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LT1640H

Negative Voltage Hot Swap Controller

Linear Technology 

LT1640L/LT1640H

APPLICATIONS INFORMATION

Electronic Circuit Breaker

The LT1640 features an electronic circuit breaker function

that protects against short circuits or excessive supply

currents. By placing a sense resistor between the VEE and

SENSE pin, the circuit breaker will be tripped whenever 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 over current condition occurs. Vi is the

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 0.02Ω

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 R7 = 100Ω, then C3 = 1µF.

INRUSH

CURRENT

2A/DIV

GATE – VEE

4V/DIV

VEE

50V/DIV

4ms/DIV

1640 F07

Figure 7. Start-Up Into a Short Circuit

(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

LT1640L

GATE

6

1

PWRGD

+

DRAIN

7

CL

100µF

100V

1N4148

*

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

1640 F08

Figure 8. Extending the Short-Circuit Protection Delay

1640lhfb

9

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