Datasheet

C T2P FLED
T2P
RT2P
V V V
12 1 1.1
R = = = 15.48 k
I 0.625 mA
- -
- -
W
RT2P MIN
TEMP
100 100
I I = 0.5 mA = 0.625 mA
100 CTR 100 20
@ ´ ´
- D -
OUT T2P-OUT
RT2P-OUT
T2P-OUT
V V (low)
5 0.4
I = = = 0.46 m A
R 10000
-
-
V
C
T2P From
TPS23754
R
T2P
R
T2P_OUT
Type2PSE
Indicator
V
OUT
Low= T2
TPS23754
TPS23754-1
TPS23756
SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013 www.ti.com
Figure 33. T2P Interface
1. T2P ON characteristic: I
T2P
= 2 mA minimum, V
T2P
= 1 V
2. Let V
C
= 12 V, V
OUT
= 5 V, R
T2P-OUT
= 10 kΩ, V
T2P-OUT
(low) = 400 mV max
(a)
3. The optocoupler CTR will be needed to determine R
T2P
. A device with a minimum CTR of 300% at 5 mA
LED bias current is selected. CTR will also vary with temperature and LED bias current. The strong variation
of CTR with diode current makes this a problem that requires some iteration using the CTR versus I
DIODE
curve on the optocoupler datasheet.
(a) Using the (normalized) curves, a current of 0.4 to 0.5 mA is required to support the output current at the
minimum CTR at 25°C.
(a) Pick an I
DIODE
. For example one around the desired load current.
(b) Use the optocoupler datasheet curve to determine the effective CTR at this operating current. It is
usually necessary to apply the normalized curve value to the minimum specified CTR. It might be
necessary to ratio or offset the curve readings to obtain a value that is relative to the current that the
CTR is specified at.
(c) If I
DIODE
× CTR
I_DIODE
is substantially different from I
RT2P_OUT
, choose another I
DIODE
and repeat.
(b) This manufacturer’s curves also indicate a –20% variation of CTR with temperature. The approximate
forward voltage of the optocoupler diode is 1.1 V from the datasheet.
(c) V
FLED
≉ 1.1 V
(d) Select a 15.4-kΩ resistor. Even though the minimum CTR and temperature variation were considered,
the designer might choose a smaller resistor for a little more margin.
Advanced ORing Techniques
See Advanced Adapter ORing Solutions using the TSP23753, TI document number SLVA306A for ORing
applications that also work with the TPS23754 device. The material in sections Adapter ORing and Protection,
D1 are important to consider as well. The following applications are unique to the TPS23754 device with the
introduction of PPD.
Option 2 ORing with PoE acting as a hot backup is eased by connecting PPD to V
DD
per Figure 34. This PPD
connection enables the class regulator even when APD is high. The R-Zener network (1.8 kΩ – 24 V) is the
simplest circuit that will satisfy MPS requirements, keeping the PSE online. This network may be switched out
when the APD is not powered with an optocoupler. This works best with a 48-V adapter and the APD-
programmed threshold as high as possible. An example of an adapter priority application with smooth switchover
between a 48-V adapter and PoE is shown on the right side of Figure 34. D
APD
is used to reduce the effective
APD hysteresis, allowing the PSE to power the load before V
VDD1
-V
RTN
falls too low and causes a hotswap
foldback.
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