LM5145 datasheet(43/73 Pages) TI2

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According to the design tool from the capacitor vendor, these capacitors are each effectively 32µF and

41µF at 5V and 3.3V DC voltage, respectively.

3. Use Equation 35 to estimate the peak-to-peak output voltage ripple of channel 1 at nominal input voltage.

(35)

where

• RESR is the effective equivalent series resistance (ESR) of the output capacitors.

• 128µF is the total effective (derated) ceramic output capacitance at 5V.

4. Use Equation 36 to calculate the output capacitor RMS ripple current at maximum input voltage. Verify that

the output capacitor RMS ripple current at maximum input voltage is within the capacitor ripple current rating.

(36)

8.2.1.2.6 Ceramic 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 37 to calculate the input

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

(37)

3. Use Equation 38 to find the required input capacitance.

(38)

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 four 10µF, 50V, X7R, 1210 ceramic input

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

5. Use four 10nF, 50V, 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 100MHz. The result is lower power loop parasitic inductance, thus minimizing

switch-node voltage overshoot and ringing for lower EMI signature. Refer to Figure 8-25 and Figure 8-27 for

additional context.

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