MAX5922
+48V, Single-Port Network Power Switch
For Power-Over-LAN
18 ______________________________________________________________________________________
This overcurrent timeout enables the MAX5922 to operate
in a periodic overcurrent condition without causing a fault
(see Figure 9).
Power-OK (POK)
POK goes open-drain t
POK_HIGH
(88ms) after V
OUT
rises to within V
THPOK
(0.75V) from V
IN
. POK goes low
t
POK_LOW
(1.4ms) after V
OUT
drops 0.82V below V
IN
.
Zero-Current Detection
Zero-current detection is enabled if ZC_EN is high and
only after the startup period has finished (indicated by
POK going high). When V
SENSE
falls below the zero-
current threshold (V
ZCTH
) for a continuous t
ZCDEL
=
350ms, a zero-current fault is generated. The MOSFET
is turned off and ZC is latched low. The MAX5922A and
the MAX5922B/MAX5922C (with DET_DIS = low) imme-
diately begin a PD detection sequence, regardless of
the status of the LATCH input. ZC is unlatched and
goes high impedance after a PD is detected. ZC is also
high impedance during initial power-up. When
DET_DIS is high (MAX5922B/MAX5922C), a zero-cur-
rent fault shuts down the MOSFET and the EN pin
needs to be toggled to unlatch the fault.
At any time during a zero-current condition, if V
SENSE
goes above V
ZCTH
for the zero-current deglitch time
(t
ZC_DEG
= 10ms), the zero-current counter resets to
zero and a zero-current fault is not generated. Bring
ZC_EN low to disable the zero-current detection func-
tion. ZC stays high impedance in this mode.
Thermal Shutdown
If the MAX5922 die temperature reaches +150°C, an
overtemperature fault is generated. The MOSFET turns
off and FAULT goes low. The MAX5922 die tempera-
ture must cool down below 120°C before the overtem-
perature fault condition is removed (see Fault
Management section).
Fault Report (
FAULT
)
FAULT goes low when there is an overcurrent fault and/or
an overtemperature fault. FAULT is open-drain otherwise.
After a fault, the FAULT signal is latched low. FAULT is
unlatched at the beginning of the next power mode.
Fault Management
The MAX5922 offers either latched-off or auto-retry fault
management configurable by the LATCH input.
Bringing LATCH high puts the device into latch mode
while pulling LATCH low selects the autoretry option.
In latch mode, the MAX5922 turns the MOSFET off and
keeps it off after an overcurrent fault or an over-temper-
ature fault. After the fault condition goes away, recycle
the power supplies or toggle the EN pin low and high
again to unlatch the part. However, the part waits a
t
RESTART
period (1.92s) before recovering from a fault
condition and resuming normal operation.
In autoretry mode, the MAX5922 turns the MOSFET off
after an overcurrent or overtemperature fault. After the
fault condition is removed, the device waits a t
RESTART
period (1.92s) and then automatically restarts and
enters the PD detection mode (MAX5922A and
MAX5922B/MAX5922C with DET_DIS low). If DET_DIS
is high (MAX5922B/MAX5922C), the MAX5922B/
MAX5922C automatically restart into the power-up
mode after t
RESTDRT
. If the fault was due to an
overtemperature condition, the MAX5922 waits for its
die temperature to cool down below the hysteresis level
before starting the t
RESTART
time.
Detection Collision Avoidance
Detection collision avoidance is enabled by connecting
the DCA pin directly to V
DIG
and disabled by connect-
ing it to DGND. When DCA is high, the MAX5922A acti-
vates a back-off time, t
DCA
(2.8s), after every failed
detection during PD detection mode. During this back-
off time, the MAX5922A turns off the MOSFET and dri-
ves the OUT pin to high impedance. This function is
required by the IEEE 802.3af standard if the PSE
resides in a midspan system. After t
DCA
, the
MAX5922A starts a PD detection sequence. The
MAX5922B has this function internally disabled and the
MAX5922C has this function internally enabled.
