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SC4508A Datasheet PDF : 21 Pages
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
core losses (flux swing) and the winding AC resistance
Co
losses are higher. A compromise is to choose the
inductance such that peak-to-peak inductor ripple-
current is 20% to 30% of the rated output load current
Lesl
or the inductor DC current. Assuming that the inductor
current ripple (peak-to-peak) value is ∆IL = δ *Idc, the
Resr
inductance value will then be:
L=
VIN − VO
fs • ∆IL
(
VO
VIN
+
+
VD
VD
)
for
Buck
Figure 1. An equivalent circuit of C
o
L
=
VIN
fs • ∆IL
( VO + VD )
VIN + VO + VD
for
Buck-Boost
The peak current in the inductor becomes (1+δ/2)*Idc
and the RMS current is:
IL,rms = Idc
1
+
δ2
12
The Idc is the inductor average or DC current.
The following are to be considered when choosing
inductors:
a) Inductor core material: For high efficiency applications
above 350KHz, ferrite, Kool-Mu and polypermalloy
materials should be used. Low-cost powdered iron cores
can be used for cost sensitive-applications below 350KHz
but with attendant higher core losses.
b) Select inductance value: Sometimes the calculated
inductance value is not available off-the-shelf. The
designer can choose the adjacent (larger) standard
inductance value. The inductance varies with
temperature and DC current. It is a good engineering
practice to re-evaluate the resultant current ripple at
the rated DC output current.
c) Current rating: The saturation current of the inductor
should be at least 1.5 times of the peak inductor current
under all conditions.
Output Capacitor (Co) and Vout Ripple
The output capacitor provides output current filtering in
steady state and serves as a reservoir during load
transient. The output capacitor can be modeled as an
ideal capacitor in series with its parasitic ESR (R ) and
esr
ESL (L ) (Figure 1).
esl
If the current through the branch is ib(t), the voltage
across the terminals will then be:
∫ vo(t) =
Vo
+
1
Co
t
ib(t)dt + Lesl
0
dib (t )
dt
+
Resrib (t)
This basic equation illustrates the effect of ESR, ESL
and C on the output voltage.
o
The first term is the DC voltage across Co at time t=0.
The second term is the voltage variation caused by the
charge balance between the load and the converter
output. The third term is voltage ripple due to ESL and
the fourth term is the voltage ripple due to ESR. The
total output voltage ripple is then a vector sum of the
last three terms.
Since the inductor current is a triangular waveform in
buck configuration with peak-to-peak value δ*Io, the
ripple-voltage caused by inductor current ripples is:
∆v C
≈
δIo
8Cofs
the ripple-voltage due to ESL is:
∆v ESL
=
Leslfs
δIo
D
and the ESR ripple-voltage is:
∆vESR = ResrδIo
Aluminum capacitors (e.g. electrolytic, solid OS-CON,
POSCAP, tantalum) have high capacitances and low
ESL’s. The ESR has the dominant effect on the output
ripple voltage. It is therefore very important to minimize
the ESR.
2007 Semtech Corp.
8
www.semtech.com
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