LT4250 LINER [Linear Technology], LT4250 Datasheet - Page 13

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

SENSE
The SENSE pin is monitored by the circuit breaker (CB)
comparator, the analog current limit (ACL) amplifier, and
the fast current limit (FCL) comparator. Each of these
three measures the potential of SENSE relative to V
SENSE exceeds 50mV, the CB comparator activates the
230µA TIMER pull-up. At 100mV, the ACL amplifier servos
the MOSFET current, and at 200mV the FCL compara-
tor abruptly pulls GATE low in an attempt to bring the
MOSFET current under control. If any of these conditions
persists long enough for TIMER to charge C
Equation (2)), the LTC4251/LTC4251-1/LTC4251-2 latch
off and pull GATE low.
If the SENSE pin encounters a voltage greater than 100mV,
the ACL amplifier will servo GATE downwards in an attempt
to control the MOSFET current. Since GATE overdrives the
MOSFET in normal operation, the ACL amplifier needs time
to discharge GATE to the threshold of the MOSFET. For a
mild overload, the ACL amplifier can control the MOSFET
current, but in the event of a severe overload the current
may overshoot. At SENSE = 200mV, the FCL comparator
takes over, quickly discharging the GATE pin to near V
potential. FCL then releases, and the ACL amplifier takes
over. All the while TIMER is running. The effect of FCL is
to add a nonlinear response to the control loop in favor
of reducing MOSFET current.
Owing to inductive effects in the system, FCL typically
overcorrects the current limit loop, and GATE undershoots.
A zero in the loop (resistor R
capacitor) helps the ACL amplifier recover.
SHORT-CIRCUIT OPERATION
Circuit behavior arising from a load-side low impedance
short is shown in Figure 4. Initially, the current overshoots
the analog current limit level of V
as the GATE pin works to bring V
The overshoot glitches the backplane in the negative
direction, and when the current is reduced to 100mV/R
the backplane responds by glitching in the positive
direction.
applicaTions inForMaTion
C
GS
SENSE
in series with the gate
under control (Trace 3).
= 100mV (Trace 2)
T
to 4V (see
EE
. If
EE
S
TIMER commences charging C
analog current limit loop maintains the fault current at
100mV/R
the backplane voltage (Trace 1) sags under load. When C
reaches 4V, GATE turns off, the load current drops to zero
and the backplane rings up to over 100V. The positive peak
is usually limited by avalanche breakdown in the MOSFET,
and can be further limited by adding a transient voltage
suppressor across the input from – 48V to –48RTN, such
as Diodes Inc. SMAT70A.
A low impedance short on one card may influence the
behavior of others sharing the same backplane. The initial
glitch and backplane sag as seen in Figure 4, Trace 1, can
rob charge from output capacitors on adjacent cards. When
the faulty card shuts down, current flows in to refresh the
capacitors. If LTC4251, LTC4251-1 or LTC4251-2s are used
throughout, they respond by limiting the inrush current to
a value of 100mV/R
will recharge long before C
MOSFET SELECTION
The external MOSFET switch must have adequate safe
operating area (SOA) to charge the load capacitance on
start-up and handle short-circuit conditions until TIMER
latchoff. These considerations take precedence over DC
current ratings. A MOSFET with adequate SOA for a given
application can always handle the required current, but
the opposite cannot be said. Consult the manufacturer’s
MOSFET data sheet for safe operating area and effective
transient thermal impedance curves.
200mV/DIV
50V/DIV
10V/DIV
–48RTN
5V/DIV
SENSE
TIMER
GATE
S
, which in this case is 5A (Trace 2). Note that
Figure 4. Output Short-Circuit Behavior
(All Waveforms are Referenced to V
LTC4251/LTC4251-1/
S
ANALOG
CURRENT LIMIT
FAST CURRENT
LIMIT
. If C
SUPPLY RING
OWING
TO CURRENT
OVERSHOOT
ONSET OF OUTPUT
SHORT-CIRCUIT
C
TIMER
T
2ms/DIV
RAMP
is sized correctly, the capacitors
T
times out.
SUPPLY RING
OWING TO
MOSFET
TURN-OFF
T
LATCH OFF
LTC4251-2
(Trace 4) while the
425112 F04
EE)
TRACE 1
TRACE 2
TRACE 3
TRACE 4
13
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T