Datasheet LT3517 (Analog Devices) - 10

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