8.3.3 Fall Time (tFALL) and Quick Output Discharge (QOD)
The TPS22917 device includes a QOD pin that can be configured in one of three ways:
- QOD pin shorted to VOUT pin. Using this method, the discharge rate after the switch becomes disabled is controlled with the value of the internal resistance QOD.
- QOD pin connected to VOUT pin using an external resistor RQOD. After the switch becomes disabled, the discharge rate is controlled by the value of the total discharge resistance. To adjust the total discharge resistance, Equation 3 can be used:
= QOD + RQOD
- RDIS = Total output discharge resistance (Ω)
- QOD = Internal pulldown resistance (Ω)
- RQOD = External resistance placed between the VOUT and QOD pins (Ω)
- QOD pin is unused and left floating. Using this method, there will be no quick output discharge functionality, and the output will remain floating after the switch is disabled.
The fall times of the device depend on many factors including the total discharge resistance (RDIS) and the output capacitance (CL). To calculate the approximate fall time of VOUT use Equation 4.
= 2.2 × (RDIS
) × CL
- tFALL = Output Fall Time from 90% to 10% (μs)
- RDIS = Total QOD + RQOD Resistance (Ω)
- RL = Output Load Resistance (Ω)
- CL = Output Load Capacitance (μF)