11
LTC1421/LTC1421-2.5
APPLICATIONS INFORMATION
WUU
U
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 32µs via a push
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 V
OUTLO
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 V
CCLO
to set the internal trip point
to 2.90V.
sense resistor is greater than 50mV for more than 20µs.
When the circuit breaker trips, both N-channel MOSFETs
are quickly turned off, FAULT and PWRGD go low and
RESET is pulled low 32µs 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 V
CCHI
and
V
CCLO
is cycled. If the circuit breaker feature is not used,
short V
CCLO
to SETLO and V
CCHI
to SETHI.
If more than 20µs 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
V
CCLO
and V
CCHI
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
5V
1/6 LS7404
OPEN
COLLECTOR
GND
32µs
200ms
FB
11 7
12
1421 F09
R1
1k
R1 USED FOR 3.3V
SUPPLY ONLY
RESET
RESET
FB
RESET
LOGIC
23
R
SENSE
C
F
Q1
22
R
F
21 20
V
CCLO
SETLO GATELO V
OUTLO
LTC1421
1421 F10
43
21
Figure 10. Short-Circuit Protection Circuit
Figure 11. AUXV
CC
Circuitry
GATE DRIVE
CIRCUITRY
10k
1µF
AUXV
CC
1421 F11
V
CCLO
24
LTC1421
23
GATELO GATEHI
21 17
Auxiliary V
CC
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 AUXV
CC
,
which is in turn powered from V
CCLO
through an internal
Schottky diode and current limiting resistor (Figure 11).
