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1640AL

Negative Voltage Hot Swap Controller

Linear Technology 

LT1640AL/LT1640AH

APPLICATIO S I FOR ATIO

Electronic Circuit Breaker

The LT1640A features an electronic circuit breaker func-

tion that protects against short circuits or excessive sup-

ply currents. By placing a sense resistor between the VEE

and SENSE pin, the circuit breaker will be tripped when-

ever the voltage across the sense resistor is greater than

50mV for more than 3µs as shown in Figure 7.

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

pulled to VEE and the external N-channel turns off. The

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 3µs 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 20µA, 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 100Ω for R7

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

to estimate the delay time at the SENSE pin:

t

=

–R

•

C

•

In



1–

V(t) – V(tO)

Vi – V(tO) 

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

50mV. V(tO) is the voltage drop across the sense resistor

before the short or overcurrent condition occurs. Vi is the

voltage across the sense resistor when the short current

or overcurrent is applied on it.

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

sense resistor is used. An extended delay circuit needs to

be designed for a 50µs delay time after the load jumps to

5A. In this case:

V(t) = 50mV

V(tO) = 20mV

Vi = 5A • 0.02Ω = 100mV

If we choose R = 100Ω, we will get C = 1µF.

1640A F07

Figure 7. Short-Circuit Protection Waveforms

(SHORT PIN)

GND

GND

UV = 37V

OV = 71V

R4

562k

1%

R5

9.09k

1%

R6

10k

1%

3

UV

2

OV

VEE

4

SENSE

5

C3

8

VDD

LT1640AL

GATE

6

1

PWRGD

+

DRAIN

7

CL

100µF

100V

*

R7

R1

3

0.02Ω

5%

4

– 48V

1

2

* DIODES INC. SMAT70A

C1

150nF

25V

R2 R3

10Ω 18k C2

5% 5% 3.3nF

100V

Q1

IRF530

1640A F08

Figure 8. Extending the Short-Circuit Protection Delay

9

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