8.2.1.4 Output Capacitor

The output capacitor is selected based upon the desired output ripple and transient response. The LMR12015/20's loop compensation is designed for ceramic capacitors. A minimum of 22 µF is required at 2 MHz (33 uF at 1 MHz) while 47 – 100 µF is recommended for improved transient response and higher phase margin. The output voltage ripple of the converter is:

Equation 24.

When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output ripple is approximately sinusoidal and 90° phase shifted from the switching action. Another benefit of ceramic capacitors is their ability to bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not.

The transient response is determined by the speed of the control loop and the ability of the output capacitor to provide the initial current of a load transient. Capacitance can be increased significantly with little detriment to the regulator stability. However, increasing the capacitance provides dimininshing improvement over 100 uF in most applications, because the bandwidth of the control loop decreases as output capacitance increases. If improved transient performance is required, add a feed forward capacitor. This becomes especially important for higher output voltages where the bandwidth of the LMR12015/20 is lower. See Feedforward Capacitor (Optional) and Frequency Synchronization sections.

Check the RMS current rating of the capacitor. The RMS current rating of the capacitor chosen must also meet the following condition:

Equation 25.

where

• I_OUT is the output current, and
• r is the ripple ratio.