Datasheet MAX253 (Maxim) - 16

MAX253 Transformer Driver for Isolated RS-485 Interface age. In addition, include in the calculations an is a good choice for through-hole applications, and the allowance for worst-case losses in the rectifiers. Since NIEC* SB05W05C dual in an SOT-23 package is rec- the turns ratio determined in this manner will ordinarily ommended for surface-mount applications. Use the produce a much higher voltage at the secondary under higher frequency setting to reduce ripple. conditions of high input voltage and/or light loading, be careful to prevent an overvoltage condition from occur-
Output Filter Capacitor
ring (see Output Voltage vs. Load Current in the Typical In applications sensitive to output-ripple noise, the out- Operating Characteristics). put filter capacitor C2 should have a low effective series resistance (ESR), and its capacitance should Transformers used with the MAX253 will ordinarily be remain fairly constant over temperature. Sprague 595D wound on high-permeability magnetic material. To min- surface-mount solid tantalum capacitors and Sanyo imize radiated noise, use common closed-magnetic- OS-CON through-hole capacitors are recommended path physical shapes (e.g., pot cores, toroids, E/I/U due to their extremely low ESR. Capacitor ESR usually cores). A typical core is the Philips 213CT050-3B7, rises at low temperatures, but OS-CON capacitors pro- which is a toroid 0.190” in diameter and 0.05” thick. vide very low ESR below 0°C. For operation with this core at 5.5V maximum supply voltage, the primary should have approximately 22 In applications where output ripple is not critical, a turns on each side of the center tap, or 44 turns total. 0.1µF chip or ceramic capacitor is sufficient. Refer to This will result in a nominal primary inductance of Table 4 for suggested capacitor suppliers. Use the approximately 832µH. The secondary can be scaled to higher frequency setting to reduce ripple. produce the required DC output.
Input Bypass Capacitor Diode Selection
The input bypass capacitor C1 is not critical. Unlike The MAX253’s high switching frequency demands switching regulators, the MAX253’s supply current is high-speed rectifiers. Schottky diodes are recom- fairly constant, and is therefore less dependent on the mended. Ensure that the Schottky diode average cur- input bypass capacitor. A low-cost 0.1µF chip or rent rating exceeds the load-current level. The 1N5817 ceramic capacitor is normally sufficient for input bypassing.
Table 4. Suggested Capacitor Suppliers PRODUCTION METHOD CAPACITORS
Matsuo 267 series (low ESR) USA Phone: (714) 969-2491, FAX: (714) 960-6492 Sprague Electric Co. Surface Mount 595D/293D series (very low ESR) USA Phone: (603) 224-1961, FAX: (603) 224-1430 Murata Erie Ceramic USA Phone: (800) 831-9172, FAX: (404) 436-3030 Sanyo High-Performance OS-CON series (very low ESR) Through Hole USA Phone: (619) 661-6835, FAX: (619) 661-1055 Japan Phone: 81-7-2070-1005, FAX: 81-7-2070-1174 Nichicon PL series (low ESR) Through Hole USA Phone: (708) 843-7500, FAX: (708) 843-2798 Japan Phone: 81-7-5231-8461, FAX: 81-7-5256-4158 * Nihon Inter Electronics Corp. USA Phone: (805) 867-2555 FAX: (805) 867-2556 Japan Phone: 81-3-3494-7411 FAX: 81-3-3494-7414 www.maximintegrated.com Maxim Integrated | 16