LT4250 LINER [Linear Technology], LT4250 Datasheet - Page 10

• Manufacturer Part Number: LT4250
• Description: Negative Voltage Hot Swap Controllers in SOT-23
• Manufacturer: LINER [Linear Technology]

LTC4251/LTC4251-1/
LTC4251-2
operaTion
Two modes of operation are possible during the time the
MOSFET is first turning on, depending on the values of
external components, MOSFET characteristics and nomi-
nal design current. One possibility is that the MOSFET
will turn on gradually so that the inrush into the load
capacitance remains a low value. The output will simply
ramp to –48V and the MOSFET will be fully enhanced.
A second possibility is that the load current exceeds the
current limit threshold of 100mV/R
LTC4251/LTC4251-1/LTC4251-2 will ramp the output by
sourcing 100mV/R
It is important to set the timer delay so that, regardless
of which start-up mode is used, the start-up time is less
than the TIMER delay time. If this condition is not met,
the LTC4251/LTC4251-1/LTC4251-2 may shutdown after
one TIMER delay.
Board Removal
If the board is withdrawn from the card cage, the UV/OV
divider is the first to lose connection. This shuts off the
MOSFET and commutates the flow of current in the con-
nector. When the power pins subsequently separate, there
is no arcing.
Current Control
Three levels of protection handle short-circuit and over-
load conditions. Load current is monitored by SENSE and
resistor R
50mV for a timed circuit breaker function; 100mV for an
analog current limit loop; and 200mV for a fast, feedfor-
ward comparator which limits peak current in the event
of a catastrophic short-circuit.
If, owing to an output overload, the voltage drop across R
exceeds 50mV, TIMER sources 230µA into C
charges to a 4V threshold and the LTC4251/LTC4251-1/
LTC4251-2 latchoff. If the overload goes away and SENSE
measures less than 50mV, C
10
S
. There are three distinct thresholds at SENSE:
S
current into the load capacitance.
T
slowly discharges (5.8µA).
S
. In this case, the
T
. C
T
eventually
S
In this way the circuit breaker function will also respond
to low duty cycle overloads, and accounts for fast heating
and slow cooling characteristic of the MOSFET.
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
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
The LTC4251/LTC4251-1/LTC4251-2 circuit breaker latch
is reset by either pulling UV/OV momentarily low, or
dropping the input voltage V
threshold of 8.2V.
Although short-circuits are the most obvious fault type,
several operating conditions may invoke overcurrent
protection. 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 detection of an
overcurrent condition. The action of TIMER and C
these events allowing the LTC4251/LTC4251-1/LTC4251-2
to “ride out” temporary overloads and disturbances that
would trip a simple current comparator and in some cases,
blow a fuse.
T
reaches 4V.
S
current limit loop then takes over, and servos
S
reaches 100mV, the current
IN
below the internal UVLO
T
during this time and the
S
. Note that because
T
rejects
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