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MAX1627ESA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX1627ESA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page Diode Selection The MAX1626/MAX1627’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817–1N5822 family or surface-mount equiva- lents, are recommended. Ultra-high-speed rectifiers with reverse recovery times around 50ns or faster, such as the MUR series, are acceptable. Make sure that the diode’s peak current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds V+. Schottky diodes are preferred for heavy loads due to their low forward voltage, especially in low-voltage applications. For high-temperature applica- tions, some Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-high- speed rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance. Capacitor Selection Choose filter capacitors to service input and output peak currents with acceptable voltage ripple. Equivalent series resistance (ESR) in the capacitor is a major contributor to output ripple, so low-ESR capaci- tors are recommended. Sanyo OS-CON capacitors are best, and low-ESR tantalum capacitors are second best. Low-ESR aluminum electrolytic capacitors are tol- erable, but do not use standard aluminum electrolytic capacitors. Voltage ripple is the sum of contributions from ESR and the capacitor value: To simplify selection, assume initially that two-thirds of the ripple results from ESR and one-third results from capacitor value. Voltage ripple as a consequence of ESR is approximated by: Estimate input and output capacitor values for given voltage ripple as follows: where I ∆L is the change in inductor current (around 0.5IPEAK under moderate loads). These equations are suitable for initial capacitor selec- tion; final values should be set by testing a prototype or evaluation kit. When using tantalum capacitors, use good soldering practices to prevent excessive heat from damaging the devices and increasing their ESR. Also, ensure that the tantalum capacitors’ surge-current ratings exceed the start-up inrush and peak switching currents. Pursuing output ripple lower than the error compara- tor’s hysteresis (0.5% of the output voltage) is not prac- tical, since the MAX1626/MAX1627 will switch as needed, until the output voltage crosses the hysteresis threshold. Choose an output capacitor with a working voltage rating higher than the output voltage. The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on V+ and CS, caused by the circuit’s switching action. Use a low-ESR capacitor. Two smaller-value low-ESR capacitors can be connected in parallel for lower cost. Choose input capacitors with working volt- age ratings higher than the maximum input voltage. Place a surface-mount ceramic capacitor very close to V+ and GND, as shown in Figure 7. This capacitor bypasses the MAX1626/MAX1627, and prevents spikes and ringing on the power source from obscuring the C LI VV C LI VV V VV IN L RIPPLE CIN IN OUT L RIPPLE COUT OUT IN IN OUT = = − 1 2 2 1 2 2 ∆ ∆ , , VRIPPLE,ESR ≈ ()( ) RESR IPEAK VRIPPLE ≈+ ,, VV RIPPLE ESR RIPPLE C 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers ______________________________________________________________________________________ 11 Table 2. Component Suppliers COMPANY PHONE FAX (803) 946-0690 AVX USA or (803) 626-3123 (800) 282-4975 Coilcraft USA (847) 639-6400 (847) 639-1469 Coiltronics USA (516) 241-7876 (516) 241-9339 Dale USA (605) 668-4131 (605) 665-1627 International USA (310) 322-3331 (310) 322-3332 Rectifier IRC USA (512) 992-7900 (512) 992-3377 Motorola USA (602) 303-5454 (602) 994-6430 Nichicon USA (847) 843-7500 (847) 843-2798 Japan 81-7-5231-8461 81-7-5256-4158 Nihon USA (805) 867-2555 (805) 867-2698 Japan 81-3-3494-7411 81-3-3494-7414 Sanyo USA (619) 661-6835 (619) 661-1055 Japan 81-7-2070-6306 81-7-2070-1174 (408) 988-8000 Siliconix USA or (408) 970-3950 (800) 554-5565 Sprague USA (603) 224-1961 (603) 224-1430 Sumida USA (847) 956-0666 (847) 956-0702 Japan 81-3-3607-5111 81-3-3607-5144 United USA (714) 255-9500 (714) 255-9400 Chemi-Con |
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