LM5145 datasheet(35/73 Pages) TI2

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

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

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

8.1.1.4 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 power 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. Use Equation 15 to calculate the input capacitor RMS current

for a single-channel buck regulator.

CIN,rms

OUT

1 D

(15)

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

capacitors is approximately equal to 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 ceramic capacitors. Neglecting inductor ripple current, the input capacitors source current of amplitude

(IOUT − IIN) during the D interval and sink 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. Following, the resultant capacitive

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

Equation 16 to calculate the peak-to-peak ripple voltage amplitude.

OUT

ESR

(16)

Use Equation 17 to calculate the input capacitance required for a particular load current, based on an input

voltage ripple specification of ΔVIN.

OUT

ESR

OUT

1 D

˜ '

(17)

Place low-ESR ceramic capacitors in parallel with a 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. While dependent on switching frequency and load current level, four 10μF, 50V, X7R, ceramic

decoupling capacitors are usually sufficient for 12V battery automotive applications. As outlined in Section

8.1.1.5, select the input bulk capacitor equal to three to four times the derated ceramic value and make sure the

bulk capacitor is rated for the full 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 previous equations represent valid

calculations when one channel is disabled and the other channel is loaded.

8.1.1.5 EMI Filter

Switching regulators exhibit a negative input impedance characteristic, which is lowest at the minimum input

voltage and full load. An underdamped LC filter exhibits a high output impedance at the resonant frequency

of the filter. For stability, the filter output impedance, approximated by the characteristic impedance of the LC

components, must be less than the absolute value of the regulator input impedance.

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