LTC1422 datasheet(9/16 Pages) LINER

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LTC1422

APPLICATIONS INFORMATION

When the timer is off, the internal N-channel Q1 shorts the

TIMER pin to ground. When the timer is turned on, a 2

µA

current from VCC is connected to the TIMER pin and the

voltage on the external capacitor C2 starts to ramp up with

a slope given by: dV/dt = 2

µA/C2. When the voltage

reaches the trip point (1.232V), the timer will be reset by

pulling the TIMER pin back to ground. The timer period is

given by: (1.232V • C2)/2

µA. For a 200ms delay, use a

0.33

µF capacitor.

Electronic Circuit Breaker

The LTC1422 features an electronic circuit breaker func-

tion that protects against short circuits or excessive cur-

rents on the supply. By placing a sense resistor between

the supply input and SENSE pin, the circuit breaker will be

tripped whenever the voltage across the sense resistor is

greater than 50mV for more than 10

µs. When the circuit

breaker trips, the GATE pin is immediately pulled to

ground and the external N-channel is quickly turned off.

When the ON pin is cycled off for greater than 40

µs and

then on as shown in Figure 7, time point 7, the circuit

breaker is reset and another timing cycle is started.

At the end of the timer cycle (time point 8), the charge

pump will turn on again. If the circuit breaker feature is not

required, the SENSE pin should be shorted to VCC.

If more than 10

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 8.

Connection Sense with ON Pin

The ON pin can be used to sense board connection to the

backplane as shown in Figure 9.

Using staggered connection pins, ground mates first to

discharge any static build up on the board, followed by the

VCC connection and all other pins. When VCC makes

connection, the bases of transistors Q3 and Q4 are pulled

high turning them on and pulling the ON pin to ground.

When the base connector pins of Q3 and Q4 finally mate

to the backplane, the bases are shorted to ground. This

turns off Q3 and Q4 and allows the ON pin to pull high and

start a power-up cycle. The base connection pins of Q3 and

Q4 should be located at opposite ends of the connector

RESET

VOUT

GATE

1422 F07

TIMER

VCC – VSENSE

VCC

1 2

because most people will rock the board back and forth to

get it seated properly.

A software-initiated power-down cycle can be started by

momentarily turning on transistor Q2, which will pull the

ON pin to ground. If the ON pin is held low for greater than

µs, the GATE pin is pulled to ground. If the low pulse on

the ON pin is less than 15

µs, a soft reset is generated.

Hot Swapping Two Supplies

With two external pass transistors, the LTC1422 can

switch two supplies. In some cases, it is necessary to bring

up the dominant supply first during power-up and ramp it

down last during the power-down phase. The circuit in

Figure 10 shows how to program two different delays for

the pass transistors. The 5V supply is powered up first. R1

Figure 7. Current Fault Timing

SENSE

VCC

GATE

LTC1422

1422 F08

Figure 8. Extending the Short-Circuit Protection Delay

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