The Quick Output Discharge (QOD) transistor is engaged indefinitely whenever the switch is disabled and the recommended V_BIAS voltage is met. During this state, the QOD resistance (R_PD) discharges V_OUT to GND. TI does not recommend to apply a continuous DC voltage to OUT when the device is disabled.

The QOD transistor can remain active for a short period of time even after V_BIAS loses power. This brief period of time is defined as t_DIS. For best results, TI recommends the device get disabled before V_BIAS goes below the minimum recommended voltage. The waveform in Figure 9-6 shows the behavior when power is applied and then removed in a typical application.

Figure 9-6 Power Applied and Then Removed in a Typical Application

At the end of the t_DIS time, it is not assured that V_OUT is 0 V because the final voltage is dependent upon the initial voltage and the C_L capacitor. The final V_OUT can be calculated with Equation 3 for a given initial voltage and C_L capacitor.

Equation 3.

where

• V_f is the final V_OUT voltage
• V_o is the initial V_OUT voltage
• R is the the value of the output discharge resistor, R_PD (see the Electrical Characteristics table)
• C is the output bulk capacitance on OUT