LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-2
18
425212fe
For more information www.linear.com/LTC4252-1
applicaTions inForMaTion
and eliminates current spikes at insertion. A large external
gate-source capacitor is thus unnecessary for the purpose
of compensating C
GD
. Instead, a smaller value (≥ 10nF)
capacitor C
C
is adequate. C
C
also provides compensation
for the analog current limit loop.
GATE has two comparators: the GATE low comparator
looks for < 0.5V threshold prior to initial timing or a GATE
start-up cycle; the GATE high comparator looks for < 2.8V
relative to V
IN
and, together with the DRAIN low compara-
tor, sets PWRGD status during GATE startup.
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
EE
. When
SENSE exceeds 50mV, the CB comparator activates the
230µA TIMER pull-up. At 100mV (60mV for the LTC4252A),
the ACL amplifier servos the MOSFET current and, at
200mV, the FCL comparator 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
T
to 4V (see Equation3), the LTC4252 shuts
down and pulls GATE low.
If the SENSE pin encounters a voltage greater than V
ACL
,
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
EE
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
C
in series with the gate ca-
pacitor) helps the ACL amplifier to recover.
SHORT
-CIRCUIT OPERATION
Circuit behavior arising from a load side low impedance
short is shown in Figure 6 for the LTC4252. Initially, the
current overshoots the fast current limit level of V
SENSE
=
200mV (Trace 2) as the GATE pin works to bring V
GS
under
control (Trace 3). The overshoot glitches the backplane
in the negative direction and when the current is reduced
to 100mV/R
S
, the backplane responds by glitching in the
positive direction.
TIMER commences charging C
T
(Trace 4) while the analog
current limit loop maintains the fault current at 100mV/R
S
,
which in this case is 5A (Trace 2). Note that the backplane
voltage (Trace 1) sags under load. T
imer pull-up is ac-
celerated by V
OUT
. When C
T
reaches 4V, GATE turns off,
PWRGD pulls high, the load current drops to zero and the
backplane rings up to over 100V. The transient associated
with the GATE turn off can be controlled with a snubber to
reduce ringing and a transient voltage suppressor (such as
Diodes Inc. SMAT70A) to clip off large spikes. The choice
of RC for the snubber is usually done experimentally. The
value of the snubber capacitor is usually chosen between
10 to 100 times the MOSFET C
OSS
. The value of the snub-
ber resistor is typically between 3Ω to 100Ω.
425212 F06
–48RTN
50V/DIV
GATE
10V/DIV
SENSE
200mV/DIV
TIMER
5V/DIV
0.5ms/DIV
FAST CURRENT LIMIT
SUPPLY RING OWING TO
CURRENT OVERSHOOT
SUPPLY RING OWING TO
MOSFET TURN OFF
ANALOG CURRENT LIMIT
ONSET OF OUTPUT SHORT-CIRCUIT
C
TIMER
RAMP
LATCH OFF
Figure 6. Output Short-Circuit Behavior of LTC4252
