9
LTC1422
1422fb
APPLICATIONS INFORMATION
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When the timer is off, the internal N-channel Q1 shorts the
TIMER pin to ground. When the timer is turned on, a 2µA
current from V
CC
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 = 2µA/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)/2µA. For a 200ms delay, use a
0.33µF 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 10µs. 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 40µs 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 V
CC
.
If more than 10µs 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
V
CC
connection and all other pins. When V
CC
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
V
OUT
GATE
1422 F07
TIMER
ON
V
CC
– V
SENSE
V
CC
1 2 3 4 5 6 7 8 9 10
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
40µs, the GATE pin is pulled to ground. If the low pulse on
the ON pin is less than 15µs, 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
7
V
CC
8
GATE
LTC1422
6
C1
1422 F08
Q1
R1
R
F
C
F
R2
Figure 8. Extending the Short-Circuit Protection Delay
