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Digitally Adjustable LCD Bias Supply

_______________General Description

The MAX749 generates negative LCD-bias contrast

voltages from 2V to 6V inputs. Full-scale output voltage

can be scaled to -100V or greater, and is digitally

adjustable in 64 equal steps by an internal digital-to-

analog converter (DAC). Only seven small surface-

mount components are required to build a complete

supply. The output voltage can also be adjusted using

a PWM signal or a potentiometer.

A unique current-limited control scheme reduces supply

current and maximizes efficiency, while a high switching

frequency (up to 500kHz) minimizes the size of external

components. Quiescent current is only 60µA max and is

reduced to under 15µA in shutdown mode. While shut

down, the MAX749 retains the voltage set point, simpli-

fying software control. The MAX749 drives either an

external P-channel MOSFET or a PNP transistor.

________________________Applications

Notebook Computers

Laptop Computers

Palmtop Computers

Personal Digital Assistants

Communicating Computers

Portable Data-Collection Terminals

____________________________Features

♦ +2.0V to +6.0V Input Voltage Range

♦ Flexible Control of Output Voltage:

Digital Control

Potentiometer Adjustment

PWM Control

♦ Output Voltage Range Set by One Resistor

♦ Low, 60µA Max Quiescent Current

♦ 15µA Max Shutdown Mode

♦ Small Size – 8-Pin SO and Plastic DIP Packages

MAX749

_______________________________________________________________

Maxim Integrated Products

DHI

DLOW

GND

CTRL

ADJ

V+

DIP/SO

TOP VIEW

MAX749

__________________Pin Configuration

V+

ADJ

CTRL

DHI

DLOW

GND

+5V

-V

OUT

SENSE

COMP

ON/OFF

DIGITAL

ADJUST

0.1µF

MAX749

__________Typical Operating Circuit

Call toll free 1-800-998-8800 for free samples or literature.

19-0143; Rev 1; 2/95

PART TEMP. RANGE PIN-PACKAGE

MAX749CPA 0°C to +70°C 8 Plastic DIP

MAX749CSA 0°C to +70°C 8 SO

MAX749EPA -40°C to +85°C 8 Plastic DIP

MAX749ESA -40°C to +85°C 8 SO

MAX749C/D 0°C to +70°C Dice*

______________Ordering Information

* Contact factory for dice specifications.

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

Summary of content (12 pages)

PAGE 1
9-0143; Rev 1; 2/95 NUAL KIT MA ATION HEET S A EVALU T A WS D FOLLO Digitally Adjustable LCD Bias Supply ________________________Applications Notebook Computers ____________________________Features ♦ +2.0V to +6.
PAGE 2
MAX749 Digitally Adjustable LCD Bias Supply ABSOLUTE MAXIMUM RATINGS V+ ................................................................................-0.3V, +7V CTRL, ADJ, FB, DLOW, DHI, CS.....................-0.3V, (V+ + 0.3V) Continuous Power Dissipation (TA = +70°C) Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW SO (derate 5.88mW/°C above +70°C) .........................471mW Operating Temperature Ranges: MAX749C_A........................................................
PAGE 3
Digitally Adjustable LCD Bias Supply EFFICIENCY vs. OUTPUT CURRENT – PNP EFFICIENCY (%) -5V 75 V+ = 3V RBASE = 470Ω RSENSE = 0.25Ω TRANSISTOR: ZTX750 70 -24V -12V -12V 76 -12V -24V 85 MAX749TOC2-B 78 80 EFFICIENCY (%) 80 -5V 74 -24V 72 V+ = 3V RBASE = 160Ω RSENSE = 0.25Ω TRANSISTOR = ZTX750 70 68 -5V 75 V+ = 5V RSENSE = 0.
PAGE 4
MAX749 Digitally Adjustable LCD Bias Supply ____________________________Typical Operating Characteristics (continued) (TA = +25°C, L = 47µH, unless otherwise noted.
PAGE 5
Digitally Adjustable LCD Bias Supply 0.1µF MAX749 +2V TO +6V INPUT 22µF 6.2V V+ POWER-ON RESET RESET 6-BIT CURRENT-OUTPUT DAC 6-BIT COUNTER RSENSE REF CTRL ADJ 6.66µA TO 20µA LOGIC INCREMENT CS ON/OFF DHI SWITCHMODE POWER SUPPLY Q1 ZTX750 DLOW RBASE 470Ω BIAS RFB FB MAX749 D1 1N5819 L1 47µH VOUT (NEGATIVE) GND CCOMP 22µF 30V Figure 1.
PAGE 6
MAX749 Digitally Adjustable LCD Bias Supply +2V TO +6V INPUT 0.1µF 22µF V+ 140mV Q TRIG MINIMUM OFF-TIME ONE-SHOT FLIP-FLOP DHI Q S R Q1 ZTX750 DLOW MAXIMUM ON-TIME ONE-SHOT TRIG RSENSE CURRENT COMPARATOR RBASE 470Ω VOLTAGE COMPARATOR Q FB REF D1 1N5819 VOUT (NEGATIVE) RFB 6-BIT CURRENT-OUTPUT DAC MAX749 L1 47µH 22µF 30V GND CCOMP Figure 2.
PAGE 7
Digitally Adjustable LCD Bias Supply CTRL tR tSD ON SHUTDOWN RESET SHUTDOWN In Figure 3, the MAX749 is reset when it is taken out of shutdown, which sets the output at mid-scale. Figure 4 shows how to increment the counter. Figure 5 illustrates a reset without shutting the device down. Figure 7 provides an example of a sequence of operations: Starting from shutdown, the device is turned on, incremented, reset to mid-scale without being shut down, incremented again, and finally shut down.
PAGE 8
MAX749 Digitally Adjustable LCD Bias Supply Current-Sense Resistor +4.5V to +6V INPUT 22µF 0.1µF RSENSE V+ CS MAX749 DHI CTRL Q1 SMD10P05L DLOW L1 47µH ADJ R1 R2 D1 1N5819 VOUT (NEGATIVE) GND 22µF 30V VOUT(MIN) = -R1(13.33µA) VOUT(MAX) = -(R1+R2)(13.33µA) CCOMP Figure 8. Using a Potentiometer to Adjust the Output Voltage counter rolls over and sets the DAC to -V OUT(MIN) , where -VOUT(MIN) = RFB x 6.66µA.
PAGE 9
Digitally Adjustable LCD Bias Supply 0.2 VOUT = -15V L = 47µH 80 60 0.25 RSENSE (Ω) 100 MAX749-Fig 11 MAXIMUM OUTPUT CURRENT (mA) MAX749 Choosing an Inductor Practical inductor values range from 22µH to 100µH, and 47µH is normally a good choice. Inductors with a ferrite core or equivalent are recommended.
PAGE 10
MAX749 Digitally Adjustable LCD Bias Supply The Sumida CD54-470N (47µH, 720mA, 370mΩ) is suitable for a wide range of applications, and the larger CD105-470N (47µH, 1.17A, 170mΩ) permits higher current levels and efficiencies. Diode Selection The MAX749’s high switching frequency demands a highspeed rectifier. Schottky diodes such as the 1N58171N5822 family are recommended.
PAGE 11
Digitally Adjustable LCD Bias Supply Input Bypass Capacitor A 22µF tantalum capacitor in parallel with a 0.1µF ceramic normally provides sufficient bypassing. Mount the 0.1µF capacitor very close to the IC. Larger capacitors may be needed if the incoming supply has high impedance. Less bypass capacitance is acceptable if the circuit is run off a low-impedance supply.
PAGE 12
MAX749 Digitally Adjustable LCD Bias Supply _______________________________________________________Package Information D E DIM E1 A A1 A2 A3 B B1 C D1 E E1 e eA eB L A3 A A2 L A1 0° - 15° C e B1 eA B eB D1 Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.300 in.) INCHES MAX MIN 0.200 – – 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.045 0.012 0.008 0.080 0.005 0.325 0.300 0.310 0.240 – 0.100 – 0.300 0.400 – 0.150 0.115 PKG.