LTC4253/LTC4253A
9
425353aff
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the start-up cycle, the SS capacitor (C
SS
) is ramped by a 
22礎(chǔ) (28礎(chǔ) for the LTC4253A) current source. The GATE 
pin is held low until SS exceeds 20 " V
OS 
= 0.2V . SS is 
internally shunted by a 100k R
SS 
which limits the SS pin 
voltage to 2.2V (50k resistor and 1.4V for the LTC4253A). 
This corresponds to an analog current limit SENSE voltage 
of 100mV (60mV for the LTC4253A). If the SS capacitor 
is omitted, the SS pin ramps up in about 250祍 (140祍 
for the LTC4253A). The SS pin is pulled low under any of 
the following conditions: UVLO at V
IN
, UV , OV , during the 
initial timing cycle, a circuit breaker fault time-out or the 
RESET pin going high.
SENSE (Pin 7): Circuit Breaker/Current Limit Sense Pin. 
Load current is monitored by a sense resistor R
S
 connected 
between
 
SENSE and V
EE
, and controlled in three steps. If 
SENSE exceeds V
CB
 (50mV), the circuit breaker compara-
tor activates a (200礎(chǔ)???營(yíng)
DRN
) TIMER pull-up current. 
If SENSE exceeds V
ACL
, the analog current-limit amplifier 
pulls GATE down to regulate the MOSFET current at V
ACL
/
R
S
. In the event of a catastrophic short-circuit, SENSE may 
overshoot V
ACL
. If SENSE reaches V
FCL
 (200mV), the fast 
current-limit comparator pulls GATE low with a strong 
pull-down. To disable the circuit breaker and current limit 
functions, connect SENSE
 
to V
EE
.
V
EE
 (Pin 8): Negative Supply Voltage Input. Connect this 
pin to the negative side of the power supply.
GATE (Pin 9): N-channel MOSFET Gate Drive Output. This 
pin is pulled high by a 50礎(chǔ) current source. GATE is pulled 
low by invalid conditions at V
IN
 (UVLO), UV , OV , during the 
initial timing cycle, a circuit breaker fault time-out or the 
RESET pin going high. GATE is actively servoed to control 
the fault current as measured at SENSE. Compensation 
capacitor, C
C
, at GATE stabilizes this loop. A comparator 
monitors GATE to ensure that it is low before allowing an 
initial timing cycle, then the GATE ramps up after an over-
voltage event or restart after a current limit fault. During 
GATE start-up, a second comparator detects GATE within 
2.8V of V
IN
 before PWRGD1 can be set and power good 
sequencing starts.
DRAIN (Pin 10): Drain Sense Input. Connecting an external 
resistor, R
D
 between this pin and the MOSFETs drain (V
OUT
) 
allows voltage sensing below 6.15V (5V for LTC4253A) 
and current feedback to TIMER. A comparator detects if 
DRAIN is below 2.39V and together with the GATE high 
comparator, sets the PWRGD1 flag. If V
OUT
 is above V
DRNCL
, 
the DRAIN pin is clamped at approximately V
DRNCL
. R
D
 
current is internally multiplied by 8 and added to TIMERs 
200礎(chǔ) during a circuit breaker fault cycle. This reduces 
the fault time and MOSFET heating.
OV (Pin 11): Overvoltage Input. For the LTC4253, the 
threshold at the OV pin is set at 6.15V with 0.3V hys-
teresis. If OV > 6.15V , GATE pulls low. When OV returns 
below 5.85V , GATE start-up begins without an initial timing 
cycle. The LTC4253A OV threshold 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 OV occurs in the middle of an initial 
timing cycle, the initial timing cycle is restarted after OV 
goes away. OV does not reset the latched fault or PWRGD1 
flag. The internal UVLO at V
IN
 always overrides OV . A 1nF 
to 10nF capacitor at OV prevents transients and switch-
ing noise from affecting the OV thresholds and prevents 
glitches at the GATE.
UV (Pin 12): Undervoltage Input. For the LTC4253, the 
threshold at the UV pin is set at 3.225V with 0.3V hysteresis. 
If UV < 2.925V , PWRGD1 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 LTC4253A UV 
threshold is set at 3.08V with 324mV hysteresis. If UV < 
2.756V , PWRGD1 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.
PIN FUNCTIONS