PSoC 4 MCU

[ToVa_285016]

Hi,

   

Can anyone think of a reason that the Pseudo random and PRISM outputs result in a dimmer light than the direct PWM output when fed the same data?  At full power they are the same, but at half power the Pseudo and PRISM are significantly dimmer than the PWM.  Do they have a built in gamma correction or something?
Thank you,
Tom

[odissey1]

        Apart from some banal reasons, it can be LED threshold voltage ~1.5-2.7V. If PRS frequency is high, some of randomly-short pulses won't pass through parasitic RC, leaving peak voltage below LED threshold. For PWM at 50% this won't happen, as pulse is long enough.   

[ToVa_285016]

Hi Odissey,

   

Thanks for the post.  The PWM is going to a constant current LED driver, the LED should be receiving its full voltage (3V in this case), just in pulses, correct?  Unfortunately the PWM at 50% is noticeably dimmer.  I may have to start a case on this and get back to the forum with results.

   

Regards,

   

Tom

[odissey1]

Do you control current driver digitally (TTL ON-OFF)?  Then it may be not enough bandwidth in the driver.   To confirm, reduce PRISM carrier frequency to <100kHz, then brightness should restore. Can you post the driver type?

   

The same problem is likely exist with PWM at very low duty cycle, but then LED is so dim that it's hard to notice that it is sagging.

[ToVa_285016]

Hi,

   

Driver is a Mean Well LDD-500L Driver:

   

http://www.mouser.com/ds/2/260/LDD-L-SPEC-806086.pdf

   

It is a TTL On/Off pwm input driver.  Specified PWM frequency is 100Hz-1kHz.  If I am understanding correctly, the (carrier frequency / Period = ) PWM frequency should fall between that range, correct?  If so, that is what I am currently doing.

   

Thank you for your feedback,
Tom

[JoMe_264151]

There is a difference between the two modules: While the PWM frequency is stable and does not change, the PrISM switching frequency varies randomly and so can be sometimes outside the range of 100..1kHz.

       

Bob

[ToVa_285016]

Ok, that makes sense.  Is there a way to put boundaries on it?  Why is full on the same for both PWM and Pseudo PWM?

   

Thank you,
Tom

[JoMe_264151]

In "full" mode no switching happens. The output signal is 100% high.

       

Bob

[odissey1]

tvdesign,

   

I got this paper describing LED turn-on delay, which covers the issue with LED drivers and PWM:

   

http://www.digikey.com/en/articles/techzone/2016/oct/how-to-dim-an-led-without-compromising-light-qu...

       

"The designer is facing something of a trade-off though, because the higher the frequency, the greater the impact on contrast ratio. This is because even the best LED driver takes a finite time to respond to a PWM input. Figure 5 illustrates where these time delays occur."

   

   

Figure 5: An LED driver exhibits delays in its response to a dimming PWM signal. These delays determine the dimming system’s maximum contrast ratio. (Source: Texas Instruments)