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dominates. However, depending on package and voltage rating of the ceramic capacitor, the effective

capacitance can drop quite significantly with applied DC voltage and operating temperature.

Ignoring the ESR term in Equation 15 gives a quick estimation of the minimum ceramic capacitance necessary

to meet the output ripple specification. Two to four 47-µF, 10-V, X7R capacitors in 1206 or 1210 footprint is a

common choice for a 5-V output. Use Equation 16 to determine if additional capacitance is necessary to meet

the load-off transient overshoot specification.

A composite implementation of ceramic and electrolytic capacitors highlights the rationale for paralleling

capacitors of dissimilar chemistries yet complementary performance. The frequency response of each capacitor

is accretive in that each capacitor provides desirable performance over a certain portion of the frequency range.

While the ceramic provides excellent mid- and high-frequency decoupling characteristics with its low ESR and

ESL to minimize the switching frequency output ripple, the electrolytic device with its large bulk capacitance

provides low-frequency energy storage to cope with load transient demands.

9.1.1.3 Input Capacitors

Input capacitors are necessary to limit the input ripple voltage to the buck power stage due to switching-

frequency AC currents. TI recommends using X7S or X7R dielectric ceramic capacitors to provide low

impedance and high RMS current rating over a wide temperature range. To minimize the parasitic inductance

in the switching loop, position the input capacitors as close as possible to the drain of the high-side MOSFET

and the source of the low-side MOSFET. The input capacitor RMS current for a single-channel buck regulator is

given by Equation 17.

CIN,rms

OUT

1 D

(17)

The highest input capacitor RMS current occurs at D = 0.5, at which point, the RMS current rating of the input

capacitors must be greater than half the output current.

Ideally, the DC component of input current is provided by the input voltage source and the AC component by the

input filter capacitors. Neglecting inductor ripple current, the input capacitors source current of amplitude (IOUT −

IIN) during the D interval and sinks IIN during the 1−D interval. Thus, the input capacitors conduct a square-wave

current of peak-to-peak amplitude equal to the output current. It follows that the resultant capacitive component

of AC ripple voltage is a triangular waveform. Together with the ESR-related ripple component, the peak-to-peak

ripple voltage amplitude is given by Equation 18.

OUT

ESR

(18)

The input capacitance required for a particular load current, based on an input voltage ripple specification of

ΔVIN, is given by Equation 19.

OUT

ESR

OUT

1 D

˜ '

(19)

Low-ESR ceramic capacitors can be placed in parallel with higher valued bulk capacitance to provide optimized

input filtering for the regulator and damping to mitigate the effects of input parasitic inductance resonating

with high-Q ceramics. One bulk capacitor of sufficiently high current rating and four 10-μF 50-V X7R ceramic

decoupling capacitors are usually sufficient for 12-V battery automotive applications. Select the input bulk

capacitor based on its ripple current rating and operating temperature range.

Of course, a two-channel buck regulator with 180° out-of-phase interleaved switching provides input ripple

current cancellation and reduced input capacitor current stress. The above equations represent valid calculations

when one output is disabled and the other output is fully loaded.

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LM5148

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