0
\$\begingroup\$

I've decided on using WS2803 PWM constant current LED drivers. I'd like to cascade two of them and use the first one for 15mA output and the second one for 20mA output (each chip for driving different LEDs).

The output current is based on a resistor connected to each chip. So I thought this shouldn't be a problem, until I read this in the datasheet:

"In LED display and decorative applications, WS2803 provides nearly no variations in current from channel to channel and from IC to IC."

Does this mean I can't cascade two chips with differently set output current? Or is it just my misunderstanding, based on my fuc*ing poor english?

Also, the datasheet mentions resistors for impedance match, when cascading (up to 6 meters far). Do I care about those, when the ICs are just few centimeters far?

Thank you everyone!

\$\endgroup\$
1
  • \$\begingroup\$ What it means is that you can trust that using the same resistor value on 2 different units will give you the same current from both of them, so that it you want the same brightness you won't have to individually tune each one. \$\endgroup\$
    – brhans
    Mar 14, 2016 at 22:05

1 Answer 1

Reset to default
1
\$\begingroup\$

WS2803 provides nearly no variations in current from channel to channel

Means that the adjacent LED channels will sink nearly the same amount of current (+/- 2%) when they are set to the same duty cycle.

and from IC to IC

Refers to the promise that separate chips are guaranteed to drive the leds at nearly the same current (within +/- 4%) when sharing identical current setting resistors.

So you can configure the current separately for each chip, although not for each channel within a chip. The drive current is determined by the resistance of the current setting resistor Rext, which should be connected between the Iref pin and ground:

Where Vrext is equal to 1.25 V.
To drive the outputs of a WS2803 at 15 mA, you need a resistance of 22 * 1.25 V / 15 mA = 1833 Ω, while for 20 mA you need 22 * 1.25 V / 20 mA = 1375 Ω.

While the drive current of adjacent channels is fixed, the LED brightness can still be controlled on a per-channel basis with pulse width modulation: The leds are rapidly switched on and off at 4687 Hz, and they can be set individually to 256 different values between off all the time (0) and on all the time (255).

\$\endgroup\$
1
  • \$\begingroup\$ @user3396686 Thanks for the compliment, but you shouldn't accept an answer this soon, as somebody else might be writing a better answer than mine. In the future, wait at least for a few hours. \$\endgroup\$
    – jms
    Mar 14, 2016 at 22:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.