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LT4250HCS8 Datasheet Download

Part No.:
LT4250HCS8
Download:
Download Datasheet
Description:
[Negative 48V Hot Swap Controller]
File Size:
231 K
Page:
12 Pages
Logo:
Manufacturer:
LINER [ LINEAR TECHNOLOGY ]
PCB Prototype
 LT4250HCS8 Datasheet Page:4LT4250HCS8 Datasheet Page:5LT4250HCS8 Datasheet Page:6LT4250HCS8 Datasheet Page:7LT4250HCS8 Datasheet Page:9LT4250HCS8 Datasheet Page:10LT4250HCS8 Datasheet Page:11LT4250HCS8 Datasheet Page:12 
LT4250L/LT4250H
APPLICATIONS INFORMATION
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:
40mV
I
INRUSH
0.8 •
– I
LOAD,
R
SENSE
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 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.
DRAIN
50V/DIV
GATE
10V/DIV
I
D
(Q1)
5A/DIV
1ms/DIV
Figure 7. Short-Circuit Protection Waveforms
8
U
W
U
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
L
∆V/∆T.
If the
inrush exceeds 50mV/R
SENSE
, the current limit will acti-
vate 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 pro-
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 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 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
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.
4250lhf