9.2.2.1 Input and Output Capacitance

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

All protection circuits such as the TPS2069D has 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 terminal is high impedance (before turnon). Theoretically, the peak voltage is 2× the applied. 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 device output is shorted. Applications with large input inductance (for example, connecting the evaluation board to the bench power-supply through long cables) may require large input capacitance reduce the voltage overshoot from exceeding the absolute maximum voltage of the device. The fast current limit speed of the 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 device input aids in both speeding the response time and limiting the transient seen on the input power bus. Momentary input transients to 6.5 V are permitted.

Output voltage undershoot is caused by the inductance of the output power bus just after a short has occurred and the TPS2069D has abruptly reduced OUT current. Energy stored in the inductance drives the OUT voltage down and potentially negative as it discharges. Applications with large output inductance (such as from a cable) benefit from use of a high-value output capacitor to control the voltage undershoot. When implementing USB standard applications, a 120 µF minimum output capacitance is required. Typically a 150-µF electrolytic capacitor is used, which is sufficient to control voltage undershoots. However, if the application does not require 120 µF of capacitance, and there is potential to drive the output negative, then TI recommends a minimum of 10-µF ceramic capacitance on the output. The voltage undershoot should be controlled to less than 1.5 V for 10 µs.