CAT32

8

Doc. No. 5001, Rev. AC

Table 4: RSET Resistor Values

I

RSET

40

562

Ω

30

750

Ω

25

909

Ω

20

1.13k

Ω

15

1.50k

Ω

10

2.26k

Ω

5

4.53k

Ω

DIODE SELECTION

Schottky diodes, with their low forward voltage drop and

fast switching speed, are the ideal choice for high

efficiency applications. Table 3 shows several different

Schottky diodes that work well with the CAT32. Make

sure that the diode has a voltage rating greater than the

Table 3: Schottky Diode Suppliers

Part

Supplier

MBR0520

ON Semiconductor

MBR0530

www.onsemi.com

MBR0540

800.282.9855

ZHCS400

Zetex

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 RSET pin The RSET pin is internally

regulated to 100mV, which sets the current flowing out

of this pin, ISET, equal to 100mV/RSET. The CAT32

regulates the current into the LED pin, I

the value of ISET. For the best accuracy, a 1% or better

resistor is recommended. Table 4 shows several typical

1% R

For other LED current values, use the following equation to choose R

R

V

I

SET

LED

=×









225

01

.

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 into the LED

pin.