TK65020 데이터 시트보기 (PDF) - Toko America Inc
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TK65020 Datasheet PDF : 14 Pages
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ADVANCED INFORMATION
TK65020
SINGLE-CELL APPLICATION (CONT.)
rating is determined by wire size and power dissipation in
the wire resistance. The inductor rms current is given by:
IL(RMS) = IPK
IPK f L
D + VOUT + VF - VIN
(5)
3
where “IPK” is the same maximized value that was just used
to check against inductor peak current rating, and the term
in the numerator within the radical that is added to the
[on-time] duty ratio, “D”, is the off-time duty ratio.
Toko America, Inc. can offer a miniature matched
magnetic solution in a wide range of inductor values and
sizes to accommodate varying power level requirements.
The following series of Toko inductors work especially well
with the TK65020 : 10RF, 12RF, 3DF, D73, and D75. The
5CA series can be used for isolated-output applications,
although such design objectives are not considered here.
OTHER CONVERTER COMPONENTS
In choosing a diode, parameters worthy of consideration
are: forward voltage, reverse leakage, and capacitance.
The biggest efficiency loss in the converter is due to the
diode forward voltage. A Schottky diode is typically chosen
to minimize this loss. Possible choices for Schottky diodes
are: LL103A from ITT MELF case; 1N5017 from Motorola
(through hole case); MBR0530 from Motorola (surface
mount) or 15QS02L from Nihon EC (surface mount).
Reverse leakage current is generally higher in Schottkys
than in pin-junction diodes. If the converter spends a good
deal of the battery lifetime operating at very light load (i.e.,
the system under power is frequently in a standby mode),
then the reverse leakage current could become a substantial
fraction of the entire average load current, thus degrading
battery life. So don’t dramatically oversize the Schottky
diode if this is the case.
Diode capacitance isn’t likely to make much of an
undesirable contribution to switching loss at this relatively
low switching frequency. It can, however, increase the
snubber (look in the “Ripple and Noise Considerations”
section) dissipation requirement.
The output capacitor, the capacitor connected from the
diode cathode to ground, has the function of averaging the
current pulses delivered from the inductor while holding a
January 1999 TOKO, Inc.
relatively smooth voltage for the converter load. Typically,
the ripple voltage cannot be made smooth enough by this
capacitor alone, so an output filter is used. In any case, to
minimize the dissipation required by the output filter, the
output capacitor should still be chosen with consideration
to smoothing the voltage ripple. This implies that its
Equivalent Series Resistance (ESR) should be low. This
usually means choosing a larger size than the smallest
available for a given capacitance. To determine the peak
ripple voltage on the output capacitor for a single switching
cycle, multiply the ESR by the peak current which was
calculated in Equation 4. ESR can be a strong function of
temperature, being worst-case when cold. The capacitance
should be capable of integrating a current pulse with little
ripple. Typically, if a capacitor is chosen with reasonably
low ESR, and if the capacitor is the right type of capacitor
for the application (typically aluminum electrolytic or
tantalum), then the capacitance will be sufficient.
ESR and printed circuit board layout have strong influence
on RF interference levels. Special care must be taken to
optimize PCB layout and component placement.
THE BENEFITS OF INPUT FILTERING
In practice, it may be that the peak current (calculated in
Equation 4) flowing out of the battery and into the converter
will cause a substantial input ripple voltage dropped across
the resistance inside the battery. This becomes a more
likely case for cold temperature (when battery series
resistance is higher), higher load rating converters (whose
inductors must draw higher peak currents), and when the
battery is undersized for the peak current application.
While the simple analysis used a parameter “VIN” to
represent the converter input voltage in the equations, one
may not know what “VIN” value to use if it is delivered by a
battery that allows high ripple to occur. For example,
assume that the converter draws a peak current of 100 mA
for a 1 V input, and assume that the input is powered by a
partially discharged AAA battery which might have a series
resistance of 2 Ohms at 0 °C. (Environmentally clean, so
called “green” batteries tend to have higher source
resistance than their “unclean” predecessors). If such
partially discharged battery voltage is 1 V without load, the
converter battery voltage will sag to about 0.8 V during the
on-time. This can cause two problems: 1) with the effective
input voltage to the converter reduced in this way, the
converter output capacity will decrease, 2) if the same
battery is powering the TK65020 at the VIN pin (i.e., the
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