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ZXRD1050PQ16 Datasheet(PDF) 8 Page - Diodes Incorporated |
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ZXRD1050PQ16 Datasheet(HTML) 8 Page - Diodes Incorporated |
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8 / 28 page  Applications (continued) Inductor Selection The inductor is one of the most critical components in the DC-DC circuit.There are numerous types of devices available from many suppliers. Zetex has opted to specify off the shelf encapsulated surface mount components, as these represent the best compromise in terms of cost, size, performance and shielding. The SimpleSync TM technique uses a main inductor with an overwinding for the gate drive which is available as a standard part. However, for engineers who wish to design their own custom magnetics, this is a relatively simple and low cost construction technique. It is simply formed by terminating one of the multiple strands of litz type wire separately. It is still wound on the same core as the main winding and only has to handle enough current to charge the gate of the synchronous MOSFET. The major benefit is circuit simplification and hence lower cost of the control IC. For non-synchronous operation, the overwinding is not required. The choice of core type also plays a key role. For optimum performance, a ’swinging choke’ is often preferred. This is one which exhibits an increase in inductance as load current decreases. This has the net effect of reducing circulating current at lighter load improving efficiency. There is normally a cost premium for this added benefit. For this reason the chokes specified are the more usual constant inductance type. Peak current of the inductor should be rated to minimum 1.2IOUT (max) . To maximise efficiency, the winding resistance of the main inductor should be less than the main switch output on resistance. Schottky Diode Selection depends on whether a synchronous or non-synchronous approach is taken. For the ZXRD1000, the unique approach to the synchronous drive means minimal dead time and hence a small SOT23 1A DC rated device will suffice, such as the ZHCS1000 from Zetex. The device is only designed to prevent the body diode of the synchronous MOSFET from conducting during the initial switching transient until the MOSFET takes over. The device should be connected as close as possible to the source terminals of the main MOSFET. For non-synchronous applications , the Schottky diode must be selected to allow for the worst case conditions, when VIN is at its highest and VOUT is lowest (short circuit conditions for example). Under these conditions the device must handle peak current at close to 100% duty cycle. Frequency Adjustment The nominal running frequency of the controller is set to 200kHz in the applications shown. This can be adjusted over the range 50kHz to 300kHz by changing the value of capacitor on the CT pin. A low cost ceramic capacitor can be used. Frequency = 60000/C3 (pF) Frequency v temperature is given in the typical characteristics. Output Voltage Adjustment The ZXRD1000 is available as either a fixed 5V, 3.3V or adjustable output. On fixed output versions, the VFB pin should be connected to the output. Adjustable operation requires a resistive divider connected as follows: The value of the output voltage is determined by the equation VOUT = VFB (1 + RA RB ) VFB =1.24V Note: The adjustable circuit is shown in the following transient optimisation section. It is also used in the evaluation PCB. In both these circuits RA is assigned the label R6 and RB the label R5. Values of resistor should be between 1k and 20k to guarantee operation. Output voltage can be adjusted in the range 2V to 12V for non-synchronous applications. For synchronous applications, the minimum VOUT is set by the VGS threshold required for the synchronous MOSFET, as t he sw ing in t he gat e using t he SimpleSync TM technique is approximately VOUT. 8 ISSUE 4 - OCTOBER 2000 ZXRD1000 SERIES |
Similar Part No. - ZXRD1050PQ16 |
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Similar Description - ZXRD1050PQ16 |
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