MAX1542 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1542

TFT LCD DC-to-DC Converter with Operational Amplifiers

Maxim Integrated 

TFT LCD DC-to-DC Converter with

Operational Amplifiers

Table 1. Component List

DESIGNATION

C1

C8, C9

D1

D2, D3, D4

L1

DESCRIPTION

10µF ±10%, 6.3V X5R ceramic capacitor

4.7µF ±10%, 10V X5R ceramic capacitors

1A, 30V Schottky diode

200mA, 100V dual ultra-fast diodes

4.7µH, 1.3A inductor

PART

TDK C3216X5R0J106K

TDK C3225X5R1A475K

Toshiba CRS02

Fairchild MMBD4148SE

Sumida CLS5D11HP-4R7

Table 2. Component Suppliers

SUPPLIER

Inductors

Sumida USA

Capacitors

TDK

Diodes

Fairchild

Toshiba

PHONE

847-956-0666

847-803-6100

888-522-5372

949-455-2000

FAX

847-956-0702

847-803-6296

408-822-2104

949-859-3963

WEBSITE

www.sumida.com

www.component.tdk.com

www.fairchildsemi.com

www.toshiba.com/taec/

The regulator controls the output voltage and the power

delivered to the outputs by modulating the duty cycle

(D) of the power MOSFET in each switching cycle. The

duty cycle of the MOSFET is approximated by:

D ≈ VMAIN − VIN

VMAIN

The device regulates the output voltage through a com-

bination of an error amplifier, two comparators, and

several signal generators (Figure 3). The error amplifier

compares the signal at FB to 1.24V and varies the

COMP output. The voltage at COMP determines the

current trip point each time the internal MOSFET turns

on. As the load varies, the error amplifier sources or

sinks current to the COMP output accordingly to pro-

duce 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.

Operational Amplifiers

The MAX1542/MAX1543 include two operational ampli-

fiers that are typically used to drive the LCD backplane

VCOM and/or the gamma correction divider string. The

operational amplifiers feature ±150mA output short-cir-

cuit current, 7.5V/µs slew rate, and 12MHz bandwidth.

The rail-to-rail inputs and outputs maximize flexibility.

Short-Circuit Current Limit

The MAX1542/MAX1543 operational amplifiers limit

short-circuit current to ±150mA if the output is directly

shorted to SUP or AGND. In such a condition, the junc-

tion temperature of the IC rises until it reaches the ther-

mal shutdown threshold, typically +160°C. Once it

reaches this threshold, the IC shuts down and remains

inactive until IN falls below VUVLO.

Driving Pure Capacitive Loads

The operational amplifiers are typically used to drive

the LCD backplane (VCOM) or the gamma correction

divider string. The LCD backplane consists of a distrib-

uted series capacitance and resistance, a load easily

driven by the operational amplifiers. However, if the

operational amplifiers are used in an application with a

pure capacitive load, steps must be taken to ensure

stable operation.

As the operational amplifier’s capacitive load increases,

the amplifier bandwidth decreases and gain peaking

increases. A small 5Ω to 50Ω resistance placed between

OUT_ and the capacitive load reduces peaking but

reduces the amplifier gain. An alternative method of

reducing peaking is the use of a snubber circuit. A 150Ω

and 10nF (typ) shunt load, or snubber, does not continu-

ously load the output or reduce amplifier gain.

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