Datasheet MAX845 (Maxim) - 13

Isolated Transformer Driver for PCMCIA Applications MAX845
2) Use a test winding to measure ET product (if using an ungapped toroid) and/or AL value for the core. VIN 6 3) Determine the number of turns required for the pri- V 1 CC D1 mary winding. For an ungapped toroid, ET product from center-tap to D1 must be at least 5V-µs. Other core types must have sufficient inductance to limit MAX845 D1 and D2 output current under minimum load con- 8 D2 ditions, and must not be allowed to saturate. GND1 GND2 2 7 4) Select a rectifier topology based on performance requirements (ripple vs. loss, and space required for secondary winding). Refer to Table 2, Rectifier Figure 11a. 2-Diode Push-Pull Topology Trade-Offs. 5) Work backward from VOUT requirements to deter- mine the secondary to primary turns ratio. Include VIN 6 losses in the rectifier diodes, and estimate resistive 1 VCC D1 losses in the windings. For load currents exceed- ing 150mA, use a voltage step-down transformer to step up the output current from the MAX845. Do MAX845 not exceed the MAX845’s absolute maximum out- 8 D2 put current rating (200mA). GND1 GND2 2 7 6) Wind the transformer with the largest diameter wire that will fit the winding area. Select a wire gauge to fill the winding aperture as much as possible. Figure 11b. 4-Diode Bridge Larger diameter wire has lower resistance per unit length. Doubling the wire diameter reduces resis- tive losses by a factor of four. VIN 6 Bobbin or drum cores suffer from low coupling between 1 VCC D1 windings. This usually requires bifilar winding for the two halves of the primary. MAX845 Due to the inherent complexity of magnetic circuit 8 design, it will be necessary to build a prototype and re- D2 GND1 GND2 iterate the design. If necessary, adjust the design by 2 7 altering the number of primary or secondary turns, or the wire gauge. If using a different core material or geome- try, evaluate its ET product or AL as described above. Figure 11c. Voltage Doubler
Rectifier Topology Diodes
Figure 11 shows various rectifier topologies. Refer to Use fast-switching diode rectifiers. Ordinary silicon sig- Table 2 for selection criteria. The turns ratio of the trans- nal diodes like the 1N914 or 1N4148 may be used for former must be set to provide the minimum required out- low output current levels (less than 50mA), but Schottky put voltage at the maximum anticipated load, with the diodes have a lower forward voltage drop and should minimum expected input voltage. In addition, the calcu- be used for higher-current applications. Central lations should allow for worst-case losses in the recti- Semiconductor has low-current Schottky diodes as fiers. Since the turns ratio determined in this manner will duals in SOT-23 packages (CMPSH-3 series). The ordinarily produce a much higher voltage at the sec- Nihon SB05W05C is a common-cathode dual in a SOT- ondary under conditions of high input voltage and/or 23; it works well in the two-diode full-wave configura- light loading, be careful to prevent an overvoltage con- tion. The Motorola MBR0520 is an excellent choice for dition from occurring (see the Output Voltage vs. Load all configurations. Current graph in the Typical Operating Characteristics).
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