Datasheet

PM6644 Application information
Doc ID 023203 Rev 1 21/35
3 Application information
3.1 External component selection
3.1.1 Inductor selection
Once the switching frequency is defined, inductor selection depends on the desired inductor
ripple current and load transient performance.
Low inductance means greater ripple current and may generate greater 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 current ripple ∆I
L
between 20% and 50% of the maximum output current I
LOAD
(max.). The maximum ∆I
L
occurs at the maximum input voltage. With these considerations, the inductor value can be
calculated with the following relationship:
Equation 7
where f
SW
is the switching frequency, V
IN
is the input voltage, V
OUT
is the output voltage and
∆I
L
is the selected inductor current ripple.
In order to prevent overtemperature working conditions, the inductor must be able to provide
an RMS current greater than the maximum RMS inductor current I
LRMS
:
Equation 8
where ∆Ι
L
(max.) is the maximum current ripple:
Equation 9
If hard saturation inductors are used, the inductor saturation current should be much greater
than the maximum inductor peak current Ipeak:
Equation 10
Using soft saturation inductors it is possible to choose inductors with a saturation current
limit at nearly Ipeak. In Ta ble 1 0 there is a list of some inductor part numbers.
L
V
IN
V
OUT
–
f
sw
I
L
∆⋅
------------------------------
V
OUT
V
IN
--------------
⋅=
I
LRMS
I
LOAD
max()()
2
∆I
L
max()()
2
12
---------------------------------+=
∆I
L
max()
V
INmax
V
OUT
–
f
sw
L⋅
---------------------------------------
V
OUT
V
INmax
-------------------
⋅=
Ipeak I
LOAD
max()
∆I
L
max()
2
-------------------------+=