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TPS23757
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SLVS948D – JULY 2009 – REVISED NOVEMBER 2013
5 - 0.4
(a)
I APb =
V OUT - V APb (low)
R APb
= = 0.46 mA
10000
3. The optocoupler CTR will be needed to determine R APb . 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 data sheet.
(a) Using the (normalized) curves, a current of 0.4 mA 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 RAPb_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 data sheet.
I APb @ I MIN ′ = 0.5 mA ′
100 100
100 - D CTR TEMP 100 - 20
= 0.625 mA
I APb
(c) V FLED ? 1.1 V
V C - V APb - V FLED 12 - 1 - 1.1
R APb = = = 15.48 k W
0.625 mA
(d) Select a 15.4 k ? 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 literature number SLVA306A for ORing
applications that also work with the TPS23757. The material in sections Adapter ORing and Protection, D1 are
important to consider as well. The following applications are supported 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 29 . This PPD
connection enables the class regulator even when APD is high. The R-Zener network (1.8 k ? – 24 V) is the
simplest circuit that will satisfy MPS requirements, keeping the PSE online. This network may be switched out
when 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 29 . 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.
C VDD
10nF
D VDD
D VDD
PPD
DEN
For 48V
Adapter
PPD
DEN
D A
CLS
V SS
C IN
CLS
V SS
C IN
D A
Adapter
R APD1
APD
Figure 29. Option 2 PoE Backup ORing
APD
Adapter
Copyright ? 2009–2013, Texas Instruments Incorporated
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