XRP7604 Datasheet, PDF (10/13 Pages) Exar Corporation – 1A 29V Non-Sync. Buck High Power LED Driver

XRP7604
1A 29V Non-Sync. Buck High Power LED Driver
Equ.3: L = ^{Vo × (Vin −Vo)}
Vin × f × ΔI_L
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)
ΔI_L 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.
I_O(A)
Cin (µF)
<0.7
0.71 to 1.2
>1.2
2.2
4.7
2 x 4.7
Table 1: Cin Selection
Calculate the ripple current requirement from:
Equ.4: Irip = Io × D × (1− D)
Where D = ^Vo
Vin
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 : If ≥ 1− ^Vo × Io
Vin
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.
© 2009 Exar Corporation
10/13
Rev. 1.0.0