8.1.4 Input Capacitors

The input capacitance (C_IN) provides energy during the discontinuous portions of the switching period. For buck and buck-boost regulators, C_IN provides energy during t_ON and during t_OFF, the input voltage source charges up C_IN with the average input current (I_IN). For boost regulators, C_IN only needs to provide the ripple current due to the direct connection to the inductor. C_IN is selected given the maximum input voltage ripple (Δv_IN-PP) which can be tolerated. Δv_IN-PP is suggested to be less than 10% of the input voltage (V_IN).

An input capacitance at least 100% greater than the calculated C_IN value is recommended to account for derating due to temperature and operating voltage. When PWM dimming, even more capacitance can be helpful to minimize the large current draw from the input voltage source during the rising transistion of the LED current waveform.

The chosen input capacitors must also have the necessary RMS current rating. Ceramic capacitors are again the best choice due to their high ripple current rating, long lifetime, and good temperature performance. An X7R dieletric rating is suggested.

For most applications, it is recommended to bypass the V_IN pin with an 0.1 µF ceramic capacitor placed as close as possible to the pin. In situations where the bulk input capacitance may be far from the LM3424-Q1 device, a 10 Ω series resistor can be placed between the bulk input capacitance and the bypass capacitor, creating a 150 kHz filter to eliminate undesired high frequency noise.