7

LTC1422

Hot Circuit Insertion

When circuit boards are inserted into a live backplane, the

supply bypass capacitors on the board can draw huge

transient currents from the backplane power bus as they

charge up. The transient currents can cause permanent

damage to the connector pins and cause glitches on the

system supply, causing other boards in the system to

reset.

The LTC1422 is designed to turn a board’s supply voltage

on and off in a controlled manner, allowing the board to be

safely inserted or removed from a live backplane. The chip

also provides a system reset signal to indicate when board

supply voltage drops below a programmable voltage.

Power Supply Ramping

The onboard power supply is controlled by placing an

external N-channel pass transistor in the power path

(Figure 1). R1 provides current fault detection and R2

prevents high frequency oscillation. By ramping up the

gate of the pass transistor at a controlled rate, the transient

surge current (I = C • dV/dt) drawn from the main backplane

supply can be limited to a safe value when the board makes

connection.

1422 F02

GATE

SLOPE = 10

µA/C1

Figure 2. Supply Turn-On

Figure 3. Supply Monitor Block Diagram

equal to 10

µA/C1 (Figure 2), where C1 is the external

capacitor connected between the GATE pin and GND.

10

µA. After the ON pin has been pulled low for more than

40

µs, the GATE is immediately pulled to GND.

Voltage Monitor

The LTC1422 uses a 1.232V bandgap reference, precision

voltage comparator and a resistive divider to monitor the

output supply voltage (Figure 3).

Figure 1. Supply Control Circuitry

+

SENSE

TIMER

GND

GATE

LTC1422

ON

8

34

76

C1

C2

R3

Q1

R1

5

2

1422F01

1

FB

RESET

R2

10

Ω

R4

When power is first applied to the chip, the gate of the

N-channel (Pin 6) is pulled low. After the ON pin is held

high for at least one timing cycle, the charge pump is

turned on. The voltage at GATE begins to rise with a slope

+

SENSE

12

µA

R4

R3

7

8

GATE

LTC1422

6

2

FB

Q2

ON

1422 F03

1

5

C2

34

C1

Q1

R1

LOGIC

TIMER

1.232V

REFERENCE

–

+

COMP 2

µP

RESET

R2