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PM6644 Datenblatt(PDF) 21 Page - STMicroelectronics

Teilenummer PM6644
Bauteilbeschribung  350 mA adjustable step-down regulator
PDF  35 Pages
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Hersteller  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6644 Datenblatt(HTML) 21 Page - STMicroelectronics

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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
∆IL
between 20% and 50% of the maximum output current ILOAD(max.). The maximum ∆IL
occurs at the maximum input voltage. With these considerations, the inductor value can be
calculated with the following relationship:
Equation 7
where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage and
∆IL 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 ILRMS:
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 Table 10 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
-------------------------
+
=



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