LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-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 nominal
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 LTC4252 will fully enhance the MOSFET. A second
possibility is that the load current exceeds the soft-start
current limit threshold of [V
SS
(t)/20 – V
OS
]/R
S
. In this case
the LTC4252 will ramp the output by sourcing soft-start
limited current into the load capacitance. If the soft-start
voltage is below 1.2V, the circuit breaker TIMER is held
low. Above 1.2V, TIMER ramps up. It is important to set
the timer delay so that, regardless of which start-up mode
is used, the TIMER ramp is less than one circuit breaker
delay time. If this condition is not met, the LTC4252-1 may
shut down after one circuit breaker delay time whereas
the LTC4252-2 may continue to autoretry.
Board Removal
If the board is withdrawn from the card cage, the UV and
OV divider is the first to lose connection. This shuts off
the MOSFET and commutates the flow of current in the
connector. 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
S
. There are three distinct thresholds at SENSE:
50mV for a timed circuit breaker function; 100mV for an
analog current limit loop (60mV for the LTC4252A); and
200mV for a fast, feedforward comparator which limits
peak current in the event of a catastrophic short-circuit.
If, owing to an output overload, the voltage drop across R
S
exceeds 50mV, TIMER sources 230μA into C
T
. C
T
eventually
charges to a 4V threshold and the LTC4252 shuts off. If
the overload goes away before C
T
reaches 4V and SENSE
measures less than 50mV, C
T
slowly discharges (5.8μA). In
this way the LTC4252’s circuit breaker function responds
to low duty cycle overloads and accounts for fast heating
and slow cooling characteristics of the MOSFET.
Higher overloads are handled by an analog current limit
loop. If the drop across R
S
reaches V
ACL
, the current limiting
loop servos the MOSFET gate and maintains a constant
output current of V
ACL
/R
S
. In current limit mode, V
OUT
typically rises and this increases MOSFET heating. If V
OUT
>
V
DRNCL
, connecting an external resistor, R
D
, between V
OUT
and DRAIN allows the fault timing cycle to be shortened
by accelerating the charging of the TIMER capacitor. The
TIMER pull-up current is increased by 8 • I
DRN
. Note that
because SENSE > 50mV, TIMER charges C
T
during this
time and the LTC4252 will eventually shut down.
Low impedance failures on the load side of the LTC4252
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 com-
parator with a threshold of 200mV detects overshoot and
pulls GATE low much harder and hence much faster than
the weaker current limit loop. The V
ACL
/R
S
current limit
loop then takes over and servos the current as previously
described. As before, TIMER runs and shuts down the
LTC4252 when C
T
reaches 4V.
If C
T
reaches 4V, the LTC4252-1 latches off with a 5.8μA
pull-up current source whereas the LTC4252-2 starts a
shutdown cooling cycle. The LTC4252-1 circuit breaker
latch is reset by either pulling UV momentarily low or
dropping the input voltage V
IN
below the internal UVLO
threshold or pulling TIMER momentarily low with a switch.
The LTC4252-2 retries after its shutdown cooling cycle.
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 inser-
tion 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
T
rejects these events allowing the LTC4252 to “ride out”
temporary overloads and disturbances that could trip a
simple current comparator and, in some cases, blow a fuse.
425212fd
13
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