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:

Figure 32. 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 maximum
   1. 
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 data sheet.
   1. 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.
      1. Pick an I_DIODE. For example one around the desired load current.
      2. 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.
      3. If I_DIODE × CTR_{I_DIODE} is substantially different from I_{RT2P_OUT}, choose another I_DIODE and repeat.
   2. 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.
      
   3. V_FLED ≉ 1.1 V
      
   4. 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.