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TK652XX Datasheet(PDF) 10 Page - TOKO, Inc |
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TK652XX Datasheet(HTML) 10 Page - TOKO, Inc |
10 / 20 page ADVANCED INFORMATION Page 10 January 1999 TOKO, Inc. TK652xx filtering component values (consult the “Ripple and Noise Considerations” section) can be determined if needed or desired. The TK652xx runs with a fixed oscillator frequency and it regulates by applying or skipping pulses to the internal power switch. This regulation method is called Pulse Burst Modulation (PBM). ANALYSIS OF SWITCHING CYCLE Above is the input or inductor current waveform over a switching cycle. From an oscillator standpoint, the switching cycle consists of only an on-time and an off-time. But from an inductor current standpoint, the switching cycle breaks down into three important sections: on-time, off-time, and deadtime. The on-time of the switch and the inductor current are synonymous. During the on-time, the inductor current increases. During the off-time, the inductor current decreases as it flows into the output. When the inductor current reaches zero, that marks the end of the inductor current off-time. For the rest of the cycle, the inductor current remains at zero. Since no energy is being either stored or delivered, that remaining time is called “deadtime.” This mode of the inductor current decaying to zero every cycle is called “discontinuous mode.” In summary, energy is stored in the inductor during on-time, delivered to the output during off-time, and remains at zero during deadtime. The output current of the boost converter comes from the The TK652xx is a boost converter control IC with the power MOSFET switch built into the device. It operates from a single battery cell and steps up the output voltage to a regulated 1.8, 2.1 and 2.4 V. The device operates at a fixed nominal clock frequency of 83 kHz. In its simplest form, a boost power converter using the TK652xx requires only three external components: an inductor, a diode, and a capacitor. The analysis is easier to follow when referencing the simple boost circuit below. THEORY OF OPERATION The converter operates with one terminal of an inductor connected to the DC input and the other terminal connected to the switch pin of the IC. When the switch is turned on, the inductor current ramps up. When the switch is turned off (or “lets go” of the inductor), the voltage flies up as the inductor seeks out a path for its current. A diode, also connected to the switching node, provides a path of conduction for the inductor current to the boost converter’s output capacitor. The TK652xx monitors the voltage of the output capacitor and has a 1.8, 2.1 and 2.4 V threshold at which the converter switching becomes deactivated. So the output capacitor charges up to 1.8, 2.1 and 2.4 V and regulates there, provided that no more current is drawn from the output than the inductor can provide. The primary task, then, in designing a boost converter with the TK652xx is to determine the inductor value (and its peak current rating to prevent inductor core saturation problems) which will provide the amount of current needed to guarantee that the output voltage will be able to maintain regulation up to a specified maximum load current. Secondary necessary tasks also include choosing the diode and the output capacitor. Then the snubber and SINGLE-CELL APPLICATION FIGURE 1: SIMPLE BOOST CONVERTER VIN LBI GND LOI VOUT SW + VOUT I PEAK di/dt = - (V OUT + Vf - VIN)/ L t (off) t (deadtime) t (on) di/dt = VIN / L |
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