SC2446 Semtech Corporation, SC2446 Datasheet - Page 13

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

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 with
peak-to-peak value *I
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
no load to full load, the ESR value should satisfy
Then, the required ESR value of the output capacitors
should be
The voltage rating of aluminum capacitors should be at
least 1.5V
greater than
Usually it is necessary to have several capacitors of the
same type in parallel to satisfy the ESR requirement. The
voltage ripple cause by the capacitor charge/discharge
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Application Information (Cont.)
2004 Semtech Corp.
o
, the ESR should satisfy
(say 3%) of the steady state output voltage) from
o
. The RMS current ripple rating should also be
R
esr
= min{R
v
v
ESR
R
R
ESL
v
esr
esr
o
C
, the ripple-voltage caused by
2
2
1
esr1
R
I
L
o
8
3
esl
esr
C
,R
.
I
I
f
V
V
I
o
o
o
s
o
esr2
f
o
o
. I
s
o
D
.
.
,
I
o
}.
,
13
should be an order of magnitude smaller than the voltage
ripple caused by the ESR. To guarantee this, the
capacitance should satisfy
In many applications, several low ESR ceramic capacitors
are added in parallel with the aluminum capacitors in
order to further reduce ESR and improve high frequency
decoupling. Because the values of capacitance and ESR
are usually different in ceramic and aluminum capacitors,
the following remarks are made to clarify some practical
issues.
Remark 1: High frequency ceramic capacitors may not carry
most of the ripple current. It also depends on the capacitor
value. Only when the capacitor value is set properly, the
effect of ceramic capacitor low ESR starts to be significant.
For example, if a 10 F, 4m
connected in parallel with 2x1500 F, 90m
capacitors, the ripple current in the ceramic capacitor is
only about 42% of the current in the electrolytic capacitors
at the ripple frequency. If a 100 F, 2m ceramic capacitor
is used, the ripple current in the ceramic capacitor will be
about 4.2 times of that in the electrolytic capacitors. When
two 100 F, 2m ceramic capacitors are used, the current
ratio increases to 8.3. In this case most of the ripple
current flows in the ceramic decoupling capacitor. The ESR
of the ceramic capacitors will then determine the output
ripple-voltage.
Remark 2: The total equivalent capacitance of the filter
bank is not simply the sum of all the paralleled capacitors.
The total equivalent ESR is not simply the parallel
combination of all the individual ESR’s either. Instead they
should be calculated using the following formulae.
where R
electrolytic capacitors, and R
capacitance of the ceramic capacitors respectively. (Figure
7)
C
R
eq
eq
(
(
: )
: )
1a
R
R (
a 1
R (
a 1
and C
R
2
a 1
b 1
C
R (
R (
a 1
R
a 1
a 1
1a
b 1
R
)
b 1
R
2
are the ESR and capacitance of
R
C
2
b 1
C
b 1
o
2
)
C
b 1
)
2
a 1
)
2
2
2
2
C
C
2
C
C
a 1
10
f
1b
b 1
a 1
s
a 1
2
R
2
2
C
and C
C
C
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esr
b 1
b 1
b 1
ceramic capacitor is
(
2
.
C
2
a 1
(
R (
C
1b
(
C
a 1
b 1
C
are the ESR and
a 1
C
b 1
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SC2446
b 1
C
)
2
2
C
b 1
b 1
electrolytic
)
R
)
2
a 1
C
a 1
2
)