CAT32TDI-GT3 datasheet(8/14 Pages) CATALYST

номер детали	CAT32TDI-GT3

подробное описание детали	CMOS White LED Driver

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производитель	CATALYST [Catalyst Semiconductor]

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CAT32

Doc. No. MD-5001 Rev. AD

8

Characteristics subject to change without notice

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

MBR0530 s

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 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, ILED, to

225 times the value of ISET. For the best accuracy, a

1% or better resistor is recommended. Table 4 shows

several typical 1% RSET values.

Table 4: RSET Resistor Values

ILED (mA)

RSET

40

562Ω

30

750Ω

25

909Ω

20

1.13kΩ

15

1.50kΩ

10

2.26kΩ

5

4.53kΩ

For other LED current values, use the following

equation to choose RSET.

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.

LED DIMMING WITH PWM SIGNAL

PWM brightness control provides the widest dimming

range (greater than 20:1). By turning the LEDs ON

and OFF using the control signal the LEDs operate at

either zero or full current, but their average current

changes with the PWM signal duty cycle. Typically, a

5kHz to 40kHz PWM signal is used. PWM dimming

with the CAT32 can be accomplished two different

ways.

The SHDN

¯¯¯¯¯ pin can be driven directly or a resistor

can be added to drive the RSET pin. If the SHDN

¯¯¯¯¯

pin is used, increasing the duty cycle will increase

the LED brightness. Using this method, the LEDs

can be dimmed and turned off completely using the

same control signal. A 0% duty cycle signal will turn

off the CAT32, reducing the total quiescent current

to near zero.

If the RSET pin is used, increasing the duty cycle will

decrease the brightness. Using this method, the LEDs

are dimmed using RSET and turned off completely

using SHDN

¯¯¯¯¯. If the RSET pin is used to provide PWM

dimming, the approximate value of RPWM should

be calculated (where VMAX is the “HIGH” value of the

PWM signal):

In addition to providing the widest dimming range,

PWM brightness control also ensures the “purest”

white LED color over the entire dimming range. The

true color of a white LED changes with operating

current, and is the “purest” white at a specific forward

current, usually 15mA or 20mA. If the LED current is

less than or more than this value, the emitted light

becomes more blue. Applications involving color

LCDs can find the blue tint objectionable.

When a PWM control signal is used to drive the

SHDN

¯¯¯¯¯ pin of the CAT32, the LEDs are turned off and

on at the PWM frequency. The current through them

alternates between full current and zero current, so

the average current changes with duty cycle. This

ensures that when the LEDs are on, they can be

driven at the appropriate current to give the purest

white light. LED brightness varies linearly with the

PWM duty cycle.

LED

SET

I

V

1

.

0

×

255

=

R

)

1

–

V

15

.

0

V

(

×

R

=

R

MAX

SET

PWM

Аналогичный номер детали - CAT32TDI-GT3

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ON Semiconductor	CAT32TDI-GT3	473Kb / 13P	CMOS White LED Driver Doc. No. MD-5005 Rev. V
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