LTC1643L-1_15 Datasheet, PDF (14/16 Pages) Linear Technology – PCI-Bus Hot Swap Controller

LTC1643L/LTC1643L-1/LTC1643H
APPLICATIONS INFORMATION
For the 12V supply, P-channel transistor Q4 is placed in
parallel with the internal switch. When the LTC1643H is
turned off, the GATE pin is held low and transistor Q3 is
turned on which pulls the gate of Q4 high, turning it off.
When the LTC1643H is turned on, the GATE pin goes high,
turning off Q3 and allowing R8 to pull the gate of Q4 low
to turn it on.
Because Q4 is in parallel with the internal 12V switch, the
load current will be shared in proportion to their respective
R_DS(ON) values. For example, if the R_DS(ON) of the external
switch is 0.2Ω and the internal switch is 0.4Ω, then, at
1.5A load current, the external switch will provide 1A and
the internal switch 500mA. The circuit breaker current will
be reached when the internal current reaches 1A and the
external current is 2A or 3A load current.
For the –12V supply, N-channel transistor Q6 is used to
provide the extra load current. When the LTC1643H is
turned on, the internal V_EE switch is turned on and the
–12V output starts to pull down through D1 and turns on
Q5. When Q5 turns on, the gate of Q6 starts to rise, turning
it on. When the LTC1643H turns off, the V_EEOUT pin is
pulled up to ground, diode D1 is reversed biased, and
transistor Q5 turns off, which allows resistor R10 to turn
off Q6.
The internal –12V switch provides the current limit for the
supply, but because the high R_DS(ON) of the internal switch
(1.2Ω) is in series with D1, very large currents can flow
through Q6 before the circuit breaker trips. However, if a
short to ground occurs on the –12V output, diode D1 will
prevent Q5 from turning on, which will prevent Q6 from
turning on.
SYSTEM
POWER
SUPPLY
R3
PCI
10k
POWER
SYSTEM
R4
CONTROLLER 10k
Q4
Si9400
12V
1.5A
Q3
TP0610T
10Ω
R8
C3
100k
0.1µF
R2
Q2
1 ^0.005Ω 2 ^IRF7413
3
4
1
2
3
R1
4
0.007Ω
Q1
IRF7413
R6
10Ω
R5
10Ω
19
12V_IN 3V_IN
8
GND
2 _VEEIN
5
ON
10 11 3
3V_SENSE GATE 3V_OUT
13
5V_IN
LTC1643H
6
FAULT
7 _PWRGD
12
16
5V_SENSE 12V_OUT
15
V_EEOUT
4
TIMER
R7
100Ω
C1
0.047µF
Q5
TP0610T
R9
D1
200Ω ^1N4148
C2
0.1µF
R10
200Ω
Q6
Si9410DY
5V
5A
3.3V
7.6A
GND
–12V
1.5A
1643 F08
Figure 8. Increasing 12V and – 12V Current Capability
14