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XRP7604 Datasheet(PDF) 10 Page - Exar Corporation |
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XRP7604 Datasheet(HTML) 10 Page - Exar Corporation |
10 / 13 page X XR RP P7 76 60 04 4 1 1A A 2 29 9V V N No on n--S Sy yn nc c.. B Bu uc ck k H Hiig gh h P Po ow we err L LE ED D D Drriiv ve err © 2009 Exar Corporation 10/13 Rev. 1.0.0 Equ.3: L I f Vin Vo Vin Vo L Δ × × − × = ) ( Where Vin is converter input voltage and Vo is converter output voltage. Since voltage across Rset is small, Vo approximately equals Vf (for a string of series connected LEDs Vo equals total Vf) ΔIL is inductor current ripple (nominally set to 30% of Io) Inductor Isat and Irms must allow sufficient safeguard over output current Io. As a rule of thumb these parameters should be 50% higher than Io. Where high efficiency is required a low DCR inductor should be used. INPUT CAPACITOR SELECTION Select the input capacitor for capacitanceand ripple current rating. Use the capacitances listed in the table 1 as a starting point and if needed increase Cin. IO(A) Cin (µF) <0.7 2.2 0.71 to 1.2 4.7 >1.2 2 x 4.7 Table 1: Cin Selection Calculate the ripple current requirement from: Equ.4: ) 1 ( D D Io Irip − × × = Where Vin Vo D = Ceramic capacitors are recommended for input filtering due to their low Equivalent Series Resistance (ESR), Equivalent Series inductance (ESL) and small form factor. SCHOTTKY RECTIFIER SELECTION Select the Schottky D1 for Voltage rating VR and current rating If. Recommended schottky diode voltage rating for 12V and 24V applications is 30V and 40V respectively. Current rating can be calculated from: Equ.5 : Io Vin Vo If × − ≥ 1 Note that in applications where duty cycle is low, Schottky losses comprise a larger percentage of converter losses. In order to improve the efficiency in these applications choose a Schottky that meets the calculated current rating and has a lower Vf. FEEDBACK RESISTOR RFB R2 is part of XRP7604 loop compensation network. Use a 30k R2 for Vin of 20V and larger. Use R2 of 60K for Vin less than 20V. CAPACITOR C5 This is the decoupling capacitor for the power supply for the internal driver. Use a 0.1uF and place as closely to VDR and SVIN pins as possible. |
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