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TK65025 Datasheet(PDF) 8 Page - TOKO, Inc |
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TK65025 Datasheet(HTML) 8 Page - TOKO, Inc |
8 / 12 page Page 8 February, 1997 Toko, Inc. TK65025 series (surface mount); Matsuo 267 series (surface mount); Sanyo OS-CON series (miniature through hold). I O = V BB 2 D D 2 f L 1- D 2 f L R S + R L + R SW () 2 V O + R OF I O(TGT) + D 2 f L V BBR U () + V F − V BB 1- D 2 f L R S + R L () − f C S V BB 2 + V O + V F ()2 + V O + V F − V BB ()2 [] 2V O + V F () Higher-Order Design Equation The equation above was developed as a closed form approximation for the design variable that required the least approximation to allow a closed form. In this case, that variable was “I O” (e.g., as opposed to “L”). The approximations made in the equation development have the primary consequence that error is introduced as resistive losses become relatively large. As it is normally a practical design goal to ensure that resistive losses will be relatively small, this seems acceptable. The variables used are: I O Output current capability I O(TGT) Targeted output current capability V O Output voltage V F Diode forward voltage V BB Battery voltage, unloaded D Oscillating duty ratio of main switch f Oscillator frequency L Inductance value R S Source resistance (battery + filter) R L Inductor winding resistance R SW Switch on-state resistance R OF Output filter resistance R U ESR of upstream output capacitor C S Snubber capacitance Deriving a design solution with this equation is neces- sarily an iterative process. Use worst case tolerances as described for inductor selection, plugging in different val- ues for “L” to approximately achieve an “I O” equal to the targeted value. Then, fine tune the parasitic values as needed and, if necessary, readjust “L” again and reiterate the process. DUAL-CELL APPLICATION There are special considerations involved in designing a converter with the TK65025 for use with two battery cells. With two battery cells the TK65025 can provide substan- tially more output current than a single cell input for the same efficiency. The concern is the possibility of saturating the inductor. For a single cell input it was only necessary to choose the current capability in accordance with the maximum peak current that could be calculated using Eq. (4). For a two cell input the peak current is not so readily determined because the inductor can go into continuous mode. When this happens, the increase of current during the on-time remains more-or-less the same (i.e., approximately equal to the peak current as calculated using Eq. (4)), but the inductor current doesn’t start from zero. It starts from where it had decayed to during the previous off-time. There is no deadtime associated with a single switching period when in continuous mode because the inductor current never decays to zero within one cycle. The cause for continuous mode operation is readily seen by noting that the rate of current increase in the inductor during the on-time is faster than the rate of decay during the off-time. The reason for that is because there is more voltage applied across the switch during the on-time (two battery cells) than during the off-time (3 volts plus a diode drop minus two battery cells). That situation, in conjunction with a switch duty ratio of about 50%, implies that the current can’t fall as much as it can rise during a cycle. So when a switching cycle begins with zero current in the inductor, it ends with current still flowing. Continuous mode operation implies that the inductor value no longer restricts the output current capability. With discontinuous mode operation, it was necessary to choose a lower inductor value to achieve a higher output current rating. (Eq. (6) specifically shows “I O” as a function of “L”.) This also implied higher ripple current from the battery. In continuous mode operation, one can choose a larger inductor value intentionally if it is desirable to minimize ripple current. The catch is that high inductance and high current rating together generally imply larger inductor size. But generally this unrestricted inductor value allows more freedom in the converter design. The dual cell input and the continuous current rating imply that the peak current in the inductor will be at least twice as high as it would for a single cell input using the same inductor value. The Toko D73 and D75 series inductors are particularly suited for the higher output current capability of the dual cell configuration. For operation at a fixed maximum load, the inductor can be kept free of saturation by choosing its peak current (6) |
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