TK65025 データシートの表示（PDF） - Toko America Inc
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TK65025 Datasheet PDF : 12 Pages
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diode cathode to ground, has the function of averaging the
current pulses delivered from the inductor while holding a
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 ESR
(equivalent series resistance) 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 Eq. (4). ESR can be a strong function of
temperature, being worst case when cold. The capaci-
tance should be capable of integrating a current pulse with
little ripple. Typically, if a capacitor is chosen with reason-
ably low ESR and if the capacitor is the right type of
capacitor for the application (typically aluminum electro-
lytic or tantalum), then the capacitance will be sufficient.
ESR and printed circuit board layout have strong influ-
ence 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 Eq. (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
inductor’s must draw higher peak currents), and when the
battery is undersized for the peak current application.
While the simple analysis used a parameter “VI” to
represent the converter input voltage in the equations,
one may not know what “VI” 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 100mA
for a 1V input, and assume that the input is powered by a
partially discharged AAA battery which might have a
series resistance of 2 Ω at 0°C. (Environmentally clean, so
called “green” batteries tend to have higher source resis-
tance than their “unclean” predecessors.) If such partially
discharged battery voltage is 1V without load, the con-
verter battery voltage will sag to about 0.8V 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 current capability will decrease,
(2) if the same battery is powering the TK65025 at the VIN
pin (i.e., the normal case), then the IC may become
inoperable due to insufficient VIN. This is why the applica-
tion test circuit features an RC filter into the VIN pin. The
February, 1997 Toko, Inc.
TK65025
current draw is very small, so the voltage drop across this
filter resistor is negligible. The filter serves to average out
the input ripple caused by the battery resistance. Note
that this filter is optional and the net effect of its use is the
extension of battery life by allowing the battery to be
discharge more deeply.
A more power-efficient method comes at the price of a
large capacitor. This can be placed in parallel with the
battery to help channel the converter current pulses away
from the battery. The capacitor must have low ESR
compared to the battery resistance in order to accomplish
this effectively.
Still another solution is to filter the DC input with an LC
filter. However, it is more likely that the filter will either be
too large or too lossy. It is of questionable benefit to
smooth the input if the DC loss through the filter is large.
Assuming that input ripple voltage at the battery termi-
2
nal and converter input is large, and that we filter the VIN
pin of the IC as in the test circuit, then the parameter “VI”
in the previous equations is not usable, and we will need
to use parameters to represent both the source voltage
and the source resistance.
Switch On-Resistance, Inductor Winding
Resistance, and Capacitor ESR
The on-resistance of the TK65025’s internal switch is
about 1Ω maximum. Using the previously stated example
of 100mA peak current, the voltage drop across the
switch would reach 100mV during the on-time. This
subtracts from the voltage which is impressed across the
inductor to store energy during the on-time, so less
energy is delivered to the output during the off-time.
It is quite possible for the inductor winding resistance to
meet or exceed 1Ω, also. Voltage drop across the
winding resistance of the inductor also subtracts from the
voltage used to store energy in the core. So it also
degrades efficiency.
As the inductor delivers energy into the output capaci-
tor during the off-time, its current decays at a rate propor-
tional to the voltage drop across it. The idealized equa-
tions assume that the voltage at the switching node is
clamped at a diode drop above the output voltage. How-
ever, the ESR of the output capacitor can increase the
voltage drop across the inductor by the additional voltage
dropped across the ESR when the peak current flows in
it. For example, the voltage across a capacitor with an
ESR of 2Ω (not unusual at cold temperature) would jump
by 200mV when 100mA peak current began to flow in it.
This extra voltage drop would cause the inductor current
to ramp down more quickly, thus, depleting the available
output current. Possible choices for low ESR capacitors
are: Panasonic TE series (surface mount); AVX TPS
Page 7
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