10.2.2 Output Capacitor Selection

The advanced, fast-response, voltage mode, control scheme of the TPS8269x allows the use of a tiny ceramic output capacitor (C_O). For most applications, the output capacitor integrated in the TPS8269x is sufficient.

At nominal load current, 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 ESL and the ripple current that flows through the output capacitor impedance. At light loads, the output capacitor limits the output ripple voltage and provides holdup during large load transitions.

The TPS8269x is designed as a Point-Of-Load (POL) regulator, to operate stand-alone without requiring any additional capacitance. Adding a 4.7μF ceramic output capacitor (X7R or X5R dielectric) generally works from a converter stability point of view, helps to minimize the output ripple voltage in PFM mode and improves the converter's transient response under when input and output voltage are close together.

For best operation (i.e. optimum efficiency over the entire load current range, proper PFM/PWM auto transition), the TPS8269xSIP requires a minimum output ripple voltage in PFM mode. The typical output voltage ripple is ca. 1% of the nominal output voltage V_O. The PFM pulses are time controlled resulting in a PFM output voltage ripple and PFM frequency that depends (first order) on the capacitance seen at the MicroSiP^TM DC/DC converter's output.

In applications requiring additional output bypass capacitors located close to the load, care should be taken to ensure proper operation. If the converter exhibits marginal stability or erratic switching frequency, experiment with additional low value series resistance (e.g. 50 to 100mΩ) in the output path to find a remedy.

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

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

• PFM/PWM efficiency
• PFM/PWM and forced PWM load transient response

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 has usually more than 45° of phase margin.