CAT32 Datasheet, PDF (8/14 Pages) Catalyst Semiconductor – CMOS White LED Driver

CAT32
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
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 3: Schottky Diode Suppliers
Part
Supplier
MBR0520
ON Semiconductor
MBR0530
www.onsemi.com
MBR0540
800.282.9855
ZHCS400
Zetex
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
Table 4: RSET Resistor Values
I (mA)
LED
40
30
25
20
15
10
5
RSET
562Ω
750Ω
909Ω
1.13kΩ
1.50kΩ
2.26kΩ
4.53kΩ
regulates the current into the LED pin, I_LED, to 225 times
the value of ISET. For the best accuracy, a 1% or better
resistor is recommended. Table 4 shows several typical
1% R_SET values.
For other LED current values, use the following equation to choose R_SET.
R_SET
=
225
×


0.1V
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 into the LED
pin.
Doc. No. 5001, Rev. AC
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