LTC1421 datasheet(11/24 Pages) LINER

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LTC1421/LTC1421-2.5

APPLICATIONS INFORMATION

is connected between REF (Pin 8) and the negative supply

and the trip point of Comparator 2 set to GND.

Soft Reset Generation

A soft reset that doesn’t cycle the supply voltage can be

generated externally using Pin 11 (FB) as shown in Figure

9. For a 5V supply the FB pin is left floating to set the

internal supply monitor trip voltage to 4.65V. However, if

the FB pin is pulled to ground for more than 64

µsviaapush

button or open-collector logic gate, the internal trip point

will go to 5.88V and the RESET pin will pull low. RESET will

remain low for 200ms after the FB pin is released. The

RESET signal will also be pulled low when the voltage at

the VOUTLO pin dips below 4.65V for more than 32µs.

When using a 3.3V supply, a 1k resistor must be con-

nected from the FB pin to VCCLO to set the internal trip point

to 2.90V.

sense resistor is greater than 50mV for more than 20

µs.

When the circuit breaker trips, both N-channel MOSFETs

are quickly turned off, FAULT and PWRGD go low and

RESET is pulled low 32

µs later. FAULT can be connected

to a LED or a logic signal back to the host to indicate a faulty

board. The chip will remain in the tripped state until a

power-on reset is generated, or the power on VCCHI and

VCCLO is cycled. If the circuit breaker feature is not used,

short VCCLO to SETLO and VCCHI to SETHI.

If more than 20

µs of response time is needed to reject

supply noise, an external resistor and capacitor can be

added to the sense circuit as shown in Figure 10.

Figure 9. Generating a Soft Reset

Undervoltage Lockout

On power-up, an undervoltage lockout circuit prevents the

GATELO and GATEHI charge pumps from turning on until

VCCLO and VCCHI have both exceeded 2.45V.

Electronic Circuit Breaker

The LTC1421 features an electronic circuit breaker func-

tion that protects against short circuits or excessive cur-

rents on the supplies. By placing a sense resistor between

the supply input and set pin of either supply, the circuit

breaker will be tripped whenever the voltage across the

LTC1421

3.3V

1/6 LS7404

OPEN

COLLECTOR

GND

µs

200ms

1421 F09

R1 USED FOR 3.3V

SUPPLY ONLY

RESET

LOGIC

RSENSE

VCCLO

SETLO

GATELO

VOUTLO

LTC1421

1421 F10

Figure 10. Short-Circuit Protection Circuit

Figure 11. AUXVCC Circuitry

GATE DRIVE

CIRCUITRY

10k

µF

AUXVCC

1421 F11

VCCLO

LTC1421

GATELO GATEHI

Auxiliary VCC

When a short circuit occurs on the board, it is possible to

draw enough current to cause the backplane supply

voltage to collapse. If the input supply voltage collapses to

a low enough voltage and the LTC1421 gate drive circuitry

is unable to shut off the N-channel pass transistors, the

system might freeze up in a permanent short condition.

To prevent this from occurring, the gate discharge cir-

cuitry inside the LTC1421 is powered from AUXVCC,

which is in turn powered from VCCLO through an internal

Schottky diode and current limiting resistor (Figure 11).

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