout

♦ Pin-Programmable 640kHz/1.2MHz Switching

Frequency

♦ Programmable Soft-Start

♦ 0.1µA Shutdown Current

♦ Small 10-Pin Thin DFN Package

MAX8727

TFT-LCD Step-Up DC-DC Converter

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-3480; Rev 0; 10/04

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.

EVALUATION KIT

AVAILABLE

Pin Configuration

LX LX

FB

GND

GND

FREQ

IN

COMP

SS

1

4

5

2

3

9

8

67

10

V

OUT

V

IN

2.6V TO 5.5V

SHDN

MAX8727

Minimal Operating Circuit

PART TEMP RANGE PIN-PACKAGE

MAX8727ETB -40°C to +85°C 10 Thin DFN 3mm x 3mm

TOP VIEW

COMP 1

FB 2

3

SHDN

GND 4

GND 5

MAX8727

THIN DFN

3mm x 3mm

10

SS

FREQ

9

IN

8

LX

7

LX

6

MAX8727

TFT-LCD Step-Up DC-DC Converter

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VIN= V

SHDN

= 3V, FREQ = GND, TA= 0°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.

LX to GND ..............................................................-0.3V to +26V

IN, SHDN, FREQ, FB to GND ...................................-0.3V to +6V

COMP, SS to GND .........................................-0.3V to V

IN

+ 0.3V

LX Switch Maximum Continuous RMS Current .....................2.4A

Continuous Power Dissipation (T

A

= +70°C)

10-Pin Thin DFN (derate 24.4mW/°C above +70°C) ....1951mW

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

Input Voltage Range

Output Voltage Range 24 V

IN Undervoltage-Lockout
Threshold

IN Quiescent Current

IN Shutdown Current SHDN = GND 0.1 10.0 µA

ERROR AMPLIFIER

FB Regulation Voltage Level to produce V

FB Input Bias Current VFB = 1.24V 50 125 250 nA

FB Line Regulation Level to produce V

Transconductance 100 200 300 µS

Voltage Gain 700 V/V
Shutdown FB Input Voltage SHDN = GND 0.05 0.10 0.15 V

OSCILLATOR

Frequency

Maximum Duty Cycle 87 90 93 %

n-CHANNEL MOSFET

Current Limit VFB = 1V, 75% duty cycle 3.0 3.8 4.6 A
On-Resistance 125 250 Ω

Leakage Current VLX = 24V 30 45 µA

Current-Sense Transresistance 0.11 0.21 0.31 V/A

SOFT-START

Reset Switch Resistance 100 Ω

Charge Current VSS = 1.2V 2.5 4.5 7.5 µA

PARAMETER CONDITIONS MIN TYP MAX UNITS

V

< 18V 2.6 5.5

OUT

18V < V

rising, typical hysteresis is 50mV 2.20 2.38 2.57 V

V

IN

VFB = 1.3V, not switching 0.225 0.440

= 1.0V, switching 2 5

V

FB

FREQ = GND 540 640 740

FREQ = IN 1000 1220 1500

< 24V 4.0 5.5

OUT

= 1.24V 1.22 1.24 1.26 V

COMP

= 1.24V, VIN = 2.6V to 5.5V 0.05 0.15 %/V

COMP

V

mA

kHz

MAX8727

TFT-LCD Step-Up DC-DC Converter

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VIN= V

SHDN

= 3V, FREQ = GND, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)

ELECTRICAL CHARACTERISTICS

(VIN= V

SHDN

= 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

CONTROL INPUTS

SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V

SHDN, FREQ Input High Voltage VIN = 2.6V to 5.5V

SHDN, FREQ Input Hysteresis VIN = 2.6V to 5.5V

FREQ Pulldown Current 2.3 6.0 9.5 µA
SHDN Input Current SHDN = GND 0.001 1 µA

PARAMETER CONDITIONS MIN TYP MAX UNITS

0.3 ×

V

IN

0.7 ×

V

IN

0.1 ×

V

IN

Input Voltage Range

Output Voltage Range 24 V

IN Undervoltage-Lockout
Threshold

IN Quiescent Current

IN Shutdown Current SHDN = GND 10 µA

ERROR AMPLIFIER

FB Regulation Voltage Level to produce V

FB Input Bias Current VFB = 1.24V 250 nA

FB Line Regulation Level to produce V

Transconductance 100 300 µS
Shutdown FB Input Voltage SHDN = GND 0.05 0.15 V

OSCILLATOR

Frequency

Maximum Duty Cycle 86 94 %

n-CHANNEL MOSFET

Current Limit VFB = 1V, 75% duty cycle 3.0 5.1 A
On-Resistance 250 mΩ

Current-Sense Transresistance 0.11 0.31 V/A

SOFT-START

Reset Switch Resistance 100 Ω

Charge Current VSS = 1.2V 2.5 7.5 µA

PARAMETER CONDITIONS MIN TYP MAX UNITS

V

< 18V 2.6 5.5

OUT

18V < V

rising, typical hysteresis is 50mV 2.20 2.57 V

V

IN

VFB = 1.3V, not switching 0.44

= 1.0V, switching 5

V

FB

FREQ = GND 490 770

FREQ = IN 900 1600

< 24V 4.0 5.5

OUT

= 1.24V 1.215 1.260 V

COMP

= 1.24V, VIN = 2.6V to 5.5V 0.15 %/V

COMP

V

V

V

V

mA

kHz

MAX8727

TFT-LCD Step-Up DC-DC Converter

4 _______________________________________________________________________________________

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

ELECTRICAL CHARACTERISTICS (continued)

(VIN= V

SHDN

= 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

Typical Operating Characteristics

(Circuit of Figure 1. VIN= 5V, V

MAIN

= 15V, TA= +25°C unless otherwise noted.)

CONTROL INPUTS

SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V

SHDN, FREQ Input High Voltage VIN = 2.6V to 5.5V

PARAMETER CONDITIONS MIN TYP MAX UNITS

EFFICIENCY vs. LOAD CURRENT

(1.2MHz OPERATION)

100

L = 3.6µH

90

EFFICIENCY vs. LOAD CURRENT

(640kHz OPERATION)

100

L = 6.8µH

MAX8727 toc01

90

MAX8727 toc02

0.7 ×

V

IN

0.3 ×

V

IN

V

V

LOAD REGULATION

0.5

0

MAX8727 toc03

80

VIN = 5.0V

70

EFFICIENCY (%)

60

50

11000

VIN = 3.3V

10010

LOAD CURRENT (mA)

80

70

EFFICIENCY (%)

60

50

1 1000

SWITCHING FREQUENCY

1400

vs. INPUT VOLTAGE

1300

1200

1100

1000

900

800

SWITCHING FREQUENCY (kHz)

700

600

500

2.5 5.5

FREQ = IN

FREQ = GND

INPUT VOLTAGE (V)

4.0

3.5

MAX8727 toc04

3.0

2.5

2.0

1.5

SUPPLY CURRENT (mA)

1.0

0.5

0

5.04.54.03.53.0

2.5 5.5

vs. SUPPLY VOLTAGE

VIN = 5.0V

VIN = 3.3V

10010

LOAD CURRENT (mA)

SUPPLY CURRENT

SWITCHING

NONSWITCHING

SUPPLY VOLTAGE (V)

-0.5

-1.0

LOAD REGULATION (%)

-1.5

-2.0
1 1000

LOAD CURRENT (mA)

VIN = 5.0V

VIN = 3.3V

10010

SUPPLY CURRENT vs. TEMPERATURE

(SWITCHING)

5

MAX8727 toc05

5.04.54.03.53.0

4

3

2

SUPPLY CURRENT (mA)

1

0

-40 100

VIN = 5.0V

VIN = 3.3V

TEMPERATURE (°C)

MAX8727 toc06

806040200-20