In case the FLT pin is already used for the input OVP sensing, UCC28781-Q1 provides the third and fourth OTP functions on the CS pin. The two configurations do not affect the current sense signal on the CS pin and the OPP level, because the two sensing circuits are only biased after PWML is off. Figure 7-42 shows the two application circuits. For the third OTP configuration, when the PWMH pin is pulled high, R_NTC and R_OPP form a resistor divider to create a temperature-dependent voltage signal on the CS pin. When the voltage exceeds the 1.2-V threshold sampled before the end of the demagnetization time (T_DM) for two successive cycles, the OTP fault will be triggered. The OTP sensing circuit will not affect the operation of the peak current loop, since the PWMH is pulled low in the PWML on time duration. For auto-recovery mode, the long 1.5-s timer starts and the controller stays in fault state without switching. This long recovery time provides a temperature hysteresis to help the hot-spot temperature cool down before the next V_O restart attempt. Compared with the first OTP configuration on the FLT pin, this configuration allows the OTP armed in both AAM and ABM, so the OTP can still be triggered at around 25% output load. Compared with the second OTP configuration from FLT pin, this configuration supports both auto-recovery and latch-off modes.

The fourth configuration with a small-signal PMOS is the most comprehensive way to cover a wide output load range and support both auto-recovery and latch-off modes at the same time. The RUN pin is used to bias the sensing circuit, and the PMOS gate is controlled by the PWML pin to only allow the detection to occur when PWML is low.

Figure 7-42 Two Connections to Implement OTP on the CS Pin