LT4256-3IGN datasheet(12/20 Pages) LINER

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LT4256-3

42563f

APPLICATIO S I FOR ATIO

Short-Circuit Protection

The LT4256-3 features a programmable foldback current

limit with an electronic circuit breaker that protects against

short circuits or excessive load currents. The current limit

is set by placing a sense resistor (R5) between VCC and

SENSE. The current limit threshold is calculated as:

ILIMIT = 55mV/R5

(5)

To limit excessive power dissipation in the pass transistor

and to reduce 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

internally on FB.

If the LT4256-3 goes into current limit when the voltage on

FB is 0V, the current limit circuit drives GATE to force a

constant 14mV drop across the sense resistor. As the

output at FB increases, the voltage across the sense

resistor increases until FB reaches 2V, at which point the

voltage across the sense resistor is held constant at 55mV

(see Figure 8).

For a 0.025

Ω sense resistor, the typical current limit is set

at 2200mA and folds back to 560mA when the output is

shorted to ground. Thus, MOSFET peak power dissipation

under short-circuit conditions is reduced from 106W to

27W. See the Layout Considerations section for important

information about board layout to minimize current limit

threshold error.

The LT4256-3 also features a variable overcurrent re-

sponse time. The time required for the part to regulate the

GATE voltage is a function of the voltage across the sense

resistor connected between VCC and SENSE. This helps to

eliminate sensitivity to current spikes and transients that

might otherwise unnecessarily trigger a current limit re-

sponse and increase MOSFET dissipation. Figure 9 shows

the response time as a function of the overdrive at SENSE.

Figure 8. Current Limit Sense Voltage vs Feedback Pin Voltage

Figure 9. Response Time to Overcurrent

14mV

4256 F08

55mV

VCC – VSENSE

100

150

200

4256 F09

VCC – VSENSE (mV)

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