8

LT1640L/LT1640H

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.

Electronic Circuit Breaker

The LT1640 features an electronic circuit breaker function

that protects against short circuits or excessive supply

SENSE pin, the circuit breaker will be tripped whenever the

voltage across the sense resistor is greater than 50mV for

more than 3

µs as shown in Figure 7.

GATE pin will remain low until the circuit breaker is reset

by pulling UV low, then high or cycling power to the part.

If more than 3

µs deglitching time is needed to reject

current noise, an external resistor and capacitor can be

added to the sense circuit as shown in Figure 8. R7 and C3

act as a lowpass filter that will slow down the SENSE pin

voltage from rising too fast. Since the SENSE pin will

source current, typically 20

µA, there will be a voltage drop

on R7. This voltage will be counted into the circuit breaker

trip voltage just as the voltage across the sense resistor.

A small resistor is recommended for R7. A 100

Ω for R7

will cause a 2mV error. The following equation can be used

to estimate the delay time at the SENSE pin:

tR C In

Vt

Vt

VV t

O

iO

=









–• •

–

() – ( )

–( )

1

Where V(t) is the circuit breaker trip voltage, typically

voltage across the sense resistor when the short current

or over current is applied on it.

Example: A system has a 1A current load and a 0.02

Ω

sense resistor is used. An extended delay circuit needs to

be designed for a 50

µs delay time after the load jumps to

5A. In this case:

V(t) = 50mV

If we choose R = 100

Ω, we will get C = 1µF.

LT1640L /LT1640H

PWRGD/

PWRGD

SENSE

C1

0.033

µF

24V

100

µF

100V

Q1

IRF530

R2

10

Ω

5%

R3

10k

5%

C2

3.3nF

100V

R4

562k

1%

R5

9.09k

1%

R6

10k

1%

R1

0.02

Ω

5%

4

3

2

OV = 71V

GND

– 48V

UV = 37V

OV

UV

5

R7

C3

6

8

7

1

GATE

DRAIN

1640 F08

+

Figure 8. Extending the Short-Circuit Protection Delay

Note that the circuit breaker threshold should be set

sufficiently high to account for the sum of the load current

and the inrush current. If the load current can be controlled

by the PWRGD/PWRGD pin (as in Figure 6a), the threshold

can be set lower, since it will never need to accommodate

inrush current and load current simultaneously.

When the circuit breaker trips, the GATE pin is immediately

1640 F07

Figure 7. Short-Circuit Protection Waveforms