CAT37 datasheet(7/13 Pages) TI

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CAT37

Doc. No. MD-5005 Rev. T

Characteristics subject to change without notice

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

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 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

Supplier

MBR0520

MBR0530

MBR0540

ON Semiconductor

www.onsemi.com

800.282.9855

ZHCS400

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

ILED (mA)

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.

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.

LED

095

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