LT4250L/LT4250H
9
4250lhfa
DRAIN voltage has finished ramping, the GATE pin then
ramps to its final value.
Current Limit/Electronic Circuit Breaker
The LT4250 features a current limit function that protects
against short circuits or excessive supply currents. If the
current limit is active for more than 500μs the electronic
circuit breaker will trip. By placing a sense resistor between
the V
EE
and SENSE pin, the current limit will be activated
whenever the voltage across the sense resistor is greater
than 50mV.
Note that the current limit threshold should be set suffi-
ciently high to account for the sum of the load current
and the inrush current. The maximum value of the inrush
current is given by:
I
INRUSH
0.8 •
40mV
R
SENSE
–I
LOAD,
where the 0.8 factor is used as a worst case margin com-
bined with the minimum trip voltage (40mV).
In the case of a short circuit, the current limit circuitry
activates and immediately pulls the GATE low, servos the
SENSE voltage to 50mV, and starts a 500μs timer. The
MOSFET current is limited to 50mV/R
SENSE
(see Figure 7).
If the short circuit persists for more than 500μs, the circuit
breaker trips and pulls the GATE pin low, shutting off the
MOSFET. The circuit breaker is reset by pulling UV low,
or by cycling power to the part. If the short circuit clears
before the 500μs timing interval the current limit will
deactivate and release the GATE.
APPLICATIONS INFORMATION
Figure 7. Short-Circuit Protection Waveforms
DRAIN
50V/DIV
GATE
10V/DIV
I
D
(Q1)
5A/DIV
1ms/DIV
The LT4250 guards against voltage steps on the input
supply. A positive voltage step (increasing in magnitude)
on the input supply causes an inrush current that is
proportional to the voltage slew rate I = C
L
• ΔV/ΔT. If the
inrush exceeds 50mV/R
SENSE
, the current limit will activate
as shown in Figure 8. The GATE pin pulls low, limiting the
current to 50mV/R
SENSE
. At this level the MOSFET drain
will not follow the source as the input voltage rapidly
changes, but instead remains at the voltage stored on the
load capacitance. The load capacitance begins to charge
at a current of 50mV/R
SENSE
, but not for long. As the load
capacitance charges, C2 pushes back on the gate and limits
the MOSFET current in a manner identical to the initial start-
up condition which is less than the short circuit limiting
value of 50mV/R
SENSE
. Thus the circuit breaker does not
trip. To ensure correct operation under input voltage step
conditions, R
SENSE
must be chosen to provide a current
limit value greater than the sum of the load current and
the dynamic current in the load capacitance.
For C2 values less than 10nF a positive voltage step
increasing in magnitude on the input supply can result
in the Q1 turning off momentarily due to current limit
overshoot which can shut down the output. By adding an
additional resistor and diode, Q1 remains on during the
voltage step. This is shown as D1 and R7 in Figure 9. The
purpose of D1 is to shunt current around R7 when the
power pins first make contact and allow C1 to hold the
GATE low. The value of R7 should be sized to generate an
R7 • C1 time constant of 33μs.
Under some conditions, a short circuit at the output can
cause the input supply to dip below the UV threshold. The
LT4250 turns off once and then turns on until the electronic
circuit breaker is tripped. This can be minimized by adding
a deglitching delay to the UV pin with a capacitor from UV
to V
EE
. This capacitor forms an RC time constant with the
resistors at UV, allowing the input supply to recover before
the UV pin resets the circuit breaker.
