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LTC4251 Datasheet(PDF) 10 Page - Linear Technology

Part # LTC4251
Description  Negative Voltage Hot Swap Controllers in SOT-23
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC4251 Datasheet(HTML) 10 Page - Linear Technology

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LTC4251/LTC4251-1/
LTC4251-2
10
425112fa
APPLICATIO S I FOR ATIO
OPERATIO
Higher overloads are handled by an analog current limit
loop. If the drop across RS reaches 100mV, the current
limiting loop servos the MOSFET gate and maintains a
constant output current of 100mV/RS. Note that because
SENSE > 50mV, TIMER charges CT during this time and
the LTC4251/LTC4251-1/LTC4251-2 will eventually shut
down.
Low impedance failures on the load side of the LTC4251/
LTC4251-1/LTC4251-2 coupled with 48V or more driving
potential can produce current slew rates well in excess of
50A/
µs. Under these conditions, overshoot is inevitable. A
fast SENSE comparator with a threshold of 200mV detects
overshoot and pulls GATE low much harder and hence
much faster than can the weaker current limit loop. The
100mV/RS current limit loop then takes over, and servos
the current as previously described. As before, TIMER
runs and latches the LTC4251/LTC4251-1/LTC4251-2 off
when CT reaches 4V.
The LTC4251/LTC4251-1/LTC4251-2 circuit breaker latch
is reset by either pulling UV/OV momentarily low, or drop-
ping the input voltage VINbelowtheinternalUVLOthreshold
of 8.2V.
Although short-circuits are the most obvious fault type,
several operating conditions may invoke overcurrent pro-
tection. Noise spikes from the backplane or load, input
steps caused by the connection of a second, higher
voltage supply, transient currents caused by faults on
adjacent circuit boards sharing the same power bus, or the
insertion of non-hot swappable products could cause
higher than anticipated input current and temporary de-
tection of an overcurrent condition. The action of TIMER
and CT rejects these events allowing the LTC4251/
LTC4251-1/LTC4251-2 to “ride out” temporary overloads
and disturbances that would trip a simple current com-
parator and in some cases, blow a fuse.
SHUNT REGULATOR
A fast responding regulator shunts the LTC4251/
LTC4251-1/LTC4251-2 VIN pin. Power is derived from
–48RTN by an external current limiting resistor. The shunt
regulator clamps VIN to 13V (VZ). A 1µF decoupling
capacitor at VIN filters supply transients and contributes a
short delay at start-up. A 10k 1/2W (RIN) resistor can be
two 5k 1/4W resistors in series.
INTERNAL UNDERVOLTAGE LOCKOUT (UVLO)
Internal circuitry monitors VIN for undervoltage. The exact
thresholds are defined by VLKO and its hysteresis, VLKH.
When VIN rises above 9.2V (VLKO) the chip is enabled;
below 8.2V (VLKO-VLKH) it is disabled and GATE is pulled
low. The UVLO function at VIN should not be confused with
the UV/OV pin. These are completely separate functions.
UV/OV COMPARATORS
Two hysteretic comparators for detecting under- and
overvoltage conditions, with the following thresholds,
monitor the dual function UV/OV pin:
UV turning on at VUVHI
UV turning off at VUVLO
OV turning off at VOVHI
OV turning on at VOVLO
The UV and OV trip point ratio for LTC4251 is designed to
match the standard telecom operating range of 43V to
75V. The LTC4251-2 implements a UV threshold of 43V
only.
A divider (R1, R2) is used to scale the supply voltage.
Using R1 = 402k and R2 = 32.4k gives a typical operating
range of 43.2V to 74.4V. The under- and overvoltage
shutdown thresholds are then 39.2V and 82.5V. 1%
divider resistors are recommended to preserve threshold
accuracy. The same resistor values can be used for the
LTC4251-2.
(Refer to Block Diagram)


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