SNVS690I January 2011 – August 2021 LMZ14201H
The LMZ14201H module contains an internal 0.47 µF input ceramic capacitor. Additional input capacitance is required external to the module to handle the input ripple current of the application. This input capacitance should be as close as possible to the module. Input capacitor selection is generally directed to satisfy the input ripple current requirements rather than by capacitance value.
Worst-case input ripple current rating is dictated by Equation 10:
(As a point of reference, the worst-case ripple current will occur when the module is presented with full load current and when VIN = 2 × VO).
Recommended minimum input capacitance is 10-uF X7R ceramic with a voltage rating at least 25% higher than the maximum applied input voltage for the application. TI also recommends to pay attention to the voltage and temperature deratings of the capacitor selected. Also note ripple current rating of ceramic capacitors may be missing from the capacitor data sheet and you may have to contact the capacitor manufacturer for this rating.
If the system design requires a certain maximum value of input ripple voltage ΔVIN to be maintained then Equation 11 may be used.
If ΔVIN is 1% of VIN for a 24-V input to 12-V output application this equals 240 mV and fSW = 400 kHz.
CIN≥ 1 A × 12 V/24 V × (1– 12 V/24 V) / (400000 × 0.240 V)
CIN≥ 2.6 μF
Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input capacitance and parasitic inductance of the incoming supply lines.