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TK65025MTL Datasheet(PDF) 4 Page - TOKO, Inc |
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TK65025MTL Datasheet(HTML) 4 Page - TOKO, Inc |
4 / 12 page Page 4 February, 1997 Toko, Inc. 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. 1 3 RESET GND 6 4 VIN VOUT 2 5 GND IND S S S + S C N 300 k Ω RESET 15 10 µF 10 µF S S 1K 220 pF 10 µF S D VO IO 1K V I I B + S S L + S = 95 µH CU CD R N R R R S C S ROF 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). 1 3 RESET GND 6 4 VIN VOUT 2 5 GND IND + S S S VO V I S 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. |
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