11

LTC1422

plus a Zener voltage (D1) is more positive than the drain

of Q1 (in other words, when the switching FET Q1 has only

7.5V across its drain source).

Hot Swapping 48V Module with Isolated Controller

A power supervisory controller will sometimes reside on

an isolated supply with responsibility for other supplies.

Figure 15 shows how to Hot Swap a controller’s 5V supply

and a 48V module using two LTC1422s. Assuming the 5V

supply comes up first, the controller waits for a power

good signal from the 48V circuit. Once it receives the right

signals the controller activates the GATE IN pin of the Vicor

power module.

Power Supply Sequencer

A circuit that forces two supply voltages to power up

together is shown in Figure 16. The input supply voltages

may power up in any sequence, but both input voltages

must be within tolerance before Q1 and Q2 turn on. Back-

to-back transistors Q1 and Q2 ensure isolation between

the two supplies.

When the 5V input powers up before 3.3V, Q1 and Q2

remain off and the 5V output remains off until the 3.3V

input is within tolerance as sensed by resistors R1 and R2.

When the 3.3V input powers up before 5V, the diode D1

will pull up the 5V supply output with it. Once the 5V input

powers up and is within tolerance as sensed by R4 and R5,

Q1 and Q2 will turn on in about 1ms and pull the 5V output

up to its final voltage.

Since the pass transistor is in a common source configu-

ration, care must be taken to limit the inrush current into

capacitor C3. One way is to precharge C3 using resistor

R4. As the input supply is ramping up, current is flowing

through R4 and charging the capacitor C3. Once the input

supply crosses 38V, there is a timing cycle followed by the

ramp-up of the GATE pin. By this time the capacitor C3 is

sufficiently charged, thereby limiting the inrush current.

Another method to limit the inrush current is to slow down

the ramp-up rate of the GATE pin.

Hot Swapping 48V DC/DC Module with

Active High On/Off Control Signal

This application is identical to the previous except for the

polarity of the module’s on/off signal. The polarity reversal

is accomplished by transistor Q3 in Figure 13.

Hot Swapping Redundant 48V

In critical situations, redundant input supplies are neces-

sary. In Figure 14 a redundant 48V input is switched to a

power module. Supplies 1 and 2 are wire OR’ed using two

diodes D2 and D3. This results in the most negative of

these two supplies being used to drive the power module.

If one of the supplies is disconnected or a fuse opens, the

fault signal will be activated via diodes D4 and D5 and the

reset comparator at the FB pin. The GATE IN signal on the

Vicor module is controlled using the high voltage PNP Q2.

Once the module’s minus input pin is more negative than

the base of Q2 plus a diode drop, Q2 will turn off and the

module will turn on. This occurs when the source of Q1

1

2

3

4

8

7

6

5

SENSE

GATE

FB

RESET

ON

TIMER

GND

LTC1422

1422 F11

Q1

MMFT2N02ELT1

CURRENT LIMIT: 2.5A

Q2

MMFT2N02ELT1

R1

0.02

Ω

5%

R3

10

Ω

5%

R6

1.6M

5%

R7

360k

5%

R2

10

Ω

5%

R4

2.74k

1%

R5

1.62k

1%

Q3

PN2222

C1

0.0033

µF

16V

C2

0.1

µF

16V

C3

0.1

µF

16V

3.3V

5V

5V

+

+

Figure 11. Switching 5V and Generating 3.3V