ADM1073 Data Sheet
Rev. B | Page 14 of 24
When the device detects that the supply voltage is valid, it
ramps up the GATE voltage until the FET turns on and the load
current increases. The ADM1073 monitors the level of the
current flowing through the FET by sensing the voltage across
the external sense resistor, R
SENSE
. When the SENSE voltage
reaches 100 mV, the GATE pin is actively controlled, limiting
the load current. In this way, the maximum current permitted
to flow through the load is set by the choice of R
SENSE
.
If a change in the level of the supply voltage causes the voltage
on UV to fall below the undervoltage falling threshold (V
UVF
),
or the voltage on OV to rise above the overvoltage rising
threshold (V
OVR
), then the gate drive is disabled.
BOARD REMOVAL
If the board is removed from a card cage, the voltage on the UV
pin falls to zero (that is, outside operating range) and the GATE
drive is de-asserted, turning off the FET.
CONTROLLING THE CURRENT
The ADM1073 features the following current control functions:
• Precision maximum current limit
• Controlled time in current limit
• Limited number of consecutive maximum current events
• Current limit profiling—soft start
• Overcurrent fast limit
In the following sections, five distinct system operating
conditions are described with reference to the current control
features.
Startup into Nominal Load Capacitance
Once the supply voltage has exceeded the UV threshold, and
following the 0.6 ms UV filter time, the current to the load
ramps up linearly as the capacitor on the Soft Start (SS) pin is
charged to 2.5 V. At the same time, current is sourced into the
capacitor on the TIMER pin, both from an on-chip source and
via the drain resistor. Once the soft start voltage has reached
2.5 V, the current to the load is limited to I
MAX
(100 mV/R
SENSE
).
Assuming that the values of R
SENSE
and the TIMER capacitance
have been chosen to allow the load capacitance to charge within
one ON period (t
ON
period), the load capacitor is fully charged
before the voltage on TIMER reaches 2.5 V. At this point, the
current to the load decreases, and the FET gate voltage increases
to V
SS
, connecting the supply to the load.
Startup into Load with Large Capacitance
If the load capacitance is sufficiently large that to charge it fully
in one attempt would compromise the FET’s SOA, consecutive
maximum current events may be used. The use of this tech-
nique assumes that the load is not yet enabled, so negligible
load current is demanded. The initial current profiling is
identical to that for startup into a nominal load capacitance. If
the charge passed to the load in time t
ON
with maximum current
flowing is insufficient to fully charge the load capacitance, at the
end of the t
ON
period the load capacitance is still demanding
maximum current. The ADM1073 now controls the FET gate to
zero for a time t
OFF
, determined by the time taken for the on-
chip current sink to discharge the TIMER capacitance to 0.5 V.
At the end of time t
OFF
, the device retries, again following the
soft start current profile. In this way, a large load capacitance
can be charged using consecutive current limit periods. The
external components should be chosen to ensure that the
capacitance is fully charged within seven TIMER periods, if the
default limited consecutive retry mode is used.
Startup into a Short Circuit or over Current Fault
The load might demand large currents at initial connection.
The ADM1073 follows the Soft Start current profile as
described for startup into a nominal load. The current is limited
at I
MAX
for time t
ON
following which the FET gate is pulled low.
The FET gate is held low for time t
OFF
, before retrying, again
with the soft start current profile. The ADM1073 cycles through
7 retries, after which it latches the FET off, assuming the default
limited consecutive retry mode is used.
Voltage Step during Normal Operation
Once the load capacitance is charged at initial board insertion
and a
PWRGD
signal is issued by the ADM1073, the load
begins to demand current. Therefore, following a step increase
in the magnitude of the supply voltage, not all the FET current
is available for charging of the load capacitance. Because the
FET is fully on following a step in the supply voltage, the
current increases immediately from I
LOAD
to supply charge to
the load capacitance. If the current remains below the fast
current limit, the FET gate drive amplifier controls it back to
I
MAX
. If the current exceeds the fast current limit, the FET gate is
strongly pulled down and back into regulation with the current
at I
MAX
. The size of the voltage step and the headroom between
the load current and I
MAX
determine the time required at I
MAX
to
charge the load capacitance. External components should be
chosen to ensure that any expected step size leads to a
requirement of less than time t
ON
to charge the load capacitance.
Short Circuit or Overcurrent Fault during Operation
If a short circuit or an overcurrent fault occurs during normal
operation, the FET is fully on and initially allows increased
current to flow. If the current remains below the fast current
limit, the FET gate drive amplifier controls it back to I
MAX
. If the
current exceeds the fast current limit, the FET gate is strongly
pulled down and back into regulation with the current at I
MAX
.
Following a period, t
ON
, the ADM1073 pulls the FET gate low
for a time t
OFF
, then retries following the soft start current
profile. If the fault persists, the ADM1073 cycles through 7
retries before latching off. If the fault clears within the 7-retry
period, the ADM1073 controls the FET gate high to allow
normal operation to continue.
