10.2.2.1 Input Capacitor Selection

Because the nature of the buck converter has a pulsating input current, a low ESR input capacitor is required.

For most applications, the input capacitor that is integrated into the TPS8268x is sufficient. If the application exhibits a noisy or erratic switching frequency, experiment with additional input ceramic capacitance to find a remedy.

The TPS8268x uses a tiny ceramic input capacitor. When a ceramic capacitor is combined with trace or cable inductance, such as from a wall adapter, a load step at the output can induce ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop instability or can even damage the part. In this circumstance, additional "bulk" capacitance, such as electrolytic or tantalum, should be placed between the input of the converter and the power source lead to reduce ringing that can occur between the inductance of the power source leads and C_I.

10.2.2.2 Output Capacitor Selection

The advanced fast-response voltage mode control scheme of the TPS8268x allows the use of tiny ceramic output capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. For most applications, the output capacitor integrated in the TPS8268x is sufficient. An additional output capacitor may be used for the purpose of improving AC voltage accuracy during large load transients.

To further reduce the voltage drop during load transients, additional external output capacitance up to 30µF can be added. A low ESR multilayer ceramic capacitor (MLCC) is suitable for most applications. The total effective output capacitance must remain below 30µF.

As the device operates in PWM mode, the overall output voltage ripple is the sum of the voltage step that is caused by the output capacitor´s ESL and the ripple current that flows through the output capacitor´s impedance.

Because the damping factor in the output path is directly related to several resistive parameters (e.g. inductor DCR, power-stage r_DS(on), PCB DC resistance, load switches r_DS(on) …) that are temperature dependant, the converter´s small and large signal behavior should be checked over the input voltage range, load current range and temperature range.

The easiest test is to evaluate, directly at the converter’s output, the following items:

• efficiency
• load transient response
• output voltage ripple

During the recovery time from a load transient, the output voltage can be monitored for settling time, overshoot or ringing that helps judge the converter’s stability. Without any ringing, the loop typically has more than 45° of phase margin.