CAT37 データシートの表示（PDF） - ON Semiconductor
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CAT37 Datasheet PDF : 11 Pages
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CAT37
Application Information
Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
The low ESR and small package options available with
multilayer ceramic capacitors make them excellent choices.
The X5R and X7R capacitor types are preferred because
they retain their capacitance over wider voltage and
temperature ranges than the Y5V or Z5U types. A 1.0 mF
output capacitor is recommended for most applications.
The voltage rating of the output capacitor C2 depends on
the number of LEDs driven in the series. A 16 V ceramic
capacitor is recommended when driving 3 or 4 LEDs.
Low profile ceramic capacitors with a 1 mm maximum
height/thickness are available for designs height
requirements. Ceramic capacitors also make a good choice
for the input capacitor, which should be mounted as close as
possible to the CAT37. A 1 mF or 4.7 mF input capacitor is
recommended. Table 5 shows a list of several ceramic
capacitor manufacturers. Consult the manufacturers for
detailed information as new products and package options
are introduced regularly.
Table 5. CERAMIC CAPACITOR MANUFACTURERS
Supplier
Web
Taiyo Yuden
www.t−yuden.com
Murata
www.murata.com
Kemet
www.kemet.com
AVX
www.avxcorp.com
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are the ideal choice for high efficiency
applications. Table 6 shows several different Schottky
diodes that work well with the CAT37. Make sure that the
diode has a voltage rating greater than the output voltage.
The diode conducts current only when the power switch is
turned off (typically less than one−third the time), so a 0.4 A
or 0.5 A diode will be sufficient for most designs.
Table 6. SCHOTTKY DIODE SUPPLIERS
Part
Supplier
MBR0520
MBR0530
MBR0540
ON Semiconductor
www.onsemi.com
800.282.9855
LED Current Programming
The LED current is programmed with a single resistor
connected to the FB pin. The FB pin is internally regulated
to 95 mV, which sets the current flowing through R1 and the
LEDs as equal to 0.095/R1. For the best accuracy, a 1% or
better resistor is recommended. Table 7 shows several
typical 1% R1 values.
Table 7. R1 RESISTOR VALUES
ILED (mA)
40
R1 (W)
2.37
30
3.16
20
4.75
15
6.34
12
7.87
10
9.53
5
19.1
For other LED current values, use the following equation
to choose R1.
R1
+
0.095
ILED
Most white LEDs are driven at maximum currents of
15 mA to 20 mA. Some higher power designs will use two
parallel strings of LEDs for greater light output, resulting in
30 mA to 40 mA (two strings of 15 mA to 20 mA) flowing
through the R1 resistor.
http://onsemi.com
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