7

LT1641

APPLICATIO S I FOR ATIO

Short-Circuit Protection

The LT1641 features a programmable foldback current

limit with an electronic circuit breaker that protects against

short-circuits or excessive supply currents. The current

and SENSE (Pin 7).

To prevent excessive power dissipation in the pass tran-

sistor and to prevent voltage spikes on the input supply

during short-circuit conditions at the output, the current

folds back as a function of the output voltage, which is

sensed at the FB pin (Figure 7).

When the voltage at the FB pin is 0V, the current limit

circuit drives the GATE pin to force a constant 12mV drop

across the sense resistor. As the output voltage at the FB

pin increases, the voltage across the sense resistor in-

creases until the FB pin reaches 0.5V, at which point the

voltage across the sense resistor is held constant at 47mV.

Figure 8. Response Time to Overcurrent

12mV

0V

0.5V

1641 F07

47mV

Figure 7. Current Limit Sense Voltage vs Feedback Pin Voltage

50mV

100mV

150mV

200mV

1641 F08

12

µs

10

µs

8

µs

6

µs

4

µs

2

µs

RESPONSE TIME

The maximum current limit is calculated as:

For a 0.025

Ω sense resistor, the current limit is set at

1.88A and folds back to 480mA when the output is shorted

to ground.

The LT1641 also features a variable overcurrent response

time. The time required for the chip to regulate the GATE

pin (Pin 6) voltage is a function of the voltage across the

the SENSE pin (Pin 7). The larger the voltage, the faster the

gate will be regulated. Figure 8 shows the response time

as a function of overdrive at the SENSE pin.