ltc1646cgn-trpbf Linear Technology Corporation, ltc1646cgn-trpbf Datasheet - Page 12

• Manufacturer Part Number: ltc1646cgn-trpbf
• Description: Compactpci Dual Hot Swap Controller
• Manufacturer: Linear Technology Corporation

LTC1646
APPLICATIO S I FOR ATIO
Table 2. t
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.
12
The TIMER pin is immediately pulled low when BD_SEL#
LCL_PCI_RST#
0.022µF
0.033µF
0.047µF
0.068µF
0.082µF
C
0.01µF
HEALTHY#
0.1µF
TIMER
BD_SEL#
5V/DIV
2V/DIV
1V/DIV
5V/DIV
5V/DIV
5V/DIV
5V/DIV
TIMER
5V
3V
FAULT
Figure 4. Power-Up into a Short on 3.3V Output
GATE
TIMER
OUT
OUT
vs C
TIMER
8.25ms
11.8ms
20.5ms
t
2.5ms
5.5ms
17ms
25ms
TIMER
U
U
10ms/DIV
C
0.22µF
0.33µF
0.47µF
0.68µF
0.82µF
TIMER
1µF
W
U
82.5ms
118ms
170ms
205ms
250ms
t
55ms
TIMER
1646 F04
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).
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
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
shown in Figure 1, a sense resistor is connected between
5V
IN
50mV/DIV
– 5V
10V/DIV
5V/DIV
FAULT
SENSE
GATE
Figure 5. Overcurrent Fault on 5V
OUT
and R2 for 5V
10µs/DIV
IN
(Pin 12) power supply
OUT
, see Figure 1). As
1646 F05
1646fa