8.2.1.1.1 Input and Output Capacitance Considerations

Input and output capacitance improves the performance of the device. The actual capacitance should be optimized for the particular application. For all applications, a 0.1-μF or greater ceramic bypass capacitor between IN and GND is recommended as close to the device as possible for local noise decoupling.

All protection circuits, including those of the TPS25820/21 device, have the potential for input voltage overshoots and output voltage undershoots. Input voltage overshoots can be caused by either of two effects. The first cause is an abrupt application of input voltage in conjunction with input power-bus inductance and input capacitance when the IN pin is high-impedance (before OUT turn-on, i.e. not connected to a Type-C sink device). Theoretically, the peak voltage is 2 times the applied voltage. The second cause is due to the abrupt reduction of output short-circuit current when the device turns off and energy stored in the input inductance drives the input voltage high. Input voltage droops may also occur with large load steps and as the output is shorted. Applications with large input inductance (for instance, connecting the evaluation board to the bench power supply through long cables) may require large input capacitance to prevent the voltage overshoot from exceeding the absolute maximum voltage of the device.

The fast current-limit speed of the TPS25820/21 device to hard output short circuits isolates the input bus from faults. However, ceramic input capacitance in the range of 1 μF to 22 μF adjacent to the input aids in both response time and limiting the transient seen on the input power bus. Output voltage undershoot is caused by the inductance of the output power bus just after a short has occurred and the device has abruptly reduced the OUT current. Energy stored in the inductance drives the OUT voltage down, and potentially negative, as it discharges. An application with large output inductance (such as from a cable) benefits from the use of a high-value output capacitor to control voltage undershoot.

Since the source is considered cold socketed when not attached to a sink, the output capacitance should be placed at the IN pin rather than the OUT pin, which has been commonly used in USB Type-A ports. A 120-μF capacitance is recommended in this situation. It is also recommended to a ceramic capacitor less than 10 μF on the OUT pin for better voltage bypass and compliance to Type-C spec.