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PM6670AS 数据表(PDF) 41 Page - STMicroelectronics

部件名 PM6670AS
功能描述  Complete DDR2/3 memory power supplies controller
PDF  54 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

PM6670AS 数据表(HTML) 41 Page - STMicroelectronics

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PM6670AS
Application information
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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,
∆I
L,MAX , occurs at the maximum input voltage.
Given these considerations, the inductance value can be calculated with the following
expression:
Equation 29
where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage and
∆I
L is the inductor ripple current.
Once the inductor value is determined, the inductor ripple current is then recalculated:
Equation 30
The next step is the calculation of the maximum r.m.s. inductor current:
Equation 31
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 is important in inductor selection in term of its saturation current.
IN
OUT
L
OUT
IN
V
V
I
fsw
V
V
L
⋅
∆
⋅
−
=
MAX
,
IN
OUT
OUT
MAX
,
IN
MAX
,
L
V
V
L
fsw
V
V
I
⋅
⋅
−
=
∆
12
)
I
(
)
I
(
I
2
MAX
,
L
2
MAX
,
LOAD
RMS
,
L
∆
+
=
2
I
I
I
MAX
,
L
MAX
,
LOAD
PEAK
,
L
∆
+
=



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