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LT3475 Datasheet(PDF) 10 Page - Linear Technology |
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LT3475 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3517 10 3517ff Table 1 provides some recommended inductor vendors. Table 1. Inductor Manufacturers VENDOR PHONE WEB Sumida (408) 321-9660 www.sumida.com Toko (408) 432-8281 www.toko.com Cooper (561) 998-4100 www.cooperet.com Vishay (402) 563-6866 www.vishay.com Input Capacitor Selection For proper operation, it is necessary to place a bypass capacitor to GND close to the VIN pin of the LT3517. A 1μF or greater capacitor with low ESR should be used. A ceramic capacitor is usually the best choice. In the buck mode configuration, the capacitor at the input to the power converter has large pulsed currents due to the current returned though the Schottky diode when the switch is off. For best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. The RMS input current is: IIN(RMS) = ILED •(1– D) • D (8) where D is the switch duty cycle. A 2.2μF ceramic type capacitor is usually sufficient. Output Capacitor Selection The selection of output capacitor depends on the load and converter configuration, i.e., step-up or step-down. For LED applications, the equivalent resistance of the LED is typically low, and the output filter capacitor should be sized to attenuate the current ripple. To achieve the same LED ripple current, the required filter capacitor value is larger in the boost and buck-boost mode applications than that in the buck mode applications. For LED buck mode applications, a 1μF ceramic capacitor is usually sufficient. For the LED boost and buck-boost mode applications, a 2.2μF ceramic capacitor is usually sufficient. Very high performance PWM dimming appli- cations may require a larger capacitor value to support the LED voltage during PWM transitions. Use only ceramic capacitors with X7R, X5R or better dielec- tric as they are best for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Always check that the voltage rating of the capacitor is sufficient. Table 2 shows some recommended capacitor vendors. Table 2. Ceramic Capacitor Manufacturers VENDOR PHONE WEB Taiyo Yuden (408) 573-4150 www.t-yuden.com AVX (843) 448-9411 www.avxcorp.com Murata (770) 436-1300 www.murata.com TDK (847) 803-6100 www.tdk.com Loop Compensation The LT3517 uses an internal transconductance error ampli- fier whose VC output compensates the control loop. The external inductor, output capacitor, and the compensa- tion resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop stability. For typical LED applications, a 10nF compensation capacitor at VC is adequate and a series resistor is not required. A compensation resistor may be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on VIN or CTRL. Diode Selection The Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage. If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with sufficiently low leakage current. Table 3 has some recommended component vendors. APPLICATIONS INFORMATION |
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