10

LTC1422

1

2

3

4

8

7

6

5

SENSE

GATE

FB

RESET

ON

TIMER

GND

LTC1422

1422 F10

D1

1N4148

Q1

1/2 Si9436

Q2

1/2 Si99436

R2

0.01

Ω

5%

R3

10

Ω

5%

C1

0.33

µF

16V

R1

10k

5%

RESET

ON

R6

1M

5%

R7

10

Ω

5%

R4

2.74k

1%

TRIP POINT: 4.6V

R5

1k

1%

C3

0.047

µF

25V

C2

0.022

µF

25V

3.3V

5V

3.3V

5V

5V OUT

3.3V OUT

CURRENT LIMIT: 5A

+

+

Figure 10. Switching 5V and 3.3V

+

SENSE

R4

R3

R2

7

8

10k

GATE

LTC1422

6

2

REF

FB

1422 F09

1

5

C2

Q2: 2N7002LT1

Q3, Q4: MMBT3904LT1

Q2

ON/RESET

3

TIMER

ON

4

RESET

C1

Q1

R1

LOGIC

–

+

COMP 5

10k

10k

Q3

Q4

Figure 9. ON Pin Circuitry

and C3 are used to set the rise and fall delays on the 5V

supply. Next, the 3.3V supply ramps up with a 20ms delay

set by R6 and C2. On the falling edge, the 3.3V supply

ramps down first because R6 is bypassed by the diode D1.

Using the LTC1422 as a Linear Regulator

The LTC1422 can be used to Hot Swap the primary supply

and generate a secondary low dropout regulated supply.

Figure 11 shows how to switch a 5V supply and create a

3.3V supply using the reset comparator and one addi-

tional transistor. The FB pin is used to monitor the 3.3V

output. When the voltage on the gate of Q2 increases, the

3.3V increases. At the 3.3V threshold, the reset compara-

tor will trip. The RESET pin goes high which turns on Q3.

This lowers the voltage on the gate of Q2. This feedback

loop is compensated by the capacitor C1 and the resistors

R6 and R7.

Hot Swapping 48V DC/DC Module with

Active Low On/Off Control Signal

Using a 7.5V Zener and a resistor, the LTC1422 can switch

shown in Figure 12, the switching FET Q1 is connected as

a common source driver rather than the usual source

follower used in previous applications. This allows the

ground of the LTC1422 to sit at the negative terminal of the

48V input. The clamp circuit of R5 and D1 provides power

to the LTC1422. The resistive divider R1 and R2 at the ON

pin monitors the input supply. The switching FET Q1 is

prevented from turning on until the input supply is at least

38V. Using the reset comparator to monitor the gate

voltage allows the module to be turned on after the gate

has reached a minimum level plus one timing cycle. A high

voltage transistor Q2 is used to translate the RESET signal

to the module On/Off input.