Datasheet
Table Of Contents
- - - -
T2 P-OUT
T2P- MIN T2P
C T 2P FLED
T2 P
T2 P
I
0.46 mA
I = = = 0.46 mA, Select I = 1 mA
CTR 1.00
V V V 12 V 0.26 V 1.1 V
R = = = 10.6 kΩ
I 1mA
-
-
-
-
-
= = =
OUT T2P OUT
T2P OUT
T2P OUT
V V
5 0.4
I 0.46 mA
R 10000
V
C
V
OUT
R
T2P
I
T2P-OUT
R
T2P-OUT
V
T2P-OUT
V
Indicates
Type 2
T2P-OUT
T2P From
TPS2378
I
T2P
RTN
CDB
SS
GND
UCC3809
TPS2378
C
SS
TPS2378
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SLVSB99B –MARCH 2012–REVISED NOVEMBER 2013
Figure 24. CDB Interface
APD Pin Divider Network, R
APD1
, R
APD2
The APD pin can be used to disable the TPS2378 internal hotswap MOSFET, giving the adapter source priority
over the PoE. For an example calculation, see literature number SLVA306A.
T2P Pin Interface
The T2P pin is an active-low, open-drain output which indicates that a high power source is available. An
optocoupler can interface the T2P pin to circuitry on the secondary side of the converter. A high-gain optocoupler
and a high-impedance (e.g., CMOS) receiver are recommended. Design of the T2P optocoupler interface can be
accomplished as follows:
Figure 25. T2P Interface
1. As shown in Figure 25, let V
C
= 12 V, V
OUT
= 5 V, R
T2P-OUT
= 10 kΩ, V
T2P-OUT
= 400 mV
2. The optocoupler current transfer ratio, CTR, will be needed to determine R
T2P
. A device with a minimum CTR
of 100% at 1 mA LED bias current, I
T2P
, is selected. Note that in practice, CTR will vary with temperature and
LED bias current and aging. These variations may require some iteration using the CTR-versus- I
DIODE
curve
on the optocoupler data sheet.
(a) The approximate forward voltage of the optocoupler diode, V
FWLED
, is 1.1 V from the data sheet.
(b)
(c) Select a 10.7 kΩ resistor
Thermal considerations and OTSD
Sources of nearby local PCB heating should be considered during the thermal design. Typical calculations
assume that the TPS2378 is the only heat source contributing to the PCB temperature rise. It is possible for a
normally operating TPS2378 device to experience an OTSD event if it is excessively heated by a nearby device.
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