PM6641 datasheet(35/47 Pages) STMICROELECTRONICS

35 / 47 page

PM6641

Components selection

35/47

Components selection

The PM6641 switching regulator sections are buck converters employing a constant

frequency, current mode PWM current loop (see Chapter 7.3: SW regulators control loop on

page 24

section for details).

The duty-cycle of the buck converter is, in steady-state conditions, given by

Equation 11

The switching frequency directly affects two parameters:

●

Inductor size: greater frequencies mean smaller inductances. In notebook applications,

real estate solutions (i.e. low-profile power inductors) are mandatory also with high

saturation and root mean square (RMS) currents.

●

Efficiency: switching losses are proportional to the frequency. Generally, higher

frequencies imply lower efficiency.

8.1

Inductor selection

Once the switching frequency has been defined, the inductance value depends on the

desired inductor current ripple. Low inductance value means great ripple current that brings

to poor efficiency and great output noise. On the other hand a great current ripple is

desirable for fast transient response when a load step is applied.

Otherwise, great inductance brings to good efficiency but the load transient response is

critical, especially if VINmin - VOUT is little. The product of the output capacitor’s ESR

multiplied by the inductor ripple current must be taken in consideration; the PM6641

switching regulators current loop doesn’t need a minimum output ripple in order to work

properly, so a ceramic output capacitor can be considered a good choice.

A good trade-off between the transient response time, the efficiency, the cost and the size is

choosing the inductance value in order to maintain the inductor ripple current between 20 %

and 50 % (usually 30 %) of the maximum output current.

The maximum inductor current ripple,

∆I

L,MAX, occurs at the maximum input voltage.

With these considerations, the inductance value can be calculated with the following

expression:

Equation 12

where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage and

∆I

L is the inductor current ripple.

OUT

fsw

⋅

∆

⋅

−

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