TK65130 データシートの表示（PDF） - Toko America Inc
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TK65130 Datasheet PDF : 20 Pages
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TK651xx
SINGLE-CELL APPLICATION (CONT.)
2) if the same battery is powering the TK651xx at the VIN
pin (i.e., the normal case), then the IC may become
inoperable due to insufficient VIN. This is why the application
test circuit features an RC filter into the VIN pin. The 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 discharged 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 be either
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 terminal
and converter input is large, and that we filter the VIN pin of
the IC as in the test circuit, then the parameter “VIN” 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 CAPACITANCE ESR
The on-resistance of the TK651xx’s internal switch is
about 1 Ohm maximum. Using the previously stated
example of 100 mA peak current, the voltage drop across
the switch would reach 100 mV during the on-time. This
subtracts from the voltage which is impressed across the
inductor to store energy during the on-time. As a result,
less energy is delivered to the output during the off-time.
If the winding resistance of the inductor increases to 1 Ohm
or greater, the voltage drop across the winding resistance
also subtracts from the voltage used to store energy in the
core. Thus, efficiency degradation occurs.
at a diode drop above the output voltage. However, 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 2
Ohms (not unusual at cold temperature) would jump by
200 mV when 100 mA 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
series (surface mount); Matsuo 267 series (surface mount);
Sanyo OS-CON series.
LOI FEATURES
The Low Output Indicator (LOI) output can provide a reset
signal to a microprocessor or other external system
controller. When the output voltage falls below the LOI
threshold (during start-up of the converter or under a
current overload fault condition), the LOI signal is asserted
low, indicating that the system controller (i.e.,
microprocessor) should be in a reset mode. This method
of reset control can be used to prevent improper system
operation which might occur at low supply voltage levels.
The LOI threshold voltage is between 87% and 93% of the
regulated output voltage value. The LOI threshold also has
about 45 mV hysteresis between its on-off trigger levels.
RIPPLE AND NOISE CONSIDERATIONS
The filtered test circuit of the TK651xx is shown below in
Figure 2.
IB
VIN
RN
1K
CN
10 µF
VIN
GND
SW
L = 95 µF
CS
220 pF
D
RS
1K
LOI
GND 300 kΩ
VOUT
ROF
+ 15
CU
10 µF
+ CD
10 µF
IOUT
VOUT
As the inductor delivers energy into the output capacitor
during the off-time, its current decays at a rate proportional
to the voltage drop across it. The idealized equations
assume that the voltage at the switching node is clamped
FIGURE 2: FILTERED TEST CIRCUIT
January 1999 TOKO, Inc.
Page 13
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