ZXSC400 datasheet(8/16 Pages) ZETEX

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Dimming Control using a DC voltage

For applications where the shutdown pin is not

available a DC voltage can be used to control dimming.

By adding resistors R2 and R3 and applying a DC

voltage, the LED current can be adjusted from 100% to

0%. As the DC voltage increases, the voltage drop

across R2 increases and the voltage drop across R1

decreases, thus reducing the current through the LEDs.

Selection of R2 and R3 should ensure that the current

from the DC voltage is much less than the LED current

and much larger than the feedback current. The

component values in the diagram below represent 0%

to 100% dimming control from a 0 to 2V DC voltage.

Dimming Control using a filtered PWM signal

The filtered PWM signal can be considered as an

adjustable DC voltage by applying a RC filter. The

values shown in the diagram below are configured to

give 0% to 100% dimming for a 1kHz to 100kHz PWM

signal with a 2V amplitude. e.g. a 50% duty cycle will

give 50% dimming.

Dimming Control using a logic signal

For applications where the LED current needs to be

adjusted in discrete steps a logic signal can be applied

as shown in the diagram below. When Q1 os ‘off’,R1

sets the minimum LED current. When Q1 is ‘on’, R2 sets

the LED current that will be added to the minimum LED

current. The formula for selecting values for R1 and R2

are given below:

MOSFET ‘off’

LED(MIN)

LEB

MOSFET ‘on’

LED(MAX)

FLB

LEB

LED(MIN)

Where VFB = 300mV

ZXSC400

ISSUE 1 - JANUARY 2003

67k

10k

ZXSC400 VFB

VDC

67k

10k

ZXSC400

PWM

10k

0.1µF

VFB

ZXSC400 VFB

LOGIC

SIGNAL

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