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LTC1921 Datasheet(PDF) 7 Page - Linear Technology

Part # LTC1921
Description  Negative Voltage Hot Swap Controllers in SOT-23
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1921 Datasheet(HTML) 7 Page - Linear Technology

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4251b12f
LTC4251B/LTC4251B-1/
LTC4251B-2
PIN FUNCTIONS
SENSE (Pin 1): Circuit Breaker/Current Limit SENSE Pin.
Load current is monitored by sense resistor RS con-
nected between SENSE and VEE, and controlled in three
steps. If SENSE exceeds VCB (50mV), the circuit breaker
comparator activates a 230μA TIMER pin pull-up cur-
rent. The LTC4251B/LTC4251B-1/LTC4251B-2 latch off
when CT charges to 4V. If SENSE exceeds VACL (100mV),
the analog current limit amplifier pulls GATE down and
regulates the MOSFET current at VACL/RS. In the event
of a catastrophic short-circuit, SENSE may overshoot
100mV. If SENSE reaches VFCL (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 VEE.
Kelvin-sense connections between the sense resistor and
the VEE and SENSE pins are strongly recommended, see
Figure 6.
VEE (Pin 2): Negative Supply Voltage Input. Connect this
pin to the negative side of the power supply.
VIN (Pin 3): Positive Supply Input. Connect this pin to the
positive side of the supply through a dropping resistor.
A shunt regulator typically clamps VIN at 13V. An internal
undervoltage lockout (UVLO) circuit holds GATE low until
the VIN pin is greater than VLKO (9.2V), overriding UV/OV. If
UV is high, OV is low and VIN comes out of UVLO, TIMER
starts an initial timing cycle before initiating a GATE ramp
up. If VIN drops below approximately 8.2V, GATE pulls low
immediately.
TIMER (Pin 4): Timer Input. TIMER is used to generate
a delay at start-up, and to delay shutdown in the event of
an output overload. TIMER starts an initial timing cycle
when the following conditions are met: UV is high, OV is
low, VIN clears UVLO, TIMER pin is low, GATE is lower than
VGATEL and VSENSE – VEE < VCB. A pull-up current of 5.8μA
then charges CT, generating a time delay. If CT charges to
VTMRH (4V) the timing cycle terminates, TIMER quickly
pulls low and GATE is activated.
If SENSE exceeds 50mV while GATE is high, a 230μA
pull-up current charges CT. If SENSE drops below 50mV
before TIMER reaches 4V, a 5.8μA pull-down current slowly
discharges CT. In the event that CT eventually integrates
up to the 4V VTMRH threshold, TIMER latches high with
a 5.8μA pull-up source and GATE quickly pulls low. The
LTC4251B/LTC4251B-1/LTC4251B-2 fault latches may
be cleared by either pulling TIMER low with an external
device, or by pulling UV/OV below VUVLO.
UV/OV (Pin 5): Undervoltage/Overvoltage Input. This dual
function pin detects undervoltage as well as overvoltage.
The high threshold at the UV comparator is set at VUVHI
with VUVHST hysteresis. The high threshold at the OV
comparator is set at VOVHI with VOVHST hysteresis. If UV/
OV < VUVLO or UV/OV > VOVHI, GATE pulls low. If UV/OV
> VUVHI and UV/OV < VOVLO, the LTC4251B/LTC4251B-1/
LTC4251B-2 attempt to start-up. The internal UVLO at VIN
always overrides UV/OV. A low at UV resets an internal fault
latch. A high at OV pulls GATE low but does not reset the
fault latch. A 1nF to 10nF capacitor at UV/OV eliminates
transients and switching noise from affecting the UV/OV
thresholds and prevents glitches at the GATE pin.
GATE (Pin 6): N-Channel MOSFET Gate Drive Output.
This pin is pulled high by a 58μA current source. GATE is
pulled low by invalid conditions at VIN (UVLO), UV/OV, or
the fault latch. GATE is actively servoed to control fault
current as measured at SENSE. A compensation 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.
UV/OV refers to the UV pin for the LTC4251B-2. The OV comparator in the LTC4251B-2 is disabled. All
references in the text to overvoltage, OV, VOVHI and VOVLO do not apply to the LTC4251B-2.


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