PM6670S View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670S' PDF : 53 Pages View PDF
PM6670S
Application information
8.1.1
Inductor selection
Once the switching frequency has been defined, the inductance value depends on the
desired inductor ripple current. Low inductance value means great ripple current that brings
poor efficiency and great output noise. On the other hand a great current ripple is desirable
for fast transient response when a load step is applied.
High inductance brings higher efficiency, but the transient response is critical, especially if
VINmin - VOUT is small. Moreover a minimum output ripple voltage is necessary to assure
system stability and jitter-free operations (see output capacitor selection paragraph). The
product of the output capacitor's ESR multiplied by the inductor ripple current must be taken
into consideration. A good trade-off between the transient response time, the efficiency, the
cost and the size is choosing the inductance value in order to maintain the inductor ripple
current between 20 % and 50 % (usually 40 %) of the maximum output current.
The maximum inductor ripple current, ΔIL,MAX, occurs at the maximum input voltage.
Given these considerations, the inductance value can be calculated with the following
expression:
Equation 29
L = VIN − VOUT ⋅ VOUT
fsw ⋅ ΔIL VIN
where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage and
ΔIL is the inductor ripple current.
Once the inductor value is determined, the inductor ripple current is then recalculated:
Equation 30
ΔIL,MAX
=
VIN,MAX − VOUT
fsw ⋅ L
⋅
VOUT
VIN,MAX
The next step is the calculation of the maximum r.m.s. inductor current:
Equation 31
IL,RMS =
(ILOAD,MAX )2
+
(ΔIL,MAX )2
12
The inductor must have an r.m.s. current greater than IL,RMS in order to assure thermal
stability.
Then the calculation of the maximum inductor peak current follows:
Equation 32
IL,PEAK
= ILOAD,MAX
+
ΔIL,MAX
2
IL,PEAK is important in inductor selection in term of its saturation current.
Doc ID 14432 Rev 4
41/54
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