Datasheet

LT4256-3
12
42563fa
APPLICATIO S I FOR ATIO
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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 V
CC
and
SENSE. The current limit threshold is calculated as:
I
LIMIT
= 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 0.025Ω 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 V
CC
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
0V 2V FB
4256 F08
55mV
V
CC
– V
SENSE
50 100 150 200
4256 F09
12
10
8
6
4
2
RESPONSE TIME (µs)
V
CC
– V
SENSE
(mV)
0