LT1640L datasheet(8/16 Pages) LINER

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LT1640L/LT1640H

1640lhfb

APPLICATIONS INFORMATION

Capacitor C1 and resistor R3 prevent Q1 from momen-

tarily turning on when the power pins first make contact.

Without C1 and R3, capacitor C2 would pull the gate of Q1

up to a voltage roughly equal to VEE • C2/CGS(Q1) before

the LT1640 could power up and actively pull the gate low.

By placing capacitor C1 in parallel with the gate capaci-

tance of Q1 and isolating them from C2 using resistor R3

the problem is solved. The value of C1 should be:

INMAX

−









•+

()

where VTH is the MOSFET’s minimum gate threshold and

VINMAX is the maximum operating input voltage.

R3’s value is not critical and is given by (VINMAX + ∆VGATE)/

5mA.

The waveforms are shown in Figure 6b. When the power

pins make contact, they bounce several times. While the

contacts are bouncing, the LT1640 senses an undervoltage

condition and the GATE is immediately pulled low when

the power pins are disconnected.

Once the power pins stop bouncing, the GATE pin starts to

ramp up. When Q1 turns on, the GATE voltage is held

constant by the feedback network of R3 and C2. When the

DRAIN voltage has finished ramping, the GATE pin then

ramps to its final value.

Figure 6b. Inrush Control Waveforms

1640 F06b

INRUSH

CURRENT

1A/DIV

GATE – VEE

10V/DIV

DRAIN

50V/DIV

VEE

50V/DIV

5ms/DIV

CONTACT

BOUNCE

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