LTC4253 datasheet(33/36 Pages) LINER

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LTC4252-1/LTC4252-2

LTC4252A-1/LTC4252A-2

425212fd

APPLICATIONS INFORMATION

The internal fault latches of the LTC4252 are disabled as

the TIMER pin is always held low by the SS voltage when

Q2 and Q3 are in analog current limit.

If both power supplies from channel A and B are exactly

equal, then equal load current will flow through Q2 and

Q3 to the load module via the Hot Swap section.

If the channel A supply is greater than the channel B by

more than 100mV, the sense voltage will rise above the

fast comparator trip threshold of 200mV, the GATE will

be pulled low and Q2 is turned off. The GATE ramps up

and regulates Q2 when the channel A supply is equal to

the channel B supply. Likewise, if the channel B supply is

greater than channel A by more than 100mV, it trips the

fast comparator and GATE is pulled low and Q3 is turned

off. The GATE ramps up and regulates Q3 when the channel

B supply is equal to the channel A supply.

Resistors R4, R7 and external FETs Q4 and Q7 limit the

current flow into Q5 and Q8 during their respective sup-

ply source short. When the channel A supply is shorted

to the – 48V RTN (or GND), large current flows into Q4

momentarily and creates a voltage drop across R4, which

in turn reduces the gate-to-source voltage of Q4, limiting

the current flow. The sense voltage is lifted up and causes

the fast comparator of LTC4252 to trip and pull the GATE

low instantly. The channel A supply short will not cause

Q3 of channel B diode-OR circuit to turn off.

Similarly, when the channel B supply is shorted to the

–48V RTN (or GND), large current flows into Q7 momen-

tarily and creates a voltage drop across R7, which in turn

reduces the gate-to-source voltage of Q7, thus limiting

the current flow. The increase in sense voltage will trip

the fast comparator of LTC4252 and pull the GATE low

instantly. The channel B supply short will not cause Q2

of channel A diode-OR circuit to turn off. The load short

at the output of Q1 is protected by the Hot Swap section.

Using an EMI Filter Module

Many applications place an EMI filter module in the power

path to prevent switching noise of the module from being

injected back onto the power supply. A typical application

using the Lucent FLTR100V10 filter module is shown in

Figure 23. When using a filter, an optoisolator is required

to prevent common mode transients from destroying the

PWRGD and ON/OFF pins.

425212 F23

VEE

VIN

SENSE

TIMER

GATE

PWRGD

DRAIN

LTC4252A-1

392k

30.1k

0.68μF

CSS

68nF

10nF

–48V

0.02Ω

IRF530S

1N4003

VIN

0.1μF

100V

VIN

–

VOUT

–

10Ω

5.1k

RIN

× 1.8k

1/4W

10nF

CIN

1μF

–48RTN

(SHORT PIN)

–48RTN

(LONG PIN)

*MOC207

0.1μF

100V

100μF

100V

100μF

16V

0.1μF

100V

LUCENT

FLTR100V10

CASE

VIN

ON/OFF

VIN

–

VOUT

SENSE+

SENSE–

TRIM

VOUT

–

LUCENT

JW050A1-E

CASE

DIN

†

DDZ13B**

**DIODES, INC

†

RECOMMENDED FOR HARSH ENVIRONMENTS

Figure 23. Typical Application Using a Filter Module

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