9

LTC1642

The OV and FB Comparators

The propagation delay through the OV and FB compara-

tors on low to high transitions depends strongly on the

differential input voltage. The relationship is shown in

Figure 11. The minimum propagation delay for large

overdrives is about 20

µs. In addition the comparators

have 3mV of hysteresis.

Internal Voltage Clamp Protection

The LTC1642 includes a shunt regulator to protect itself

chip’s circuitry to 15V. When it is on the chip functions

normally with one exception: if the charge pump is on, the

GATE voltage is usually near ground but this is not

guaranteed. Use the OV pin to ensure that GATE is grounded.

The pull-up voltage on the RESET and FAULT pins follows

is on the pull-up voltage is 14.4V.

goes low at time 7, and GATE begins charging at time 8,

one RST TMR cycle after FAULT goes high.

Figure 10 shows typical waveforms when the OV divider is

driven from the N-channel’s low side. Because the voltage

driving the divider collapses after the OV comparator trips,

FAULT stays high and CRWBR stays near ground, which

prevents the pin from triggering an SCR. The GATE voltage

begins ramping up after a RST TMR timing cycle.

Automatic Restart

If there is an overvoltage, and the resistor divider feeding

OV is connected to the output of the N-channel pass

transistor, the LTC1642 will automatically restart even if

FAULT is not tied to ON. If the divider is connected to the

input side, the LTC1642 will restart itself only if FAULT is

tied to ON, and only after the overvoltage clears.

APPLICATIO S I FOR ATIO

0V

0V

0.41V

1.23V

1.23V

1642 F10

6 7

5

4

3

2

1

GATE

OV

RST TMR

FAULT

0V

CRWBR

Figure 10. Overvoltage Timing (Low Side)

OV OVERDRIVE (mV)

0

40

80

120

160

200

240

1642 F11

70

60

50

40

30

20

10

0

Figure 11. OV Comparator Propagation Delay vs

Overdrive Voltage