XRP7604 Datasheet, PDF (11/13 Pages) Exar Corporation – 1A 29V Non-Sync. Buck High Power LED Driver

XRP7604
1A 29V Non-Sync. Buck High Power LED Driver
DESIGN EXAMPLE
Design a drive circuit for a string of 5 LED at
0.75A with a 24V input voltage. Nominal LED
voltage is 3.3V.
Resistor RFB calculation
RFB = ^0.2V = 0.27Ω
0.75 A
A standard value of 0.27ohm 0805 is selected.
Inductor L1 calculation
L1
=
(5× 3.3V )× (24V − (5× 3.3V ))
24V ×1.2MHz × (0.3× 0.75A)
=
19.1μH
Use a 22uH standard inductor.
Input capacitor
A 4.7µF C_IN (C1) is needed (refer to table 1).
From Equ.4, the ripple current rating of C_IN is
a fraction of 0.75A. A 4.7uF/25V ceramic
capacitor easily meets this requirement and
offers low ESR and ESL.
Schottky current rating I_F
I_F ≥
1 − ^{(5 × 3.3)} × 0.75A = 0.42A
24V
Voltage rating should be 30V. B340A rated at
30V/3A or equivalent can be used for its
ample current rating and low forward voltage.
Rs calculation
Rs = ^{0.3V − (1.5 ×1.15 ×1.5 × 0.75A × 0.095Ω)} = 3.5kΩ
33μA
Use standard resistor value for Rs of 3.4kΩ.
Fig. 17: Circuit for design example
LAYOUT CONSIDERATION
i) Place the bypass capacitors C4 and C5 as
close as possible to the XRP7604 IC. See
figure 5 for details.
ii) Create a pad under the IC that connects the
power pad (pin 9) to the inductor. Duplicate
this pad through the pcb layers if present, and
on the bottom side of the PCB. Use multiple
vias to connect these layers to aid in heat
dissipation. Do not oversize this pad - since
the LX node is subjected to very high dv/dt
voltages, the stray capacitance formed
between these islands and the surrounding
circuitry will tend to couple switching noise
iii) Connect the Schottky diode cathode as
close as possible to the LX node and inductor
input side. Connect the anode to a large
diameter trace or a copper area that connects
the input ground to the output ground.
iv) The output capacitor, if used, should be
placed close to the load. Use short wide
copper regions to connect output capacitors to
load to minimize inductance and resistances.
v) Keep other sensitive circuits and traces
away from the LX node in particular and away
from the power supply completely if possible.
For more detail on the XRP7604 layout see the
XRP7604EVB Evaluation Board Manual
available on our web site. Each layer is shown
in detail as well as a complete bill of materials.
Fig. 18: XRP7604 Eval Board
Component Side Lay
© 2009 Exar Corporation
11/13
Rev. 1.0.0