SC2446A Semtech Corporation, SC2446A Datasheet - Page 12

• Manufacturer Part Number: SC2446A
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

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 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
voltage) from no load to full load, the ESR value should
satisfy
Then, the required ESR value of the output capacitors
should be
POWER MANAGEMENT
Application Information (Cont.)
2005 Semtech Corp.
o
, the ESR should satisfy
R
esr
= min{R
(say 3%) of the steady state output
v
v
ESR
R
R
ESL
v
esr
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esr
C
, the ripple-voltage caused by
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12
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
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
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.
o
. The RMS current ripple rating should also be
C
o
2
2
I
o
3
10
f
s
.
R
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esr
ceramic capacitor is
.
SC2446A
www.semtech.com
electrolytic
ceramic