LTC4253/LTC4253A
9
425353aff
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the start-up cycle, the SS capacitor (C
SS
) is ramped by a
22µA (28µA 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 250µs (140µs
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 (200µA+8•I
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 50µA 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 MOSFET’s 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 TIMER’s
200µA 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
