LTC4245 Datasheet, PDF (11/36 Pages) Linear Technology – Multiple Supply Hot Swap Controller with I2C Compatible Monitoring

LTC4245
PI FU CTIO S
PCI_RST#: Reset Input. Pulling this pin low causes LO-
CAL_PCI_RST# to pull low. When high, LOCAL_PCI_RST#
is the logical inverse of HEALTHY#. Tie to INTV_CC if
unused.
PGI: Power Good Input. Tie this pin to the ⎯R⎯E⎯S⎯E⎯T output
of an external supply monitor or power good output of a
DC/DC converter. When all supplies have been turned on,
a timing cycle is started at the end of which the PGI pin is
sampled. If it is low, all external MOSFETs are shut off. If
the PGI Disable control bit C3 is not set, pulling this pin
low for more than 20μs during normal operation will also
shut off all MOSFETs. Tie to INTV_CC if unused.
PRECHARGE: Bus Precharge Output. This pin can source
70mA at 1V as soon as 3V_IN is powered-up. Leave it open
if unused.
SCL: Serial Bus Clock Input. Data at the SDA pin is shifted
in or out on rising edges of SCL. This is a high impedance
pin that is generally driven by an open-collector output
from a master controller.
SDA: Serial Bus Data Input and Output. This is a high
impedance input when address, command or data bits
are shifted in. It is an open-drain output when sending
data back to the master controller or acknowledging a
write operation. An external pull-up resistor or current
source is required.
SS: Soft-Start Input. Connect a capacitor between this pin
and ground to set the rate of increase of current limit during
start-up for dI/dt limited inrush current. When an external
MOSFET is turned on, a 20μA pull-up current charges the
capacitor. The voltage ramp on the capacitor is converted
into an internal current limit increasing linearly with time.
Leave it open if dI/dt limited inrush is not required.
TIMER: Timer Input. A capacitor between this pin and
ground sets the duration of the start-up, PGI and auto-retry
timing cycles to be 23.3ms/μF, 233ms/μF and 1.17s/μF
respectively. A timing cycle consists of TIMER being
charged to 2.56V with an internal pull-up current source
and then being reset by a switch to ground. The timing
cycle ends when TIMER falls below 0.23V. The start-up,
PGI and auto-retry timing cycles use 100μA, 10μA and
2μA pull-up current sources respectively.
V_EEGATE: Gate Drive for –12V Supply External N-Chan-
nel MOSFET. An internal 20μA current source charges
the gate of the external N-channel MOSFET. An internal
clamp limits the gate voltage to 6.2V above V_EEIN. During
turn-off, a 3.5mA pull-down current discharges V_EEGATE
to V_EEIN. During short-circuit a 65mA pull-down current
between V_EEGATE and V_EEIN is activated. If a –12V supply
is not available, connect V_EEGATE to ground and use the
CFG pin appropriately.
V_EEIN: –12V Supply, Current Sense and ADC Input. An
undervoltage lockout circuit, with 38mV hysteresis, pre-
vents any external MOSFET from turning on when this
pin is above –10.5V. The V_EEIN undervoltage lockout can
be disabled by using the CFG pin. If a –12V supply is not
available, connect V_EEIN to ground and use the CFG pin
appropriately.
V_EEOUT: –12V Supply Foldback, ADC and Power Bad Input.
Connect this pin to the drain of the –12V supply external
N-channel MOSFET switch. Power is considered bad if this
pin rises above –11.1V. The comparator on this pin has a
built-in hysteresis of 54mV. The V_EEOUT power bad function
can be disabled by using the CFG pin. This pin is also an
input to the ADC and the current limit foldback circuit. A
1800Ω active pull-up discharges V_EEOUT to ground when
the external MOSFET is turned off. If a –12V supply is not
available, connect V_EEOUT to ground and use the CFG pin
appropriately.
V_EESENSE: –12V Supply Current Sense and ADC Input.
Connect this pin to the output of the –12V current sense
resistor. The current limit circuit controls the V_EEGATE pin
to limit the sense voltage between the V_EESENSE and V_EEIN
pins to 50mV or less during start-up and 150mV thereafter.
During start-up a foldback feature lowers the current limit
to 16mV as the V_EEOUT pin approaches ground. A circuit
breaker, enabled after start-up, trips when the sense voltage
exceeds 50mV for 22μs. To disable current limit, connect
this pin to V_EEIN. If a –12V supply is not available, connect
V_EESENSE to ground and use the CFG pin appropriately.
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