10.2.2.6 Output Capacitor

The second energy storing device is the output capacitor. When selecting output capacitors for large pulsed loads, the magnitude and duration of the pulsing current, together with the ripple voltage specification, determine the choice of the output capacitor. Both the ESR of the capacitor and the charge stored in the capacitor each cycle contribute to the output voltage ripple. The ripple due to the charge is approximately what results from Equation 8.

Equation 8.

where

• I_PULSE and t_ON are the peak current and on time during transmission burst.
• I_STANDBY is the current in standby mode.

The above is a worst-case approximation assuming all the pulsing energy comes from the output capacitor.

The ripple due to the capacitor ESR is defined by Equation 9.

Equation 9.

High capacitance values and low ESR can lead to instability in some internally compensated boost converters. The internal loop compensation of the TPS61251 device is optimized to be stable with output capacitor values greater than 150 μF with very low ESR.

Because big bulk capacitors can not be placed very close to the IC, it is required to put a small ceramic capacitor of about 4.7 µF as close as possible to the output terminals. This will reduce parasitic effects that can influence the functionality of the converter.