DESIGN FEATURES LTC1642: a Hot Swap Controller
with Foldback Current Limiting
and Overvoltage Protection by Pat Madden
Hot Circuit Insertion limit the charging current when a
board is plugged into a hot backplane. In this application, the
backplane voltage is 12V, but the
chip will operate with any supply
voltage between 3.0V and 16.5V. When
power is first applied to VCC, the chip
holds Q1’s gate at ground. After a
programmable debounce delay, an
internal 25ВµA current source begins
to charge the external capacitor C2,
generating a voltage ramp of 25ВµA/
C2 V/s at the GATE pin. Because Q1
acts as a source follower while its gate
ramps, the current charging the
board’s bypass capacitance, CLOAD, is
limited to 25ВµA CLOAD/C2. An internal charge pump supplies the 25ВµA
gate current, ensuring sufficient gate
drive to Q1. Resistor R3 protects
against high frequency FET oscillations; capacitor C1 sets the debounce
delay, ∆TD, before the GATE pin voltage begins ramping: ∆TD(ms) = 615 C1(ВµF). The ON pin is the chip’s control input; when it is below 1.22V, the When a circuit board is inserted into …