LTC4260IGN datasheet(13/28 Pages) LINER

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LTC4260

4260f

APPLICATIO S I FOR ATIO

undervoltage autoretry has been disabled by clearing bit

A1. When power is applied to the device, if UV is below its

3.12V threshold after INTVCC crosses its 4.5V undervolt-

age lockout threshold, an undervoltage fault will be logged

in the fault register.

Board Present Change of State

Whenever the BD_PRST pin toggles, bit D4 is set to

indicate a change of state. When the BD_PRST pin goes

high, indicating board removal, the switch turns off imme-

diately (with a 1mA current from GATE to ground) and

clears the board present bit, C4. If the BD_PRST pin is

pulled low, indicating a board insertion, all fault bits except

D4 will be cleared and the board present bit, C4, is set. If

the BD_PRST pin remains low for 100ms the state of the

ON pin will be captured in the FET On Control bit A3. This

turns the switch on if the ON pin is tied high. There is an

internal 10µA pull-up current source on the BD_PRST pin.

If the system shuts down due to a fault, it may be desirable

to restart the system simply by removing and reinserting

a load card. In cases where the LTC4260 and the switch

reside on a backplane or midplane and the load resides on

a plug-in card, the BD_PRST pin can be used to detect

when the plug-in card is removed (see Figure 4). Once the

plug-in card is reinserted the fault register is cleared

(except for D4). After 100ms the state of the ON pin is

latched into bit A3 of the control register. At this point the

system will start up again.

If a connection sense on the plug-in card is driving the

BD_PRST pin, the insertion or removal of the card may

cause the pin voltage to bounce. This will result in

clearing the fault register when the card is removed. The

pin can be debounced using a filter capacitor, CBD_PRST,

on the BD_PRST pin as shown in Figure 4. The filter time

is given by:

tFILTER = CBD_PRST • 123 [ms/µF]

FET Short Fault

A FET short fault will be reported if the data converter

measures a current sense voltage greater than or equal to

2mV while the FET is turned off. This condition sets the FET

short present bit, C5, and the FET short fault bit D5.

Figure 3. Short-Circuit Waveforms

VOUT

50V/DIV

IOUT

5A/DIV

∆VGATE

10V/DIV

TIMER

2V/DIV

100µs/DIV

4260 F03

During a short circuit, if the current limit sense voltage

exceeds 150mV, the active current limit enters a high

current protection mode that immediately turns off the

output transistor by pulling the GATE-to-SOURCE voltage

to zero. Current in the output transistor drops from tens of

amps to zero in a few hundred nanoseconds. The input

voltage will drop during the high current and then spike

upwards due to parasitic inductances when the FET shuts

off (see Supply Transients). Following this event, the part

may turn on again after a delay (typically the 100ms

normal turn-on delay if the input voltage drops below the

UVLO threshold) and enters active current limit before

shutting off.

Overvoltage Fault

An overvoltage fault occurs when the OV pin rises above

its 3.5V threshold. This shuts off the switch immediately

(with a 1mA current from GATE to ground) and sets the

overvoltage present bit, C0, and the overvoltage fault bit

D0. If the OV pin subsequently falls back below the

threshold for 100ms, the switch will be allowed to turn on

again unless the overvoltage autoretry has been disabled

by clearing bit A0.

Undervoltage Fault

An undervoltage fault occurs when the UV pin falls below

its 3.12V threshold. This turns off the switch immediately

(with a 1mA current from GATE to ground) and sets the

undervoltage present bit, C1, and the undervoltage fault bit

D1. If the UV pin subsequently rises above the threshold

for 100ms, the switch will turn on again unless the

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