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4. Use Equation 40 to calculate the output capacitor RMS ripple current using and verify that the ripple current

is within the capacitor ripple current rating.

LO1

CO1(RMS)

LO2

CO2(RMS)

1.89 A

0.55 A

2.53 A

0.73 A

(40)

10.2.1.2.6 Input Capacitors

A power supply input typically has a relatively high source impedance at the switching frequency. Good-quality

input capacitors are necessary to limit the input ripple voltage. As mentioned earlier, dual-channel interleaved

operation significantly reduces the input ripple amplitude. In general, the ripple current splits between the input

capacitors based on the relative impedance of the capacitors at the switching frequency.

1. Select the input capacitors with sufficient voltage and RMS ripple current ratings.

2. Worst case input ripple for a two-channel buck regulator typically corresponds to when one channel operates

at full load and the other channel is disabled or operates at no load. Use Equation 41 to calculate the input

capacitor RMS ripple current assuming a worst-case duty-cycle operating point of 50%.

CIN(RMS)

OUT1

D 1 D

7 A

0.5 1 0.5

3.5 A

(41)

3. Use Equation 42 to find the required input capacitance.

OUT1

ESR

OUT1

1 D

0.5

7 A

7.8 )

2.1MHz

120mV

7 A

˜ '

: ˜

(42)

where

• ΔVIN is the input peak-to-peak ripple voltage specification.

• RESR is the input capacitor ESR.

4. Recognizing the voltage coefficient of ceramic capacitors, select two 10-µF, 50-V, X7R, 1210 ceramic input

capacitors for each channel. Place these capacitors adjacent to the relevant power MOSFETs.

5. Use four 10-nF, 50-V, X7R, 0603 ceramic capacitors near each high-side MOSFET to supply the high di/dt

current during MOSFET switching transitions. Such capacitors offer high self-resonant frequency (SRF)

and low effective impedance above 100 MHz. The result is lower power loop parasitic inductance, thus

minimizing switch-node voltage overshoot and ringing for lower EMI signature. Refer to Figure 12-2 in

Section 12.1 for more detail.

10.2.1.2.7 Compensation Components

Choose compensation components for a stable control loop using the procedure outlined as follows.

1. Based on a specified open-loop gain crossover frequency, fC, of 60 kHz, use Equation 43 to calculate

RCOMP1, assuming an effective output capacitance of 130 µF. Select RCOMP1 of 20 kΩ.

OUT

COMP1

OUT

REF

3.3 V 7m

60kHz

130 )

0.6 V 1200 6

: ˜

(43)

2. Calculate CCOMP1 to create a zero at the higher of (1) one tenth of the crossover frequency, or (2) the load

pole. Select a CCOMP1 capacitor of 1 nF.

COMP1

1.3nF

60 kHz 20 k

(44)

LM25143

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