Datasheet

LTC4252B-1/LTC4252B-2
LTC4252C-1/LTC4252C-2
10
4252b12f
PIN FUNCTIONS
(MS/MS8)
V
EE
(Pin 5/Pin 4): Negative Supply Voltage Input. Connect
this pin to the negative side of the power supply.
GATE (Pin 6/Pin 5): N-Channel MOSFET Gate Drive Output.
This pin is pulled high by a 58µA current source. GATE is
pulled low by invalid conditions at V
IN
(UVLO), UV, OV, or
a circuit breaker fault timeout. GATE is actively servoed to
control
the fault current as measured at SENSE. A compen-
sation capacitor at GATE stabilizes this loop. A comparator
monitors GATE to ensure that it is low before allowing an
initial timing cycle, GATE ramp-up after an overvoltage
event or restart after a current limit fault. During GATE
start-up, a second comparator detects if GATE is within
2.8V of V
IN
before PWRGD is set (MS package only).
DRAIN (Pin 7/Pin 6): Drain Sense Input. Connecting an
external resistor, R
D
, between this pin and the MOSFET’s
drain (V
OUT
) allows voltage sensing below 6.15V (5V for
LTC4252C) and current feedback to TIMER. A comparator
detects if DRAIN is below 2.385V and together with the
GATE high comparator sets the PWRGD flag. If V
OUT
is
above V
DRNCL
, DRAIN clamps at approximately V
DRNCL
.
The current through R
D
is internally multiplied by 8 and
added to TIMER’s 230µA pull-up current during a circuit
breaker fault cycle. This reduces the fault time and MOS-
FET heating.
OV (Pin 8/Pin 7): Overvoltage Input. The active high thresh-
old at the OV pin is set at 6.15V with 0.6V hysteresis. If OV
> 6.15V, GATE pulls low. When OV returns below 5.55V,
GATE start-up begins without an initial timing cycle. The
LTC4252C OV pin is set at 5.09V with 102mV hysteresis.
If OV > 5.09V, GATE pulls low. When OV returns below
4.988V, GATE start-up begins without an initial timing
cycle. If an overvoltage condition occurs in the middle of
an initial timing cycle, the initial timing cycle is restarted
after the overvoltage condition goes away. An
overvoltage
condition does not reset the PWRGD flag. The internal UVLO
at V
IN
always overrides OV. A 1nF to 10nF capacitor at OV
prevents transients and switching noise from affecting
the OV thresholds and prevents glitches at the GATE pin.
UV (Pin 9/Pin 7): Undervoltage Input. The active low thresh-
old at the UV pin is set at 2.925V with 0.3V hysteresis. If
UV < 2.925V, PWRGD pulls
high, both GATE and TIMER
pull low. If UV rises above 3.225V, this initiates an initial
timing cycle followed by GATE start-up. The LTC4252C
UV pin is set at 3.08V with 324mV hysteresis. If UV <
2.756V, PWRGD pulls high, both GATE and TIMER pull
low. If UV rises above 3.08V, this initiates an initial timing
cycle followed by GATE start-up. The internal UVLO
at V
IN
always overrides UV. A low at UV resets an internal fault
latch. A 1nF to 10nF capacitor at UV prevents transients
and switching noise from affecting the UV thresholds and
prevents glitches at the GATE pin.
TIMER (Pin 10/Pin 8): Timer Input. TIMER is used to
generate an initial timing delay at start-up and to delay
shutdown in the event of an output overload (
circuit
breaker fault). TIMER starts an initial timing cycle when
the following conditions are met: UV is high, OV is low, V
IN
clears UVLO, TIMER pin is low, GATE is lower than V
GATEL
,
SS < 0.2V, and V
SENSE
– V
EE
< V
CB
. A pull-up current
of 5.8µA then charges C
T
, generating a time delay. If C
T
charges to V
TMRH
(4V), the timing cycle terminates, TIMER
quickly pulls low and GATE is activated.
If SENSE exceeds 50mV while GATE is high, a circuit breaker
cycle begins with a 230µA pull-up current charging C
T
.
If DRAIN is approximately 7V (6V for LTC4252C) during
this cycle, the timer pull-up has an additional current
of 8 • I
DRN
. If SENSE drops below 50mV before TIMER
reaches 4V, a 5.8µA pull-down current slowly discharges
the C
T
. In the event that C
T
eventually integrates up to the
V
TMRH
threshold, the circuit breaker trips, GATE quickly
pulls low and PWRGD pulls high. The LTC4252-1 TIMER
pin latches high with a 5.8µA pull-up source. This latched
fault is cleared by either pulling TIMER low with an external
device or by pulling UV below V
UVLO
. The LTC4252-2 starts
a shutdown cooling
cycle following an overcurrent fault.
This cycle consists of 4 discharging ramps and 3 charging
ramps. The charging and discharging currents are 5.8µA
and TIMER ramps between its 1V and 4V thresholds. At the
completion of a shutdown cooling cycle, the LTC4252-2
attempts a start-up cycle.