Datasheet

LTC1646
12
1646fa
APPLICATIO S I FOR ATIO
WUUU
Figure 4. Power-Up into a Short on 3.3V Output
GATE
5V/DIV
5V
OUT
3V
OUT
2V/DIV
TIMER
1V/DIV
BD_SEL#
5V/DIV
LCL_PCI_RST#
5V/DIV
HEALTHY#
5V/DIV
FAULT
5V/DIV
10ms/DIV 1646 F04
Table 2. t
TIMER
vs C
TIMER
C
TIMER
t
TIMER
C
TIMER
t
TIMER
0.01µF 2.5ms 0.22µF 55ms
0.022µF 5.5ms 0.33µF 82.5ms
0.033µF 8.25ms 0.47µF 118ms
0.047µF 11.8ms 0.68µF 170ms
0.068µF 17ms 0.82µF 205ms
0.082µF 20.5ms 1µF 250ms
0.1µF 25ms
The TIMER pin is immediately pulled low when BD_SEL#
goes high.
Short-Circuit Protection
During a normal power-up sequence, if the TIMER pin is
done ramping and a supply is still in current limit, all of the
pass transistors will be immediately turned off and FAULT
(Pin 3) will be pulled low as shown in Figure 4.
In order to prevent excessive power dissipation in the pass
transistors and to prevent voltage spikes on the supplies
during short-circuit conditions, the current limit on each
supply is designed to be a function of the output voltage.
As the output voltage drops, the current limit decreases.
Unlike a traditional circuit breaker function where huge
currents can flow before the breaker trips, the current
foldback feature assures that the supply current will be
kept at a safe level and prevents voltage glitches at the
input supply when powering up into a short circuit.
After power-up (TIMER pin voltage >1.25V), the 5V and
3.3V supplies are protected from overcurrent and short-
circuit conditions by dual-level circuit breakers. If the
sense resistor voltage of either supply current exceeds
56mV but is less than 150mV, an internal timer is started.
If the supply is still overcurrent after 21µs, the circuit
breaker trips and both supplies are turned off (Figure 5).
5V
IN
–5V
SENSE
50mV/DIV
GATE
10V/DIV
FAULT
5V/DIV
10µs/DIV
1646 F05
Figure 5. Overcurrent Fault on 5V
If a short-circuit occurs and the sense resistor voltage of
either supply current exceeds 150mV, the circuit breakers
trip without delay and the chip latches off (Figure 6). The
chip will stay in the latched-off state until OFF/ON (Pin 15)
is cycled high then low, or the 5V
IN
(Pin 12) power supply
is cycled.
The current limit and the foldback current level for the 5V
and 3.3V outputs are both a function of the external sense
resistor (R1 for 3V
OUT
and R2 for 5V
OUT
, see Figure 1). As
shown in Figure 1, a sense resistor is connected between