8.2.2.16 T2P Pin Interface
The T2P pin is an active low, open-drain output indicating a high-power source is available. An optocoupler is typically used to interface with the T2P pin to signal equipment on the secondary side of the converter of T2P status. Optocoupler current-gain is referred to as current transfer ratio (CTR), which is the ratio of transistor collector current to LED current. To preserve efficiency, TI recommends a high-gain optocoupler ( 250% ≤ CTR ≤ 500%, or 300% ≤ CTR ≤ 600% ) along with a high-impedance (for example, CMOS) receiver. Design of the T2P optocoupler interface can be accomplished as follows:
- T2P ON characteristic: IT2P = 2 mA minimum, VT2P = 1 V
- Let VC = 12 V, VOUT = 5 V, RT2P-OUT = 10 kΩ, VT2P-OUT (low) = 400 mV maximum
![TPS23754 TPS23754-1 TPS23756 TPS23754 TPS23754-1 TPS23756 il_eq18_lvs885.gif](/ods/images/SLVS885I/il_eq18_lvs885.gif)
- The optocoupler CTR will be needed to determine RT2P. 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 IDIODE curve on the optocoupler data sheet.
- 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.
- Pick an IDIODE. For example one around the desired load current.
- Use the optocoupler data sheet 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.
- If IDIODE × CTRI_DIODE is substantially different from IRT2P_OUT, choose another IDIODE and repeat.
- 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.
![TPS23754 TPS23754-1 TPS23756 TPS23754 TPS23754-1 TPS23756 il_eq19_lvs885.gif](/ods/images/SLVS885I/il_eq19_lvs885.gif)
- VFLED ≉ 1.1 V
![TPS23754 TPS23754-1 TPS23756 TPS23754 TPS23754-1 TPS23756 il_eq20_lvs885.gif](/ods/images/SLVS885I/il_eq20_lvs885.gif)
- 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.