LT4250H LINER [Linear Technology], LT4250H Datasheet - Page 8

• Manufacturer Part Number: LT4250H
• Description: Negative 48V Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LT4250L/LT4250H
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 500 s the electronic
circuit breaker will trip. By placing a sense resistor
between the V
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:
where the 0.8 factor is used as a worst case margin
combined 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 500 s timer. The
MOSFET current is limited to 50mV/R
If the short circuit persists for more than 500 s, 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 500 s timing interval the current limit will
deactivate and release the GATE.
8
I
INRUSH
50V/DIV
10V/DIV
I
5A/DIV
DRAIN
D
GATE
(Q1)
Figure 7. Short-Circuit Protection Waveforms
0 8
EE
. •
and SENSE pin, the current limit will be
U
R
40
SENSE
mV
INFORMATION
U
1ms/DIV
–
I
LOAD
W
,
SENSE
(see Figure 7).
U
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
inrush exceeds 50mV/R
vate as shown in Figure 8. The GATE pin pulls low, limiting
the current to 50mV/R
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
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
does not trip. To ensure correct operation under input
voltage step conditions, R
vide 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 on the
input supply can result in the Q1 turning off momentarily
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 33 s.
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 elec-
tronic circuit breaker is tripped. This can be minimized by
adding a deglitching delay to the UV pin with a capacitor
from UV to V
with the resistors at UV, allowing the input supply to
recover before the UV pin resets the circuit breaker.
EE
. This capacitor forms an RC time constant
SENSE
SENSE
SENSE
SENSE
SENSE
, but not for long. As the load
. At this level the MOSFET
, the current limit will acti-
. Thus the circuit breaker
must be chosen to pro-
L
• V/ T. If the
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