14
LTC1643AL
LTC1643AL-1/LTC1643AH
1643afb
R2
0.005Ω
Q2
IRF7413
Q1
IRF7413
R1
0.007Ω
R5
10Ω
R6
10Ω
R8
100k
R7
100Ω
C1
0.047µF
5V
5A
12V
1.5A
3.3V
7.6A
GND
R3
10k
R4
10k
11 3 13
12
16
15
4
109
8
1
2
5
6
7
3V
IN
GND
ON
FAULT
NOTE:
INTENDED FOR PCI APPLICATIONS ONLY
PWRGD
3V
SENSE
5V
SENSE
3V
OUT
LTC1643AH
12V
OUT
V
EEOUT
TIMER
5V
IN
GATE
1643A F08
SYSTEM
POWER
SUPPLY
PCI
POWER
SYSTEM
CONTROLLER
C2
0.1µF
C3
0.1µF
R10
200Ω
R9
200Ω
Q5
TP0610T
Q6
Si9410DY
Q4
Si9400
Q3
TP0610T
D1
1N4148
12V
IN
V
EEIN
–12V
1.5A
43
21
43
21
10Ω
Figure 8. Increasing 12V and –12V Current Capability for PCI Applications
APPLICATIONS INFORMATION
WUU
U
Increasing 12V and – 12V Current Capability
for PCI Applications
The internal switches of the LTC1643A are designed for up
to 500mA loads on 12V and 200mA on –12V. If more
current is needed, then the circuits in Figure 8 can be used.
For the 12V supply, P-channel transistor Q4 is placed in
parallel with the internal switch. When the LTC1643AH 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 LTC1643AH 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 LTC1643AH 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 LTC1643AH 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.
