sc2441a Semtech Corporation, sc2441a Datasheet - Page 27

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

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
Sections
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 effects of ESR, ESL
and C
The first term is the DC voltage across C
The second term is the ripple-voltage caused by the
inductor ripple-current. The third term is the 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.
POWER MANAGEMENT
Applications Information
2006 Semtech Corp.
o
esl
v
on the output voltage.
) (Figure 16).
o
) t (
=
Figure 16. C
V
o
+
C
1
o
i
b
ò
0
o
t
(t)
) and V
i
b
) t (
o
equivalent circuit
dt
Lesl
Resr
+
Co
L
out
esl
Ripple in Step-down
di
dt
b
) t (
+
b
(t), the voltage
R
o
esr
at time t=0.
( i
b
). t
esr
) and
27
Since the inductor current is a triangular waveform with
peak-to-peak value
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.
When determining the ESR value, both the steady state
ripple-voltage and the dynamic load transient need to be
considered. To keep the steady state output ripple-voltage
< V
To limit the dynamic output voltage overshoot/
undershoot within
voltage) under 0 to full load current swing, the ESR value
should be
The required ESR value of the output capacitors should
be
In the aluminum capacitor selection, the working voltage
rating is normally suggested to be greater than 1.5V
The allowable current ripple (RMS) should be greater than
o
, the ESR should satisfy
R
esr
= min{R
(say 3%) of the steady state output
D
D
D
* I
R
R
v
v
v
esr
ESR
esr
C
ESL
o
2
, the ripple-voltage caused by
1
»
esr1
<
<
=
=
8
D
a
C
R
d
L
,R
d
I
I
V
V
esl
I
o
o
esr
o
o
esr2
f
o
o
s
f
d
s
.
.
.
. I
d
o
}.
D
I
o
SC2441A
www.semtech.com
o
.