Datasheet
Time: 200s/div
10V/div
500mA/div
I
PI
16-V Input Step
Recovery from PI Dropout
V
(VDD-VSS)
C
IN
Completes Charge
While Converter Operates
V
(RTN-VSS)
V
RTN
< 12V @ 800s
20V/div
TPS23751
TPS23752
SLVSB97C –JULY 2012–REVISED JANUARY 2014
www.ti.com
Figure 25. Response to Output Short Circuit
The PD control has a thermal sensor that protects the internal hotswap MOSFET. Conditions like startup or
operation into a V
DD
-to-RTN short cause high power dissipation in the MOSFET. An over-temperature shutdown
(OTSD) turns off the hotswap MOSFET and class regulator, which are restarted after the device cools. The
hotswap MOSFET is re-enabled with the inrush current limit when exiting from an over-temperature event.
Pulling DEN to V
SS
during powered operation causes the internal hotswap MOSFET to turn off. The hotswap
switch is forced off under the following conditions:
1. V
APD
above V
APDEN
(approximately 1.5V)
2. V
(DEN –VSS)
< V
PD_DIS
when V
VDD–
V
VSS
is in the operational range,
3. PD OTSD is active, or
4. V
(DEN –VSS)
< PoE UVLO falling threshold (approximately 32 V).
Sleep Mode Operation (TPS23752 only)
These features implement a Sleep Mode, permitting power savings at night (or some other system-driven
criteria) by turning the active load circuits off while maintaining enough functionality for the PD to respond to a
local power-up request.
The Sleep Mode is initiated by command of a local device controller (microprocessor) when the SLPb input is
driven low. Sleep Mode is latched by this event, the converter is disabled, V
DD
regulates V
C
to 12.8V, and the
LED output is active. The LED output sinks current to light an LED biased from the V
C
pin with R
LED
as shown in
Figure 29. LED can alert a local user that Sleep Mode is active. The TPS23752 signals the PSE that it wants to
remain powered during sleep by drawing enough current to satisfy the IEEE 802.3at DC MPS requirements. If
MODE was low when SLPb fell, a pulsed V
DD
current-draw scheme is implemented; otherwise a DC current is
drawn. The input current consists of the TPS23752 bias currents and the LED sink current, assuming no
additional loading on V
C
or V
B
. The MPS current draw is inhibited when APD is active. A local pushbutton switch
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