Datasheet

t-Time-200 s/divm
I
PI
16VInputstep
V
RTN-VSS
V
VDD-VSS
C
IN
completescharge
whileconverteroperates
RecoveryfromPIdropout
V
RTN
<12V@400 sm
20V/div
10V/div
500mA/div
TPS23754
TPS23754-1
TPS23756
SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013 www.ti.com
PD Hotswap Operation
IEEE 802.3at has taken a new approach to PSE output limiting. A type 2 PSE must meet an output current
versus time template with specified minimum and maximum sourcing boundaries. The peak output current may
be as high as 50 A for 10 μs or 1.75 A for 75 ms. This makes robust protection of the PD device even more
important than it was in IEEE 802.3-2008.
The internal hotswap MOSFET is protected against output faults and input voltage steps with a current limit and
deglitched (time-delay filtered) foldback. An overload on the pass MOSFET engages the current limit, with V
RTN
-
V
VSS
rising as a result. If V
RTN
rises above about 12 V for longer than about 400 μs, the current limit reverts to
the inrush value, and turns the converter off. The 400 μs deglitch feature prevents momentary transients from
causing a PD reset, provided that recovery lies within the bounds of the hotswap and PSE protection. Figure 26
shows an example of recovery from a 16 V PSE rising voltage step. The hotswap MOSFET goes into current
limit, overshooting to a relatively low current, recovers to about 950-mA full current limit, and charges the input
capacitor while the converter continues to run. The MOSFET did not go into foldback because V
RTN
-V
VSS
was
below 12 V after the 400 μs deglitch.
Figure 26. Response to PSE Step Voltage
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 overtemperature shutdown
(OTSD) turns off the hotswap MOSFET and class regulator, which are restarted after the device cools. The
hotswap MOSFET will be re-enabled with the inrush current limit when exiting from an overtemperature event.
Pulling DEN to V
SS
during powered operation causes the internal hotswap MOSFET to turn off. This feature
allows a PD with Option three ORing per Figure 27 to achieve adapter priority. Care must be taken with
synchronous converter topologies that can deliver power in both directions.
The hotswap switch will be forced off under the following conditions:
1. V
APD
above V
APDEN
(about 1.5 V)
2. V
DEN
< V
PD-DIS
when V
VDD
– V
VSS
is in the operational range
3. PD over-temperature
4. (V
VDD
– V
VSS
) < PoE UVLO (about 30.5 V)
Converter Controller Features
The TPS23754 DC/DC controller implements a typical current-mode control as shown in the functional block
diagram. Features include oscillator, overcurrent and PWM comparators, current-sense blanker, dead-time
control, softstart, and gate driver. In addition, an internal slope-compensation ramp generator, frequency
synchronization logic, thermal shutdown, and startup current source with control are provided.
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