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PM6686 Datasheet(PDF) 38 Page - STMicroelectronics

Part # PM6686
Description  Constant ON-time control
PDF  50 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6686 Datasheet(HTML) 38 Page - STMicroelectronics

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Application information
PM6686
38/50
Doc ID 15281 Rev 4
10
Application information
10.1
External components selection
10.1.1
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
ΔI
L
between 20% and 50% of the maximum output current ILOAD (max). The maximum
ΔI
L
occurs at the maximum input voltage. With these considerations, the inductor value can be
calculated with the following relationship:
Equation 8
Where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage
and
ΔI
L 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 ILRMS:
Equation 9
Where
ΔI
L(max) is the maximum ripple current:
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.
IN
OUT
L
sw
OUT
IN
V
V
I
f
V
V
L
×
Δ
×
−
=
12
(max))
I
(
(max))
I
(
I
2
L
2
LOAD
LRMS
Δ
+
=
max
IN
OUT
sw
OUT
max
IN
L
V
V
L
f
V
V
(max)
I
×
×
−
=
Δ
2
(max)
I
(max)
I
Ipeak
L
LOAD
Δ
+
=



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