Slow-Comparator Normal Operation
After the startup period is complete, the slow compara-
tor is enabled and the device enters normal operation.
The comparator threshold voltage (V
SC,TH
) is adjustable
from 25mV to 100mV. The slow-comparator response
time decreases to a minimum of 100µs with a large
overdrive voltage. Response time is 3ms for a 1mV over-
drive. The variable speed response time allows the
MAX5955/MAX5956 to ignore low-amplitude momentary
glitches, thus increasing system noise immunity. After
an extended overcurrent condition, a fault is generated,
both PGOODs are pulled low and the MOSFET gates
are discharged with a strong 3mA pulldown current.
Fast-Comparator Startup Period
During the startup period, the fast comparator regulates
the gate voltage to ensure that the voltage across the
sense resistor does not exceed the startup fast-com-
parator threshold voltage (V
SU,TH
), V
SU,TH
is scaled to
two times the slow-comparator threshold (V
SC,TH
).
Fast-Comparator Normal Operation
In normal operation, if the load current reaches the fast-
comparator threshold, a fault is generated, both
PGOODs are pulled low, and the MOSFET gates are dis-
charged with a strong 3mA pulldown current. This hap-
pens in the event of a serious current overload or a dead
short. The fast-comparator threshold voltage (V
FC,TH
) is
scaled to four times the slow-comparator threshold
(V
SC,TH
). This comparator has a fast response time of
260ns (Figure 3).
Undervoltage Lockout (UVLO)
The UVLO prevents the MAX5955/MAX5956 from turning
on the external MOSFETs until one input voltage exceeds
the UVLO threshold (2.4V) for t
D,UVLO
. The MAX5955/
MAX5956 use power from the higher input voltage rail for
the charge pumps. This allows for more efficient charge-
pump operation. The UVLO protects the external
MOSFETs from an insufficient gate-drive voltage. t
D,UVLO
ensures that the board is fully inserted into the backplane
and that the input voltages are stable. Any input voltage
transient on both supplies below the UVLO threshold
reinitiates the t
D,UVLO
and the startup period.
Latched and Autoretry
Overcurrent Fault Management
The MAX5955B/MAX5956B latch the external MOSFETs
off when an overcurrent fault is detected. Toggling ON_
below 0.875V or one of the supply voltages
below/above the UVLO threshold for at least 100µs
clears the fault latch and reinitiates the startup period.
Similarly, the MAX5955A/MAX5956A turn the external
MOSFETs off when an overcurrent fault is detected,
then automatically restart after the autoretry delay that
is internally set to 64 times t
START
. During the autoretry
delay, toggling ON_ below 0.875V does not clear the
fault latch. The autoretry can be overridden, causing
the startup period to begin immediately by toggling one
of the supply voltages below/above the UVLO thresh-
old. When toggling a supply voltage to clear a fault,
remember that the supply voltage must go below and
then above the UVLO threshold for at least 100µs
regardless of the final value of the supply voltage.
Output Overvoltage/Undervoltage Fault
Management
The MAX5955/MAX5956 monitor the output voltages with
the MON1 and MON2 window comparator inputs. These
voltage monitors are enabled after the startup period.
Once enabled, the voltage monitor detects a fault if
V
MON
_ is less than 543mV or greater than 687mV.
When the MAX5955 protection device detects an output
overvoltage/undervoltage fault on either MON1 or
MON2, both external MOSFET gates are discharged at
3mA and both PGOODs pull low. For the MAX5955A,
the part continuously attempts to restart after each
autoretry period. The part successfully restarts after the
fault is removed and after waiting the autoretry period.
For the MAX5955B, the GATEs are latched off until the
output voltage fault is removed and the fault latch is
cleared by toggling ON_ or by cycling one of the supply
voltages above/below the UVLO threshold.
When the MAX5956 monitoring device detects an out-
put overvoltage/undervoltage fault on either MON1 or
MON2, neither external MOSFET gates are affected,
but the PGOOD_ of the channel experiencing the fault
pulls low. Thus the fault is reported on the channel with
the problem, but the MAX5956 does not allow an output
overvoltage/undervoltage fault to disrupt operation by
shutting down the channels. The MAX5956’s PGOOD_
output immediately goes high impedance after the out-
put overvoltage/undervoltage fault is removed.
The voltage monitors do not react to output glitches of
less than 20µs. A capacitor from MON_ to GND increas-
es the effective glitch filter time. The voltage monitoring
function of the MAX5955/MAX5956 can be disabled by
connecting V
IN1
to MON1 and V
IN2
to MON2.
Status Outputs (PGOOD_)
The status output is an open-drain output that pulls low
in response to one of the following conditions:
• Overcurrent fault
• Output undervoltage/overvoltage fault
PGOOD_ goes low when the corresponding channel is
forced off (ON_ < 0.875V) (Table 1).
MAX5955/MAX5956
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
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