APW7093NI-TRL 데이터 시트보기 (PDF) - Anpec Electronics
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APW7093NI-TRL Datasheet PDF : 20 Pages
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APW7093
Application Information(Cont.)
Programming the Switching Frequency and Low-ESR and low-ESL Tantalum or ceramic capacitor
Off-Time and On-Time (Cont.)
should be suitable.
When sinking current, the switching frequency
increases due to the on-resistances of the internal
switches adding to the voltage across the inductor,
reducing the on-time. Calculate TON when sinking
current using the equation:
TON
=
TOFF
VOUT − VNMOS
VIN − VOUT + VPMOS
Inductor Selection
The key inductor parameters must be specified: inductor
value (L) and peak current (IPEAK). A lower value of
inductor allows smaller size but results in higher losses
and ripple. A good compromise between size and
losses is found at approximately a 25% ripple current
to load current ratio (∆I/IOUT = 0.25).
L = VOUT × TOFF
IOUT × 0.25
Output Capacitor Selection
The output filter capacitor affects the output voltage
ripple, output load-transient response, and feedback
loop stability. The output filter capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy
stability requirements. Also, the capacitance value must
be high enough to guarantee stability and absorb the
inductor energy going from a full-load sourcing to full
load sinking condition without exceeding the maximum
output tolerance.
In applications where the output is subject to large
load transients, the output capacitor’s size typically
depends on how much ESR is needed to prevent the
output from dipping too low under a load transient.
R ESR ≤ ∆ VOUT /∆IOUT(MAX)
The peak inductor current at full load is calculated by:
IPEAK
= IOUT
+ VOUT × TOFF
2×L
where IOUT is the maximum source or sink current.
Choose an inductor with a saturation current at least
as high as the peak inductor current. Additionally, verify
the peak inductor current while sourcing output
current (IOUT = ISOURCE) does not exceed the positive
current limit. The inductor selected should exhibit low
losses at the chosen operating frequency.
The actual microfarad capacitance value required is
defined by the physical size needed to achieve low
ESR, and by the chemistry of the capacitor technology.
Thus, the capacitor is usually selected by ESR, size
and voltage rating rather than by capacitance value.
W hen using low-capacity filter capacitors such as
ceramic or polymer types, capacitor size is usually
determined by the capacity needed to prevent overshoot
and undershoot from causing problems during load
transients. Generally, once enough capacitance is
added to meet the overshoot requirement, undershoot
at the rising-load edge is no longer a problem.
Input Capacitor Selection
The input filter capacitor reduces peak currents and
noise at the voltage source. A 22µF to 47µF capacitor
may be required for higher power and dynamic loads.
Copyright © ANPEC Electronics Corp.
13
Rev. A.5 - Jun., 2005
www.anpec.com.tw
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