SC2463 Semtech, SC2463 Datasheet - Page 12

• Manufacturer Part Number: SC2463
• Description: High Performance Quad Output Switching Regulator
• Manufacturer: Semtech

I I I I I nductor (L) and Ripple Current
There will not be enough modulation headroom if the on
time is simply made equal to the minimum on time of the
SC2463. For ease of control, we recommend the required
pulse width to be at least 1.5 times the minimum on
time.
Both step-down controllers in the SC2463 operate in
synchronous continuous-conduction mode (CCM)
regardless of the output load. The output inductor
selection/design is based on the output DC and transient
requirements. Both output current and voltage ripples
are reduced with larger inductors but it takes longer to
change the inductor current during load transients.
Conversely smaller inductors results in lower DC copper
losses but the AC core losses (flux swing) and the winding
AC resistance losses are higher. A compromise is to
choose the inductance such that
ripple-current
current.
Assuming that the inductor current ripple (peak-to-peak)
value is *I
The peak current in the inductor becomes (1+ /2)*Io
and the RMS current is
The followings 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.
POWER MANAGEMENT
Applications Information (Cont.)
nductor (L) and Ripple Current
nductor (L) and Ripple Current
nductor (L) and Ripple Current
nductor (L) and Ripple Current
2007 Semtech Corp.
o
, the inductance value will then be
is 20% to 30% of the rated output load
I
, L
L
rms
V
I
o
o
1 (
f I
o
1
s
D
12
)
2
.
.
peak-to-peak inductor
12
Output Capacit
Output Capacit
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 Capacit
Output Capacit
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
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.
with peak-to-peak value *I
by inductor current ripples is
Since the inductor current is a triangular waveform
the ripple-voltage due to ESL is
and the ESR ripple-voltage is
o
esl
v
on the output voltage.
) (Figure 1).
o
) t (
V
Figure 1. An equivalent circuit of C
o
or (C
or (C
or (C
or (C
C
1
o
0
t
o o o o o
i
) and V
) and V
) and V
) and V
) and V
b
v
) t (
ESL
v
dt
C
Lesl
Resr
o
Co
, the ripple-voltage caused
L
L
8
esl
out
out
out
out
out
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esl
C
I
f
o
o
s
di
f
Ripple
Ripple
Ripple
Ripple
Ripple
s
dt
D
b
I
,
o
) t (
,
b
(t), the voltage
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SC2463
R
o
esr
at time t=0.
( i
b
). t
esr
o
) and