SC2545 データシートの表示（PDF） - Semtech Corporation
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SC2545 Datasheet PDF : 24 Pages
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SC2545
POWER MANAGEMENT
Applications Information (Cont.)
The first term is the DC voltage across Co at time t=0.
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 with
peak-to-peak
value
δ
*I ,
o
the
ripple-voltage
caused
by
inductor current ripples is
∆vC
≈
δIo
8Cofs
,
the ripple-voltage due to ESL is
∆v ESL
=
L esl fs
δIo
D
,
and the ESR ripple-voltage is
∆vESR = ResrδIo.
Aluminum capacitors (e.g. electrolytic) have high
capacitances and low ESLs. The ESR has the dominant
effect on the output ripple voltage. It is therefore very
important to minimize the ESR. Other types to choose
are solid OS-CON, POSCAP, and tantalum.
When determining the ESR value, both the steady state
ripple-voltage and the dynamic load transient need to be
considered. To meet the steady state output ripple-
voltage spec, the ESR should satisfy
Re
sr1〈
∆VO
δ IO
To limit the dynamic output voltage overshoot/
undershoot within a (say 3%) of the steady state output
voltage from no load to full load, the ESR value should
satisfy
Re
sr
2〈
3%VO
IO
The voltage rating of aluminum capacitors should be at
least 1.5V . The RMS current ripple rating should also
o
be greater than
δIo .
23
Usually it is necessary to have several capacitors of the
same type in parallel to satisfy the ESR requirement.
The voltage ripple caused 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
Co
>
10
2πfsR esr
.
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 µ F, 4m Ω ceramic
capacitor is connected in parallel with 2x1500 µ F,
90m Ω electrolytic 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 µ F,
2m Ω 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 µ F, 2m Ω
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 formula.
Then, the required ESR value of the output capacitors
should be
R = min{R ,R }.
esr
esr1 esr2
Ceq(ω)
=
(R1a + R1b )2 ω2C1a2C1b2 + (C1a +C1b )2
(R1a2C1a + R1b2C1b )ω2C1aC1b + (C1a + C1b)
2005 Semtech Corp.
11
www.semtech.com
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