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Psuedo Random and PRiSM dimmer than PWM | Cypress Semiconductor

Psuedo Random and PRiSM dimmer than PWM

Summary: 7 Replies, Latest post by Bob Marlowe on 11 Oct 2016 09:48 AM PDT
Verified Answers: 0
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tdvdesign's picture
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187 posts

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

user_342122993's picture
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583 posts

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.

tdvdesign's picture
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187 posts

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

user_342122993's picture
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583 posts

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.

 

tdvdesign's picture
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187 posts

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

user_1377889's picture
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9625 posts

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

tdvdesign's picture
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187 posts

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

user_1377889's picture
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9625 posts

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

 

Bob

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