LMZ10504EXT datasheet(9/22 Pages) NSC

9 / 22 page

A second criteria before finalizing the C

in bypass capacitor is

the RMS current capability. The necessary RMS current rat-

ing of the input capacitor to a buck regulator can be estimated

With this high AC current present in the input capacitor, the

RMS current rating becomes an important parameter. The

maximum input capacitor ripple voltage and RMS current oc-

cur at 50% duty cycle. Select an input capacitor rated for at

least the maximum calculated I

Cin(RMS).

Additional bulk capacitance with higher ESR may be required

to damp any resonance effects of the input capacitance and

parasitic inductance.

Output Capacitor Selection

In general, 22 µF to 100 µF high quality dielectric (X5R, X7R)

ceramic capacitor rated at twice the maximum output voltage

is sufficient given the optimal high frequency characteristics

and low ESR of ceramic dielectrics. Although, the output ca-

pacitor can also be of electrolytic chemistry for increased

capacitance density.

Two output capacitance equations are required to determine

the minimum output capacitance. One equation determines

the output capacitance (C

O) based on PWM ripple voltage.

The second equation determines C

O based on the load tran-

sient characteristics. Select the largest capacitance value of

the two.

The minimum capacitance, given the maximum output volt-

age ripple (

ΔV

OUT) requirement, is determined by the follow-

ing equation:

Where the peak to peak inductor current ripple (

Δi

L) is equal

to:

ESR is the total output capacitor ESR, L is the inductance

value of the internal power inductor, where L = 1.5 µH, and

SW = 1 MHz. Therefore, per the design example:

The minimum output capacitance requirement due to the

PWM ripple voltage is:

Three miliohms is a typical R

ESR value for ceramic capacitors.

The following equation provides a good first pass capacitance

requirement for a load transient:

Where I

step is the peak to peak load step, VFB = 0.8V, and

ΔV

o_tran is the maximum output voltage deviation, which is

±20 mV.

Therefore the capacitance requirement for the given design

parameters is:

In this particular design the output capacitance is determined

by the load transient requirements.

Table 1 lists some examples of commercially available ca-

pacitors that can be used with the LMZ10504EXT.

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