Datasheet

TPS2378
SLVSB99B –MARCH 2012–REVISED NOVEMBER 2013 www.ti.com
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 will be re-enabled with the inrush current limit when exiting from an over-temperature event.
Pulling DEN to VSS during powered operation causes the internal hotswap MOSFET to turn off. This feature
allows a PD with option three ORing per Figure 23 to achieve adapter priority.
The hotswap switch will be forced off under the following conditions:
1. V
APD
above V
APDEN
(~1.5 V),
2. V
(DEN –VSS)
< V
PD-DIS
when V
(VDD-VSS)
is in the operational range,
3. PD is over-temperature, or
4. 4. V
(DEN –VSS)
< PoE UVLO falling threshold (~32 V).
Startup and Power Management, CDB and T2P
CDB (converter disable) is an active-low pin that indicates when the internal hotswap MOSFET is in inrush
limiting. CDB de-asserts when inrush is over and can be used to enable a downstream converter to start up.
Common interfaces to the converter controller include the soft start or enable pins.
T2P (type 2 PSE) is an active-low multifunction pin that indicates if
[(PSE = Type_2) or (1.5 V < V
APD
)] and (pd current limit ≠ Inrush).
The APD term allows the PD to operate from an adapter at high-power if a type 2 PSE is not present, assuming
the adapter has sufficient capacity. Applications must monitor the state of T2P to detect power source transitions.
Transitions could occur when a local power supply is added or dropped, or when a PSE is enabled on the far
end. The PD may be required to adjust the load appropriately. The usage of T2P is demonstrated in Figure 1.
In order for a type 2 PD to operate at less than 13 W for the first 80 ms after power application, the various
delays must be estimated and used by the application controller to meet the requirement. The bootup time of
many application processors may be long enough to eliminate the need to do any timing.
Adapter ORing
Many PoE-capable devices are designed to operate from either a wall adapter or PoE power. A local power
solution adds cost and complexity, but allows a product to be used if PoE is not available in a particular
installation. While most applications only require that the PD operate when both sources are present, the
TPS2378 supports forced operation from either of the power sources. Figure 23 illustrates three options for diode
ORing external power into a PD. Only one option would be used in any particular design. Option 1 applies power
to the TPS2378 PoE input, option 2 applies power between the TPS2378 PoE section and the power circuit, and
option 3 applies power to the output side of the converter. Each of these options has advantages and
disadvantages. Many of the basic ORing configurations and much of the discussion contained in the application
note Advanced Adapter ORing Solutions using the TPS23753 (literature number SLVA306), apply to the
TPS2378 incorporating a DC/DC converter.
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