Datasheet

PM6641 Components selection
Doc ID 13510 Rev 3 35/47
8 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
V
INmin
- V
OUT
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, ΔI
L,MAX
, occurs at the maximum input voltage.
With these considerations, the inductance value can be calculated with the following
expression:
Equation 12
where f
SW
is the switching frequency, V
IN
is the input voltage, V
OUT
is the output voltage and
ΔI
L
is the inductor current ripple.
IN
OUT
V
V
D =
IN
OUT
L
OUTIN
V
V
Ifsw
VV
L ⋅
Δ⋅
−
=