8.2.2.13 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 (for example, CMOS) receiver are recommended. Design of the T2P optocoupler interface can be accomplished as follows:

Figure 34. T2P Interface

1. As shown in Figure 34, let V_C = 12 V, V_OUT = 5 V, R_T2P-OUT = 10 kΩ, V_T2P = 260 mV, V_T2P-OUT = 400 mV.
Equation 15.
2. The optocoupler current transfer ratio, CTR, is not 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 varies with temperature, LED bias current, and aging. These variations may require some iteration using the CTR-versus- I_DIODE curve on the optocoupler data sheet.
Equation 16.