SMH4811A
Preliminary
Under-/Over-Voltage Filtering
The SMH4811A may also be configured so that an out of
tolerance condition on UV/OV will not shut off the output
immediately. Instead, a filter delay may be inserted so that
only sustained under-voltage or over-voltage conditions
will shut off the output. When the UV/OV filter option is
enabled an out of tolerance condition on UV or OV for
longer than the filter delay time, t
UOFLTR
, activates the
FAULT# output, and the VGATE and PG outputs will be
latched in the off state. To initiate another power-up
sequence the FAULT# output must first be reset. Refer to
the appropriate section on resetting the FAULT# output.
The Under-/Over-Voltage Filtering 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
controller have been met, the SMH4811A provides a
means to soft start the external power FET. It is important
to limit in-rush current to prevent damage to the add-in
card or disruptions to the host power supply. For example,
V
DD
11
≤
V
DD
≤
13
<t
PUVF
UV
2.5V
REF
OV
PD1# +
PD2#
t
PDD
V
DD
– V
GT
VGATE
V
DD
5V
DRAIN
SENSE
2.5V
REF
50mV
REF
CBSENSE
<t
CBD
PG#
ENPG
2044 Fig02 5.0
Figure 2. Complete Power On Timing Sequence
SUMMIT MICROELECTRONICS, Inc.
2044 6.1 2/8/01
7
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
