LTC1646
W U U
U
APPLICATIO S I FOR ATIO
Table 2. t
C
vs C
TIMER
TIMER
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.
t
C
t
TIMER
TIMER
TIMER
TIMER
0.01µF
0.022µF
0.033µF
0.047µF
0.068µF
0.082µF
0.1µF
2.5ms
5.5ms
8.25ms
11.8ms
17ms
0.22µF
0.33µF
0.47µF
0.68µF
0.82µF
1µF
55ms
82.5ms
118ms
170ms
205ms
250ms
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).
20.5ms
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.
5V –5V
IN
SENSE
50mV/DIV
Inordertopreventexcessivepowerdissipationinthepass
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.
GATE
10V/DIV
GATE
FAULT
5V/DIV
5V/DIV
5V
OUT
3V
OUT
2V/DIV
10µs/DIV
1646 F05
TIMER
1V/DIV
Figure 5. Overcurrent Fault on 5V
BD_SEL#
5V/DIV
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 5VIN (Pin 12) power supply
is cycled.
LCL_PCI_RST#
5V/DIV
HEALTHY#
5V/DIV
FAULT
5V/DIV
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 3VOUT and R2 for 5VOUT, see Figure 1). As
10ms/DIV
1646 F04
Figure 4. Power-Up into a Short on 3.3V Output
shown in Figure 1, a sense resistor is connected between
1646fa
12
Linear [ Linear ]
