LT4256-3 Linear Technology Corporation, LT4256-3 Datasheet - Page 12

• Manufacturer Part Number: LT4256-3
• Description: Positive High Voltage Hot Swap Controller With Open-circuit Detect
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT4256-3
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
SENSE. The current limit threshold is calculated as:
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
12
Figure 8. Current Limit Sense Voltage vs Feedback Pin Voltage
I
LIMIT
V
= 55mV/R5
CC
– V
SENSE
55mV
14mV
0V
U
U
2V
W
4256
FB
U
F08
CC
and
(5)
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
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.
12
10
8
6
4
2
0
Figure 9. Response Time to Overcurrent
50
V
100
CC
– V
CC
SENSE
and SENSE. This helps to
(mV)
150
200
4256 F09
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