14

LTC1643AL

LTC1643AL-1/LTC1643AH

1643afa

R2

0.005

Ω

Q2

IRF7413

Q1

IRF7413

R1

0.007

Ω

R5

10

Ω

R6

10

Ω

R8

100k

R7

100

Ω

C1

0.047

µF

5V

5A

12V

1.5A

3.3V

7.6A

GND

R3

10k

R4

10k

11

3

13

12

16

15

4

10

9

8

1

2

5

6

7

GND

ON

FAULT

NOTE:

INTENDED FOR PCI APPLICATIONS ONLY

PWRGD

LTC1643AH

TIMER

GATE

1643A F08

SYSTEM

POWER

SUPPLY

PCI

POWER

SYSTEM

CONTROLLER

C2

0.1

µF

C3

0.1

µF

R10

200

Ω

R9

200

Ω

Q5

TP0610T

Q6

Si9410DY

Q4

Si9400

Q3

TP0610T

D1

1N4148

–12V

1.5A

4

3

2

1

4

3

2

1

10

Ω

Figure 8. Increasing 12V and – 12V Current Capability for PCI Applications

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

switch is 0.2

Ω 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

–12V output starts to pull down through D1 and turns on

Q5. When Q5 turns on, the gate of Q6 starts to rise, turning

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

(1.2

Ω) 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.