SMH4814
Preliminary Information
APPLICATIONS INFORMATION (CONTINUED)
Under-/Over-Voltage Sensing
Vuv-Vuvhys
UV
The Under-Voltage (UV) and Over-Voltage (OV) inputs
provide a set of comparators that act in conjunction
with an external resistive divider ladder to sense
whether or not the host supply voltage is within the
user-defined limits. The power-up sequence is initiated
when the input to the UV pin rises above Vuv and the
input to the OV pin remains below Vov for a period of
at least tPDD (Pin Detect Delay time). The tPDD filter
helps prevent spurious start-up sequences while the
card is being inserted.
tPUOVF
VGATE_HS
The default values for Vuv and Vov are 2.86V and
3.07V, respectively. This ratio allows the UV and OV
input to be tied together and accommodates standard
telecom over and under voltage input ranges.
Alternatively, Vuv and Vov may be programmed
independently to one of four values, determined by
R09[3:0].
Figure 9 – Example Under-Voltage Filter Timing
Under-/Over-Voltage Latching
By default, an out-of-tolerance condition on UV/ OV
will shut off the outputs until the offending condition
goes away. At that point, the entire turn on sequence
may start over. However, an over or under voltage
condition may also be programmed to cause a FAULT
condition, using R0D[1:0]. In this case the FAULT#
output is asserted, and the user is required to reset the
Fault condition before the device will go through
another power-up sequence.
Under-/Over-Voltage Filtering
If UV falls below Vuv-Vuvhys or OV rises above Vov
for a period of time determined by the UV/OV glitch
filter (R06[7:6]), the PUPX and VGATEX outputs may
be disabled immediately. Alternatively, the SMH4814
can be configured so that an out-of- tolerance
condition on UV or OV does not shut off the output
immediately. Instead, a filter delay may be inserted so
that only sustained under-voltage or over-voltage
conditions of longer than the filter delay time (tPUOVF in
Figure 9) can shut off the output. The UV and OV
filters are enabled with bits 0 and 1, respectively, of
register R0F. Refer to R04[3:0] for more information
on the filter delay options. Figure 9 shows a sample
waveform for when the under-voltage filter is enabled.
Under-/Over-Voltage Hysteresis
The Under and Over Voltage comparator inputs may
be configured with a programmable level of hysteresis
using Register R08. The falling voltage compare level
may be set from 32mV to 512mV below the nominal
value, in steps of 32mV. The rising voltage compare
level is fixed at either Vuv or Vov, depending on the
input. The default under and over voltage hysteresis
level is set to 160mV.
Soft Start Slew Rate Control
Once all of the preconditions for powering up the
DC/DC controllers have been met as explained in the
previous sections, the SMH4814 provides a means to
soft start the external power MOSFET. It is important
to limit in-rush current to prevent damage to the add-in
card or disruptions to the host power supply.
Summit Microelectronics, Inc
2080 2.0 07/21/05
14
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
