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ZXM64NO3X Datasheet(PDF) 5 Page - Zetex Semiconductors |
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ZXM64NO3X Datasheet(HTML) 5 Page - Zetex Semiconductors |
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5 / 28 page  DETAILED DESCRIPTION The ZXRD1000 series can be configured to use either N or P channel MOSFETs to suit most applications. The most popular f ormat , an a ll N channel synchronous solution gives the optimum efficiency. A feature of the ZXRD1000 series solution is the unique method of generating the synchronous drive, called SimpleSync . Most solutions use an additional output from the controller, inverted and delayed from the main switch drive. The ZXRD1000 series solution uses a simple overwinding on the main choke (wound on the same core at no real cost penalty) plus a small ferrite bead . This means that the synchronous FET is only enhanced when the main FET is turned off. This reduces the ‘blanking period’ required for shoot- through protection, increasing efficiency and allowing smaller catch diodes to be used, making the controller simpler and less costly by avoiding complex timing circuitry. Included on chip are numerous functions that allow flexibility to suit most applications. The nominal switching frequency (200kHz) can be adjusted by a simple timing capacitor, C3. A low battery detect circuit is also provided. Off the shelf components are available from major manufacturers such as Sumida to provide either a single winding inductor for non-synchronous applications or a coil with an over-winding for synchronous applications. The combination of these switching characteristics, innovative circuit design and excellent user flexibility, make the ZXRD1000 series DC-DC solutions some of the smallest and most cost effective and electrically efficient currently available. Using Zetex’s HDMOS low RDS(on) devices, ZXM64N02X for the main and synchronous switch, efficiency can peak at upto 95% and remains high over a wide range of operating currents. Programmable soft start can also be adjusted via the capacitor, C7, in the compensation loop. What is SimpleSync TM? Conventional Methods In the conventional approach to the synchronous DC-DC solution, much care has to be taken with the timing constraints between the main and synchronous switching devices. Not only is this dependent upon individual MOSFET gate thresholds (which vary from device to device within data sheet limits and over temperature), but it is also somewhat dependent upon magnetics, layout and other parasitics. This normally means that significant ‘dead time’ has to be factored in to the design between the main and synchronous devices being turned off and on respectively. Incorrect application of dead time constraints can potentially lead to catastrophic short circuit conditions between VIN and GND. For some battery operated systems this can not only damage MOSFETs, but also the battery itself. To realise correct ‘dead time’ implementation takes complex circuitry and hence implies additional cost. The ZETEX Method Zetex has taken a different approach to solving these problems. By looking at the basic architecture of a synchronous converter, a novel approach using the main circuit inductor was developed. By taking the inverse waveform found at the input to the main induc t o r of a non- s y nc hronous s o lut i on , a synchronous drive waveform can be generated that is always relative to the main drive waveform and inverted with a small delay. This waveform can be used to drive the synchronous switch which means no complex circuitry in the IC need be used to allow for shoot-through protection. Implementation Implementation was very easy and low cost. It simply meant peeling off a strand of the main inductor winding and isolating it to form a coupled secondary winding. These are available as standard items referred to in the applications circuits parts list.The use of a small, surface mount, inexpensive ’square loop’ ferrite bead provides an excellent method of eliminating shoot-through due to variation in gate thresholds. The bead essentially acts as a high i mp e da n c e fo r the few na n o seco nd s that shoot-through would normally occur. It saturates very quickly as the MOSFETs attain steady state operation, reducing the bead impedance to virtually zero. Benefits The net result is an innovative solution that gives a ddit i ona l bene f i t s w h i l s t l o w e ri ng overall implementation costs. It is also a technique that can be simply omitted to make a non-synchronous controller, saving further cost, at the expense of a few efficiency points. 5 ZXRD1000 SERIES ISSUE 4 - OCTOBER 2000 |
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