sc4508a Semtech Corporation, sc4508a Datasheet - Page 8

• Manufacturer Part Number: sc4508a
• Description: Sc4508a Buck Or Buck-boost Inverting Current-mode Controller
• Manufacturer: Semtech Corporation

core losses (flux swing) and the winding AC resistance
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
or the inductor DC current. Assuming that the inductor
current ripple (peak-to-peak) value is
inductance value will then be:
The peak current in the inductor becomes (1+ /2)*Idc
and the RMS current is:
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 (C
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
ESL (L
L
L
Application Information (Cont.)
POWER MANAGEMENT
2007 Semtech Corp.
V
f
f
s
IN
s
V
esl
IN
) (Figure 1).
I
V
L
I
L
O
(
V
(
V
IN
V
IN
O
V
O
V
V
V
O
D
D
V
)
D
I
o
, L
) and V
V
rms
for Buck
D
)
for Buck-Boost
I
dc
out
1
Ripple
12
2
I
L
=
*Idc, the
esr
) and
8
If the current through the branch is i
across the terminals will then be:
This basic equation illustrates the effect of ESR, ESL
and C
The first term is the DC voltage across C
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 *I
ripple-voltage caused by inductor current ripples is:
the ripple-voltage due to ESL is:
and the ESR ripple-voltage is:
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.
o
on the output voltage.
v
o
) t (
V
Figure 1. An equivalent circuit of C
o
C
1
o
v
v
ESR
0
t
ESL
i
v
b
) t (
C
dt
R
L
Lesl
Resr
8
esr
Co
esl
C
L
I
f
o
o
s
I
esl
f
o
s
D
I
di
o
dt
b
) t (
SC4508A
b
www.semtech.com
(t), the voltage
R
o
esr
at time t=0.
i
b
) t (
o
o
, the