LTC4251-1CS6#PBF Linear Technology, LTC4251-1CS6#PBF Datasheet - Page 10

• Manufacturer Part Number: LTC4251-1CS6#PBF
• Manufacturer: Linear Technology

Specifications of LTC4251-1CS6#PBF

• Family Name: LTC4251-1
• Package Type: TSOT-23
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Product Height (mm): 0.9mm
• Product Length (mm): 2.9mm
• Mounting: Surface Mount
• Pin Count: 6
• Lead Free Status / RoHS Status: Compliant

LTC4251/LTC4251-1/
LTC4251-2
OPERATIO
Higher overloads are handled by an analog current limit
loop. If the drop across R
limiting loop servos the MOSFET gate and maintains a
constant output current of 100mV/R
SENSE > 50mV, TIMER charges C
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/R
the current as previously described. As before, TIMER
runs and latches the LTC4251/LTC4251-1/LTC4251-2 off
when C
APPLICATIO S I FOR ATIO
10
SHUNT REGULATOR
A fast responding regulator shunts the LTC4251/
LTC4251-1/LTC4251-2 V
–48RTN by an external current limiting resistor. The shunt
regulator clamps V
capacitor at V
short delay at start-up. A 10k 1/2W (R
two 5k 1/4W resistors in series.
INTERNAL UNDERVOLTAGE LOCKOUT (UVLO)
Internal circuitry monitors V
thresholds are defined by V
When V
below 8.2V (V
low. The UVLO function at V
the UV/OV pin. These are completely separate functions.
T
IN
reaches 4V.
S
rises above 9.2V (V
current limit loop then takes over, and servos
IN
LKO
filters supply transients and contributes a
U
-V
U
LKH
IN
) it is disabled and GATE is pulled
to 13V (V
U
IN
S
IN
IN
reaches 100mV, the current
LKO
pin. Power is derived from
should not be confused with
for undervoltage. The exact
LKO
and its hysteresis, V
Z
T
) the chip is enabled;
W
). A 1µF decoupling
during this time and
S
. Note that because
IN
) resistor can be
U
(Refer to Block Diagram)
LKH
.
The LTC4251/LTC4251-1/LTC4251-2 circuit breaker latch
is reset by either pulling UV/OV momentarily low, or drop-
ping the input voltage V
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 C
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.
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 V
UV turning off at V
OV turning off at V
OV turning on at V
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.
T
rejects these events allowing the LTC4251/
UVHI
OVLO
UVLO
OVHI
IN
below the internal UVLO threshold
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