SMH4802 Datasheet, PDF (8/20 Pages) Summit Microelectronics, Inc. – Programmable -48V Hot-Swap Controller with Forced Shut Down

SMH4802
Preliminary Information
APPLICATIONS INFORMATION
Powering VDD
The 12V shunt regulator between the VDD and VSS pins
allows the SMH4802 to operate over a wide range of
supply voltages. It is necessary to use a series dropping
resistor (RD) between the host power supply and the VDD
pin in order to bias the shunt regulator and limit current into the
device.
System Enable
The EN/TS input provides an active high comparator input
that may be used as a master enable or temperature
sense input.
Under-/Over-Voltage Sensing
The Under-Voltage (UV) and Over-Voltage (OV) inputs
provide a set of comparators that act in conjunction with an
external resistor divider network to sense when the host
supply voltage exceeds the user defined limits. If the input
to the UV pin rises above 2.5V, and the input to the OV pin
falls below 2.5V, the power-up sequence may be initiated.
If UV falls below 2.5V, or OV rises above 2.5V, the PG#
and VGATE outputs will be shut down immediately.
Under-/Over-Voltage Filtering
The SMH4802 may also be configured so that an out of
tolerance condition on UV/OV will not shut off the output
immediately. A filter delay can be inserted so that only
sustained under-voltage or over-voltage conditions will
shut off the output. An out of tolerance condition on UV/
OV for longer than the filter delay time (tUOFLTR) will latch
the VGATE and PG outputs in the off state if the UV/OV
filter option is enabled. The Under-/Over-Voltage Filter-
ing feature is disabled in the default configuration of the
device.
Under-Voltage Hysteresis
The Under-Voltage comparator input may be configured
with a programmable level of hysteresis. The compare
level may be set in steps (up to 15) of 62.5mV below 2.5V.
The default under-voltage hysteresis level is set to
62.5mV.
Soft Start Slew Rate Control
Once all of the preconditions for powering up the DC/DC
converters have been met, the SMH4802 provides a means
to soft start the external power FET limiting the in-rush
current. Current limiting is generally needed due to the bulk
capacitance across the power rails of the DC/DC convert-
ers. The VGATE output of the SMH4802 is current limited
to I_VGATE, allowing the slew rate to be easily modified using
external passive components.
Load Control — Turning on a DC/DC Converter
Once power has been ramped to the DC/DC converter,
two conditions must be met before the PG# output can be
asserted: the DRAIN SENSE voltage must be below 2.5V,
and the VGATE voltage must be greater than V – V .
DD
GT
The DRAIN SENSE input ensures the power MOSFET is
not absorbing too much steady state power from operat-
ing at a high V_DS. (This sensor remains active at all times,
except during the current regulation period).
The VGATE sensor ensures the power MOSFET is
operating well into its saturation region before allowing
the loads to be switched on. Once VGATE reaches V_DD
– V_GT this sensor is latched.
After the external MOSFET is properly switched on, the
PG# output will be asserted after a delay of t_PGD. The delay
time is programmable from 5ms to 160ms.
NOTE: The PG# output has a 12V withstand capability, so
high voltages must not be connected to this pin. A bipolar
transistor or an opto-isolator can be used to boost the
withstand voltage to that of the host supply.
Force Shutdown — Secondary Feedback
The Force Shutdown signal (FS#) is an active low input
that provides a method of receiving feedback from the
secondary side of the DC/DC controllers. A built-in hold-
off timer allows the SMH4802 to ignore the state of the FS#
input until the time period expires. The FS# input must be
driven high by the end of this time period. If not, a low level
on this input will shut off the VGATE and PG# outputs.
The purpose of the hold-off timer is to allow enough time
for devices on the secondary side of the DC/DC controller
to power-up and stabilize. This unique feature of the
SMH4802 allows supervisory circuits, such as an
SMS44, to control the shutdown of the primary side soft
start circuit, even though the secondary side initially has
no power.
Circuit Breaker Operation
The SMH4802 provides a number of circuit breaker
functions to protect against over current conditions. A
sustained over-current event could damage the host
supply and/or the load circuitry.
The board’s load current passes through a series resistor
(R ) connected between the MOSFET source (which is tied
S
to CBSENSE) and V_SS. The breaker trips (Figure 4)
whenever the voltage drop across R_S is greater than 50mV
for more than t_CBD (a programmable filter delay ranging from
10µs to 500µs).
8
2062 2.3 6/19/03
SUMMIT MICROELECTRONICS, Inc.