SC2544 데이터 시트보기 (PDF) - Semtech Corporation
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SC2544 Datasheet PDF : 24 Pages
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SC2544
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 G *Io, the ripple-voltage
caused by inductor current ripples is
'Y&
|
G,R
&RIV
the ripple-voltage due to ESL is
'Y (6/
/ HVO IV
G ,R
'
and the ESR ripple-voltage is
'Y(65 5HVUG,R
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
5H VU¢ '92
G ,2
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
5H VU ¢ 92
,2
Then, the required ESR value of the output capacitors
should be
Resr = min{Resr1,Resr2 }.
The voltage rating of aluminum capacitors should be
at least 1.5Vo. The RMS current ripple rating should
also be greater than
G,R
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
&R
!
SIV5 HVU
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 P F, 4m : ceramic capacitor is
connected in parallel with 2x1500 P F, 90m :
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 P 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 P 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 ESRs either.
Instead they should be calculated using the following
formula.
&HT& ZHT Z
5D 5E Z&D&E &D &E
5D&D 5E&E Z&D&E &D &E
ã 2005 Semtech Corp.
11
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