The ON_OFF pin can be configured as a latch pin or OVP pin. In the latch configuration, the half-bridge converter remains off even after the faults are cleared. A new soft-start sequence will not be initiated until the latch is reset. One latch configuration is illustrated in Figure 7-20 where a large latch resistor R_L (for example, 50 kΩ) and a diode are tied to the ON_OFF pin.

Figure 7-20 ON/OFF Pin Latch Function

When any of the faults is detected including OVP, hiccup mode OCP and 150 °C OTP, the ON_OFF pin current source, I_OVL (50-µA typical), is activated, that raises the ON_OFF pin voltage quickly. As a result, the latch diode is reverse biased. The current source I_OVL remains active even if the fault is cleared because the ON_OFF pin voltage is latched above V_{ON_OFF} (1.25-V typical). To reset the latch operation, simply pulling down the UVLO pin voltage below V_{ON_OFF} disables the current source and thus the ON_OFF pin voltage falls quickly. A new soft-start sequence will be initiated as soon as the latch is reset and the faults are cleared.

Use Equation 28 to design the external voltage divider in latch mode.

Equation 28.

where

• V_F is the forward voltage drop of the latch diode
• V_{IN_L} is the desired input voltage latch threshold, and R_L is the latch resistor.

Note that the current source I_OVL does not provide any hysteresis when ON_OFF pin is configured in latch mode.

The ON_OFF pin can also be configured as an OVP pin as shown in Figure 7-21. In this configuration, the external voltage divider should be designed such that the ON_OFF pin voltage is greater than V_{ON_OFF} when an over-voltage condition occurs.

Figure 7-21 ON/OFF Pin Configured as OVP Pin

The OVP hysteresis is accomplished with the I_OVL current source. When the ON_OFF pin voltage exceeds V_{ON_OFF}, the I_OVL current source is activated which quickly raises the voltage at the pin. The half-bridge converter is turned off and the SS and SSSR capacitors are fully discharged. When the ON_OFF pin voltage falls below V_{ON_OFF}, the current source is deactivated causing the voltage at the pin to quickly fall followed by a new soft-start sequence. In addition to the OVP fault, hiccup mode and internal 150-°C thermal shutdown faults will also cause the half-bridge converter to turn off. Once the faults are cleared, a new soft-start sequence automatically begins. Because the hiccup mode or 150-°C thermal shutdown fault also activates the current source, it is important to make sure that the ON_OFF pin voltage doesn't rise above V_{ON_OFF} when the input voltage is high, which otherwise would lead to latch operation. Avoid this scenario by selecting a proper voltage divider.

Use Equation 29 and Equation 30 to select the voltage divider for the OVP configuration.

Equation 29.

where

• V_HYS(OVP) is the OVP hysteresis

Equation 30.

where

• V_IN(OFF) is the OVP rising threshold