8.2.1.3 Input Capacitor

An input capacitor is necessary to ensure that V_IN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, voltage rating, RMS current rating, and equivalent series inductance (ESL). The input voltage rating is specifically stated by the capacitor manufacturer. Make sure to check any recommended deratings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The input capacitor maximum RMS input current rating (I_RMS-IN) must be greater than:

Equation 10.

Neglecting inductor ripple simplifies the above equation to:

Equation 11.

It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always calculate the RMS at the point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. As a rule of thumb, a large leaded capacitor will have high ESL and a 1206 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LMR10530, leaded capacitors may have an ESL so large that the resulting impedance (2 πfL) will be higher than that required to provide stable operation. TI strongly recommends usin ceramic capacitors due to their low ESR and low ESL. A 22-µF multilayer ceramic capacitor (MLCC) is a good choice for most applications. In cases where large capacitance is required, use surface mount capacitors such as Tantalum capacitors and place at least a 4.7-µF ceramic capacitor close to the V_IN pin. For MLCCs TI recommends using X7R or X5R dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating conditions.