8

LTC1642

Overvoltage Protection

The LTC1642 can protect a load from overvoltages by

turning off the pass transistor if the supply voltage ex-

ceeds a programmable limit, and by triggering a crowbar

SCR if the overvoltage lasts longer than a programmable

time. The part can also be configured to automatically

restart when the overvoltage clears.

The overvoltage protection circuitry is shown in Figure 8.

The external components comprise a resistor divider

driving the OV pin, timing capacitor C5, NPN emitter

follower Q2, and crowbar SCR Q3. Because the MCR12DC

is not a sensitive-gate device, the optional resistor shunt-

ing the SCR gate to ground is omitted. The internal

components comprise a comparator, 1.23V bandgap ref-

erence, two current sources, and a timer at the CRWBR

output is high and internal logic pulls the GATE down and

starts the timer. This timer has a 0.410V threshold and

uses the CRWBR pin; when CRWBR reaches 0.410V the

increases to 1.5mA. Emitter follower Q2 boosts this cur-

rent to trigger crowbar SCR Q3. The ramp time

∆t needed

to trip the comparator is :

∆t(ms) = 9.1•C5(µF).

Once the CRWBR timer trips the LTC1642 latches off: after

the overvoltage clears GATE and FAULT remain at ground

and CRWBR continues sourcing 1.5mA. To restart the part

after the overvoltage clears, hold the ON pin low for at least

2

µs and then bring it high. The GATE voltage will begin

ramping up one system timing cycle later. The part will

restart itself if FAULT and ON are connected: GATE begins

ramping up one system timing cycle after the overvoltage

clears.

Figure 9 shows typical waveforms when the divider is

causing the chip to pull the GATE pin to ground and start

the CRWBR timer. At time 2, before the timer’s compara-

tor trips, OV falls below its threshold; the timer resets and

GATE begins charging one system timing cycle later at

time 3. Another overvoltage begins at time 4, and at time

5 the CRWBR timer trips; FAULT goes low and the CRWBR

pin begins sourcing 1.5mA. Even after OV falls below

1.23V at time 6, GATE and FAULT stay low, and CRWBR

continues to source 1.5mA. FAULT goes high when ON

GATE

ON

14

SENSE

15

CRWBR

1

4

FAULT

6

OV

9

16

R2

0.010

Ω

Q1

FDR9410A

GND

LTC1642

8

RST TMR

3

R1

127k

1%

R5

12.4k

1%

12V

2.5A

1642 F08

R3

100

Ω

R4

330

Ω

C2

0.047

µF

+

C7

0.1

µF

C1

0.33

µF

ALL RESISTORS

±5% UNLESS NOTED

RESET TIME = 200ms

CROWBAR DELAY TIME = 90

µs

Q2

2N2222

Q3

MCR12DC

C5

0.01

µF

Figure 8. Overvoltage Protection Circuitry

1642 F09

1.23V

0.41V

0V

0V

1.23V

GATE

0V

OV

6 7

8

5

4

3

2

1

CRWBR

RST TMR

ON

FAULT

Figure 9. Overvoltage Timing (High Side)

APPLICATIO S I FOR ATIO