Rainbow-electronics MAX1543 Uživatelský manuál stáhnout pdf (Strana 12)

MAX1542/MAX1543

TFT LCD DC-to-DC Converter with

Operational Amplifiers

12 ______________________________________________________________________________________

Delay Control Circuit

A capacitor from DEL to AGND selects the switch control

block supply startup delay. After the input voltage

exceeds V

UVLO

, a 5µA current source charges C

DEL

Once the capacitor voltage exceeds the turn-on thresh-

old (1.24V) COM can be connected to SRC, depending

on the state of CTL. Before startup and when IN is less

than V

UVLO

, DEL is internally connected to AGND to dis-

charge C

DEL

. Select C

DEL

using the following equation:

MAX1542 Control Block Switch

The switch control input (CTL) is not activated until

DEL

exceeds the turn-on voltage (1.24V) and the input

voltage (V

) exceeds V

UVLO

(2.5V). Once activated,

CTL controls the P-channel MOSFET, between COM

and SRC. A high at CTL turns on Q1 between SRC and

COM, and a low at CTL turns Q1 off (Figure 4).

MAX1543 Control Block Switch

The switch control input (CTL) is not activated until the

input voltage (V

) exceeds V

UVLO

(2.5V) and V

DEL

exceeds the turn-on voltage (1.24V). During UVLO or

when DEL is below the turn-on threshold, COM is

pulled low to PGND through Q3 and a 1kΩ resistance.

Once activated, CTL controls the COM MOSFETs,

switching COM between SRC and DRN. A high at CTL

turns on Q1 and disables Q2. A low at CTL turns on Q2

and turns off Q1 (Figure 4).

Undervoltage Lockout (UVLO)

The UVLO comparator of the MAX1542/MAX1543 com-

pares the input voltage at IN with the UVLO threshold

C DELAY TIME

DEL

=×()

124

LXIN

AGND

PGND

DEL

CTL

COM

SRC

COMP

POS2

OUT2

POS1

OUT1

NEG1

SUP

MAIN

+8V AT 250mA

TO VCOM

BACKPLANE

-7V AT 20mA

G_OFF

NEG2

G_ON

+22V AT 20mA

MAX1542

100kΩ

C11

220pF

C10

33nF

2.6V TO 5.5V

10µF

4.7µF

0.1µF

4.7µF

75kΩ

4.7µF

0.1µF

40kΩ

13.7kΩ

40kΩ

Figure 1. MAX1542 Typical Application Circuit

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