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TK65025MTL Datasheet(PDF) 5 Page - TOKO, Inc |
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TK65025MTL Datasheet(HTML) 5 Page - TOKO, Inc |
5 / 12 page 2 February, 1997 Toko, Inc. Page 5 TK65025 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 of the switch, 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 the on-time, delivered to the output during the off-time, and remains at zero during the deadtime. Unless otherwise specified, the term off-time refers to the inductor current, not to the switch. Inductor Selection It is under the condition of lowest input voltage that the boost converter output current capability is the lowest for a given inductance value. Three other significant param- eters with worst case values for calculating the inductor value are: highest switching frequency, lowest duty ratio (of the switch on-time to the total switching period), and highest diode forward voltage. Other parameters which can affect the required inductor value, but for simplicity will not be considered in this first analysis are: the series resistance of the DC input source (i.e., the battery), the series resistance of the internal switch, the series resis- tance of the inductor itself, ESR of the output capacitor, input and output filter losses, and snubber power loss. The converter reaches maximum output current capability when the switch runs at the oscillator frequency, without pulses being skipped. The output current of the boost converter is then given by the equation: I O = V I 2 D 2 2 f LV O + V F − V I ()2 (1) where “V I” is the input voltage, “D” is the on-time duty ratio of the switch, “ f ” is the switching (oscillator) frequency, “L” is the inductor value, “V O” is the output voltage, and “VF” is the diode forward voltage. It is important to note that this equation makes the assumption stated in equation form: V I ≤ V O + V F () 1- D () (2) The implication from Eq. (2) is that the inductor will operate in discontinuous mode. From a practical Theory of Operation The converter operates with one terminal of an inductor connected to the DC input and the other terminal con- nected 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 TK65025 monitors the voltage of the output capacitor and has a 3 volt threshold at which the converter switching becomes disactivated. So the output capacitor charges up to 3 volts and regulates there, provided that we don’t draw more current from the output than the inductor can provide. The primary task, then, in designing a boost converter with the TK65025 is to deter- mine the inductor value 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 tasks include choosing the diode, output capacitor, snubber, and filtering if desired. The TK65025 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). Reset Feature The TK65025 also features an output voltage monitor which provides a reset signal to a microprocessor or other external system controller. When the output voltage is below the reset threshold (which is less than the regulation threshold), the reset signal is asserted low, indicating that the system controller (e.g., microprocessor) should be in a reset mode. Such a condition might exist during startup of the converter or under an overload fault condition. This method of reset control can be used to prevent improper system operation which might occur at low supply voltage levels. The TK65025 has a reset threshold between 2.48 and 2.70 volts. Analysis of a Switching Cycle Although the derivation of equations is not discussed, the user will more easily be able to understand (and if desired, reproduce) the design equations if we begin by more precisely describing how the converter operates over a switching cycle. From an oscillator standpoint, the switching cycle con- sists of only an on-time and an off-time. But from an |
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