PM6681A STMicroelectronics, PM6681A Datasheet - Page 30

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

Design guidelines
9
9.1
9.2
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Design guidelines
The design of a switching section starts from two parameters:
●
●
Switching frequency
It's possible to set 3 different working frequency ranges for the two sections with FSEL pin
(table 1).
Switching frequency mainly influences two parameters:
●
●
Inductor selection
Once that switching frequency is defined, inductor selection depends on the desired
inductor ripple current and load transient performance.
Low inductance means great ripple current and could generate great output noise. On the
other hand, low inductor values involve fast load transient response.
A good compromise between the transient response time, the efficiency, the cost and the
size is to choose the inductor value in order to maintain the inductor ripple current ∆I
between 20 % and 50 % of the maximum output current I
occurs at the maximum input voltage. With this considerations, the inductor value can be
calculated with the following relationship:
Equation 13
where f
∆IL is the selected inductor ripple current.
In order to prevent overtemperature working conditions, inductor must be able to provide an
RMS current greater than the maximum RMS inductor current I
Equation 14
Where ∆I
Input voltage range: in notebook applications it varies from the minimum battery
voltage, V
Maximum load current: it is the maximum required output current, I
Inductor size: for a given saturation current and RMS current, greater frequency allows
to use lower inductor values, which means smaller size.
Efficiency: switching losses are proportional to frequency. High frequency generally
involves low efficiency.
sw
L(max)
is the switching frequency, V
INmin
is the maximum ripple current:
to the AC adapter voltage, V
I
LRMS
=
L
I (
=
LOAD
IN
V
IN
f
sw
is the input voltage, V
(max))
−
×
V
∆
OUT
I
L
INmax
2
×
+
V
(
V
OUT
∆
.
IN
I
L
(max))
12
LOAD(max)
OUT
2
LRMS
is the output voltage and
. The maximum ∆I
:
LOAD(max)
.
PM6681A
L
L