APW7120
Application Information (Cont.)
ignored.ThereforetheACpeak-to-peakoutput voltage
is shown below:
Input Capacitor Selection (Cont.)
T=1/FOSC
DVOUT = D I × ESR
(V) .......... .(5)
The load transient requirements are a function of the
slew rate (di/dt) and the magnitude of the transient
load current. These requirements are generally met
with a mix of capacitors and careful layout. Modern
components and loads are capable of producing
transient load rates above 1A/ns. High frequency
capacitors initially supply the transient and slow the
current load rate seen by the bulk capacitors. The bulk
filter capacitor valuesare generally determined by the
ESR (Effective Series Resistance) and voltage rating
requirements rather than actual capacitance
requirements.
VUGATE
DT
I
IOUT
IOUT
IL
IQ1
I
ICOUT
VOUT
VOUT
Figure 1 Buck Converter Waveforms
Highfrequencydecouplingcapacitorsshouldbeplaced
as close to the power pins of the load as physically
Output Capacitor Selection
An output capacitor is required to filter the output and possible. Be careful not to add inductance in the
circuit board wiring that could cancel the usefulness
supplytheloadtransient current.Thefilteringrequirements
are a functionof the switchingfrequencyand theripple of these low inductance components.
current. The output ripple is the sum of the voltages,
An aluminum electrolytic capacitor’s ESR value is
having phase shift, acrossthe ESR and the ideal output
capacitor. The peak-to-peak voltage of the ESR is cal-
culated as the following equations :
related to the case size with lower ESR available in
larger case sizes. However, the Equivalent Series
Inductance (ESL) of these capacitors increases with
case size and can reduce the usefulness of the
capacitor to high slew-rate transient loading. In most
cases, multiple electrolytic capacitors of small case
size perform better than a single large case capacitor.
VOUT = D × VIN
VOUT × (1- D)
(V) .......... . (1)
(A) .......... .(2)
(V) .......... ..(3)
DI =
FOSC × L
VESR = D I ×ESR
Output Inductor Selection
The peak-to-peak voltage of the ideal output capacitor
is calculated as the following equations :
The output inductor is selected to meet the output
voltage ripple requirements and minimize the
converter’s response time to the load transient. The
inductor value determines the converter’s ripple
current and the ripple voltage, see equations (2) and
(5). Increasing the value of inductance reduces the
ripple current and voltage. However, the large inductance
D I
DVCOUT =
(V) ....... (4)
8 × FOSC × COUT
For general applications using bulk capacitors, the
DVCOUT is much smaller than the VESR and can be
Copyright ã ANPEC Electronics Corp.
14
www.anpec.com.tw
Rev. A.4 - Jan., 2006
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
