SC4538_08 Datasheet, PDF (12/14 Pages) Semtech Corporation – High Effi ciency Boost Converter for up to 10 LEDs

SC4538
Applications Information (continued)
Diode Selection
A Schottky diode with a reverse voltage of 40V and a
forward current rating of 1A should be used with this
device for optimum performance. The Central
Semiconductor Corporation CMOSH-4E is a suitable diode
for this application.
PCB Layout Considerations
Poor layout can degrade the performance of the DC-DC
converter and can be a contributory factor in EMI prob-
lems, ground bounce, thermal issues, and resistive voltage
losses. Poor regulation and instability can result. A typical
layout is shown in Figure 2.
Selection of Other Components
R sets the maximum load current for the SC4538. Use
ISET
the following equation to select the proper value:
R = 230 × V /I
ISET
ISET LOAD
where
V = 0.5V (typ).
ISET
Refer to Figure 1 for selecting values for other current set-
tings. Notice that the error increases as the desired I
BL
current decreases.
The following design rules are recommended:
• Place the inductor and filter capacitors as close
to the device as possible and use short, wide
traces between the power components.
• Route the output voltage feedback path away
from the inductor and LX node to minimize
noise and magnetic interference.
• Use a ground plane to further reduce noise
interference on sensitive circuit nodes.
100
10
RISET
Typical
Current
Tolerance
1
1
10
100
R_ISET (kΩ)
Figure 1 – Set Resistor Value Selection Graph
Figure 2– Layout
12