CAT37_08 Datasheet, PDF (7/13 Pages) Texas Instruments – CMOS White LED Driver Boost Converter

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 1.0µF 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 16V
ceramic capacitor is recommended when driving 3 or
4 LEDs.
Low profile ceramic capacitors with a 1mm 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 1µF or
4.7µF input capacitor is recommended. Table 1 shows
a list of several ceramic capacitor manufacturers.
Consult the manufacturers for detailed information as
new products and package options are introduced
regularly.
Table 1: Ceramic Capacitor Manufacturers
Supplier
Taiyo Yuden
Murata
Kemet
AVX
Web
www.t-yuden.com
www.murata.com
www.kemet.com
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 2 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 0.4A or
0.5A diode will be sufficient for most designs.
Table 2: Schottky Diode Suppliers
Part
MBR0520
MBR0530
MBR0540
ZHCS400
Supplier
ON Semiconductor
www.onsemi.com
800.282.9855
Zetex
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 3 shows several typical 1% R1
values.
Table 4: R₁ Resistor Values
I_LED (mA)
40
30
25
20
15
10
5
R₁ (Ω)
2.37
3.16
4.75
6.34
7.87
9.53
19.1
For other LED current values, use the following
equation to choose R1.
0.095
R1 =
I_LED
Most white LEDs are driven at maximum currents of
15mA to 20mA. Some higher power designs will use
two parallel strings of LEDs for greater light output,
resulting in 30mA to 40mA (two strings of 15mA to
20mA) flowing through the R1 resistor.
© Catalyst Semiconductor, Inc.
7
Characteristics subject to change without notice
Doc. No. MD-5005 Rev. T