PM6686 STMicroelectronics, PM6686 Datasheet - Page 38

• Manufacturer Part Number: PM6686
• Description: Dual Step-down Controller With Adjustable Voltages, Adjustable Ldo And Auxiliary Charge Pump Controller For Notebook
• Manufacturer: STMicroelectronics

Application information
10
10.1
10.1.1
38/50
Application information
External components selection
Inductor
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 Δ I
between 20% and 50% of the maximum output current ILOAD (max). The maximum Δ I
occurs at the maximum input voltage. With these considerations, the inductor value can be
calculated with the following relationship:
Equation 8
Where f
and Δ I
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 9
Where Δ I
Equation 10
If hard saturation inductors are used, the inductor saturation current should be much greater
than the maximum inductor peak current Ipeak:
Equation 11
Using soft-saturation inductors it’s possible to choose inductors with saturation current limit
nearly to Ipeak.
Below there is a list of some inductor manufacturers.
L
SW
is the selected inductor ripple current.
L
(max) is the maximum ripple current:
is the switching frequency, V
I
LRMS
Δ
I
Ipeak
Doc ID 15281 Rev 4
L
(max)
=
L
I (
=
=
LOAD
=
I
V
LOAD
IN
f
IN
V
sw
IN
(max))
−
is the input voltage, V
×
max
(max)
V
f
Δ
sw
OUT
I
L
−
×
2
L
V
×
+
+
OUT
V
Δ
(
V
OUT
Δ
I
IN
L
I
×
L
(max)
2
(max))
V
12
V
IN
OUT
max
2
LRMS
OUT
:
is the output voltage
PM6686
L
L