Datasheet LT3518 (Analog Devices) - 10

LT3518
applications inForMation
For a buck or a buck-boost configuration, the output voltage Table 1 provides some recommended inductor vendors. is typically level-shifted to a signal with respect to GND as illustrated in the Figure 4. The output can be expressed as:
Table 1. Inductor Manufacturers VENDOR PHONE WEB
V R1 Sumida (408) 321-9660 www.sumida.com OUT = • 1.01V + VBE(Q1) (5) R2 Toko (408) 432-8281 www.toko.com Cooper (561) 998-4100 www.cooperet.com Vishay (402) 563-6866 www.vishay.com + R1 RSENSE
Input Capacitor Selection
VOUT LED For proper operation, it is necessary to place a bypass – ARRAY LT3518 Q1 capacitor to GND close to the VIN pin of the LT3518. A FB 1µF or greater capacitor with low ESR should be used. A R2 ceramic capacitor is usually the best choice. 3518 F04 In the buck mode configuration, the capacitor at the input to the power converter has large pulsed currents due to
Figure 4
the current returned though the Schottky diode when the switch is off. For best reliability, this capacitor should have
Inductor Selection
low ESR and ESL and have an adequate ripple current The inductor used with the LT3518 should have a satura- rating. The RMS input current is: tion current rating of 2A or greater. For buck mode LED drivers, the inductor value should be chosen to give a IIN(RMS) =ILED • (1– D) •D (8) ripple current “∆I” of ~30% to 40% of the LED current. In the buck mode, the inductor value can be estimated where D is the switch duty cycle. A 2.2µF ceramic type using the formula: capacitor is usually sufficient. D
Output Capacitor Selection
L µ ( H) ( ) = BUCK • tSW(µs) • VIN – VLED (6) ∆I The selection of output capacitor depends on the load and converter configuration, i.e., step-up or step-down. D VLED BUCK = For LED applications, the equivalent resistance of the LED VIN is typically low, and the output filter capacitor should be sized to attenuate the current ripple. VLED is the voltage across the LED string, VIN is the input voltage to the converter, and tSW is the switching period. To achieve the same LED ripple current, the required filter In the boost configuration, the inductor can be estimated capacitor value is larger in the boost and buck-boost mode using the formula: applications than that in the buck mode applications. For LED buck mode applications, a 1µF ceramic capacitor L µ D ( H) = BOOST • tSW(µs)• VIN (7) is usually sufficient. For the LED boost and buck-boost ∆I mode applications, a 2.2µF ceramic capacitor is usually sufficient. Very high performance PWM dimming appli- D VLED – VIN BOOST = cations may require a larger capacitor value to support VLED the LED voltage during PWM transitions. 3518ff 10 For more information www.linear.com/LT3518 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 Typical Application Related Parts