TK65025 Ver la hoja de datos (PDF) - Toko America Inc
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TK65025 Datasheet PDF : 12 Pages
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TK65025
SINGLE-CELL APPLICATION
The TK65025 is a boost converter control IC with the
power MOSFET switch built into the device. It operates
from one or two battery cells and steps up the output
voltage to a regulated 3.0 Volts. The device operates at a
fixed nominal clock frequency of 83 kHz. The analysis is
easier to follow when referencing the test circuit below.
IB
VI
RN
1K C N
10 µF
L = 95 µH
VIN
1
GND
2
IND
3
CS
220 pF
D
RS
1K
RESET
RESET
6
GND
5
VOUT
4
CU
10 µF
RR
300 kΩ
ROF
15
IO
VO
CD
10 µF
The Test Circuit shown here is identical with the one
shown on page 2 of the TK65025 data sheet.
RIPPLE AND NOISE CONSIDERATIONS
In its simplest form, a power converter using the TK65025
requires only three external components: an inductor, a
diode, and a capacitor (see figure below).
VIN
1
GND
2
IND
3
RESET
6
GND
5
VOUT
4
VI
VO
Compared to the test circuit, this means eliminating the
following circuitry: the RC filter into the Vin pin, the RC
snubber, the RC filter at the converter output, and the
pullup resistor to the reset pin. The RC filter at the Vin pin
is used only to prevent the ripple voltage at the battery
terminals from prematurely causing under-voltage lockout
of the IC. This is only needed when the inductor value is
relatively small and the battery resistance is relatively high
and the Vin range must extend as low as possible. The RC
snubber dampens the ringing which occurs during the
deadtime, but this provides only a limited noise reduction,
so it isn’t required. The RC filter at the converter output
attenuates the conducted noise - the converter doesn’t
require this either. Finally, the pullup resistor at the reset
pin is needed only if the reset output signal is used. Most
of this circuitry which appears in the test circuit has been
added to minimize ripple and noise effects. But when this
is not critical, the circuit can be minimized.
When any DC-DC converter is used to convert power
in RF circuits (e.g., pagers) the spectral noise generated
by the converter, whether conducted or radiated, is of
concern. The oscillator of the TK65025 has been trimmed
and stabilized to 83 +/– 4 kHz with the intention of greatly
minimizing interference at the common IF frequency of
455 kHz. In comparison with conventional IC solutions,
where the oscillator frequency is not controlled tightly, the
TK65025 can achieve as much as 20-30 dB improve-
ments in RF interference reduction by means of its accu-
rately controlled oscillator frequency. This IF frequency is
halfway between the fifth and sixth harmonics of the
oscillator. The fifth harmonic of the maximum oscillator
frequency and the sixth harmonic of the minimum oscilla-
tor frequency still leave a 39 kHz band centered around
455 kHz within which a fundamental harmonic of the
oscillator will not fall. Since the TK65025 operates by
pulse burst modulation (PBM), the switching pattern can
be a subharmonic of the oscillator frequency. The sim-
plest example and the one most to be avoided is that of the
converter causing every other oscillator pulse to be skipped.
That means that the switching pattern would have a
fundamental frequency of one-half the oscillator frequency,
or 41.5 kHz - the eleventh harmonic of which lands at
456.5 kHz, right in the IF band. Fortunately, the energy is
rather weak at the eleventh harmonic - and even more
fortunate is the ease with which that regulation mode is
avoided. Due to a finite hysteresis in the regulator com-
parator, when an additional output filter is used (e.g., the
RC filter of the test circuit, or an LC filter) this minimizes the
ripple at the regulation node which limits the rate at which
the oscillator can be gated. In practice, this means that
rather than exhibiting a switching pattern of skipping every
other oscillator pulse, it would be more likely to exhibit a
switching pattern of three or four pulses followed by that
many pulses skipped. Although this also tends to increase
the output ripple, it is low frequency and has low magni-
tude (e.g., 10 kHz and 10 mV) which tends to be of little
consequence.
Page 4
February, 1997 Toko, Inc.
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