LTC4252-1 Datasheet, PDF (13/36 Pages) Linear Technology – Negative Voltage Hot Swap Controllers

U
OPERATIO
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 capaci-
tance 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If the board is withdrawn from the card cage, the UV and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 eventu-
ally 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 re-
sponds to low duty cycle overloads and accounts for fast
heating and slow cooling characteristics of the MOSFET.
LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-2
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 resis-
tor, 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 even-
tually 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 drop-
ping the input voltage V_IN below the internal UVLO thresh-
old 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
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_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.
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