CAT37

7

Doc No. 5005, Rev. R

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.

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.

For other LED current values, use the following equa-

tion to choose R1.

R1=

0.095

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.

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

µF 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 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 1

µF or 4.7 µF

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

Table 3. R1 Resistor Values

I

R1 (

Ω)

40

2.37

30

3.16

20

4.75

15

6.34

12

7.87

10

9.53

5

19.1

Table 2. Schottky Diode Suppliers

Part

Supplier

MBR0520

ON Semiconductor

MBR0530

www.onsemi.com

MBR0540

800.282.9855

ZHCS400

Zetex