Rainbow-electronics MAX1543 Manual do Utilizador descarregar pdf (Página 13)

MAX1542/MAX1543

TFT LCD DC-to-DC Converter with

Operational Amplifiers

______________________________________________________________________________________ 13

(2.5V rising, 2.35V falling, typ) to ensure that the input

voltage is high enough for reliable operation. The

150mV (typ) hysteresis prevents supply transients from

causing a restart. Once the input voltage exceeds the

UVLO threshold, startup begins. When the input volt-

age falls below the UVLO threshold, the controller turns

off the N-channel MOSFET, the switch control block

turns off Q1, and the operational amplifier outputs float.

For the MAX1543, the switch control block also turns off

Q2 and turns on Q3 when the input voltage falls below

the UVLO threshold (Figure 4).

Oscillator Frequency (FREQ)

The MAX1542 internal oscillator is preset to 1.2MHz. The

internal oscillator frequency is pin programmable for the

MAX1543. Connect FREQ to ground or leave it uncon-

nected for 640kHz operation and connect it to V

for

1.2MHz operation. FREQ has a 5µA (typ) pulldown current.

Fault Protection

Once the soft-start routine is complete, if the output of

the main regulator is below the fault detection threshold,

the MAX1542/MAX1543 activate the fault timer. If the

fault condition continuously exists throughout the fault

timer duration, the MAX1542/MAX1543 set the fault

latch, which shuts down the device. After removing the

fault condition, cycle the input voltage (IN) below V

UVLO

to clear the fault latch and reactivate the device.

Digital Soft-Start

The MAX1542/MAX1543 digital soft-start period dura-

tion is 14ms (typ). During this time, the MAX1542/

MAX1543 directly limit the peak inductor current, allow-

ing from zero up to the full current-limit value in eight

equal current steps (I

LIM

/8). The maximum load current

is available after output voltage reaches the full regula-

tion threshold (which terminates soft-start), or after the

soft-start timer expires.

LXIN

AGND

PGND

DEL

CTL

DRN

SRC

COMP

POS2

OUT2

POS1

OUT1

NEG1

SUP

100kΩ

C10

33nF

2.6V TO 5.5V

MAIN

+8V AT 250mA

TO VCOM

BACKPLANE

0.1µF

4.7µF

0.1µF

C11

220pF

10µF

0.1µF

75kΩ

13.7kΩ

40kΩ

-7V AT 20mA

G_OFF

NEG2

G_ON

+22V AT 20mA

FREQ

COM

4.7µF

MAX1543

Figure 2. MAX1543 Typical Application Circuit

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 19 20

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