Datasheet

Data Sheet ADM1075
Rev. B | Page 25 of 52
the voltage connected to the UVL pin falls below 0.9 V, and the
gate is shut down using the 10 mA pull-down device. The fault
is cleared after UVH pin rises above 1.0 V.
Similarly, when an overvoltage event occurs and the voltage on
the OV pin exceeds 1 V, the gate is shut down using the 10 mA
pull-down device.
R
SENSE
Q1
C1
SENSE–
VEE
GATE
VIN
SENSE+
ADM1075
–48V RTN (0V)
R
SHUNT
UVH
UVL
OV
–48V
+
–
+
–
+
–
GATE
ENABLE
LOGIC
1V
0.9V
1V
09312-048
Figure 51. Undervoltage and Overvoltage Supply Monitoring
The maximum rating on the UVH pin is 4 V and the UVH
threshold is 1 V. This limits the maximum input voltage to
minimum input voltage ratio to 4:1. For example, if the UVH
threshold is set at 20 V, the maximum input voltage is 80 V so
as not to exceed the maximum ratings of the pin. If a wider
input range is required, some protection circuitry is required
on the UV pins to limit them to less than 4 V.
PWRGD
The
PWRGD
output indicates the status of the output voltage.
As shown in Figure 52, the
PWRGD
output is derived from the
DRAIN pin voltage. It is an open-drain output that pulls low
when the voltage on DRAIN is less than 2 V and the GATE pin
voltage is near its 12 V rail (power good). When a fault occurs
or hot swap is turned off, the open-drain pull-down is disabled,
allowing
PWRGD
to go high (power bad).
PWRGD
is guaran-
teed to be in a valid state for V
IN
≥ 1 V.
FET
DRAIN
HOT SWAP
DISABLE
SIGNAL
DIODE CLAMPS
DRAIN T
O 2V
S
R
Q
Q
R
DRAIN
I
DRAIN
=
50µA MAX
DRAIN
2V
11V
GATE
PWRGD
09312-049
Figure 52. Generation of
PWRGD
Signal
DRAIN
Because the source of the FET is always at or near the most
negative system supply, the drain voltage is a close approxima-
tion to the V
DS
of the F E T. When the voltage at the DRAIN pin
is less than 2 V, it is assumed the FET is turned on. The DRAIN
pin is used by the power-good circuitry to determine when
PWRGD
can be asserted. A resistor is required on the DRAIN
pin to limit current on the pin to 50 μA. A 2 MΩ resistor is
suitable to limit the current in most cases.
SPLYGD
The
SPLYGD
output indicates when the input supply is within
the programmed voltage window. This is an open-drain output.
An external pull-up resistor is required on this pin.
LATCH
The
LATCH
output signals that the device has latched off after
an overcurrent fault. This pin is also used to configure the
desired retry scheme. See the Hot Swap Fault Retry section for
additional details.
SHDN
The
SHDN
pin is a level-triggered input that allows the user to
command a shutdown of the hot swap function. When this
input is set low, the GATE output is switched to VEE to turn the
FET off. This pin has an internal pull-up of approximately 8 µA,
allowing it to be driven by an open-drain pull-down output or a
push-pull output. The input threshold is ~1 V.
This pin is also used to configure the desired retry scheme. See
the Hot Swap Fault Retry section for additional details.
Care should be taken if using the
SHDN
pin as an on/off pin.
Pulling the
SHDN
low always turns off the gate. However,
taking
SHDN
high again turns on hot swap only if there have
been less than seven faults/shutdown events within a 10 second
period. The retry scheme is configured to set GPO2/
ALERT2
low after seven faults. The
SHDN
pin cannot clear the GPO2/
ALERT2
fault. The retry counter is cleared after 10 seconds of
power good. Therefore, this is not an issue if there is never
going to be more than seven
SHDN
events within a 10 second
period.
The UVH or UVL pin may work better as a system on/off pin if
required. Toggling the UVx pin clears any faults (including
GPO2/
ALERT2
low after seven retry attempts). A switch
shorting UVH or UVL to VEE works as an on/off switch.
RESTART
The
RESTART
pin is a falling edge triggered input that allows
the user to command a 10 second automatic restart. When this
input is set low, the gate turns off for 10 seconds, and then powers
back up. The pin is falling edge triggered; therefore, holding
RESTART
low for more than 10 seconds generates only one
restart. This pin has an internal pull-up of approximately 8 µA,
allowing it to be driven by an open-drain pull-down output or a
push-pull output. The input threshold is ~1 V.
This pin is also used to configure the desired retry scheme. See
the Hot Swap Fault Retry section for additional details.