LP5012: PWM smoothing

Hi Peter,

Sorry for late reply. To confirmation, the 'reservoir' capacitor you mentioned is adding capacitors between output pin with GND, or paralleling with LED?

Hi Hardy,

No problem - thanks for getting back to me.

My thought was to connect the capacitor in parallel with the LED.

However, if you feel that connecting it between output and ground would be a better approach, please let me know. 

Either method would be ok from our point of view.

Thanks and best regards,

Peter

Hi Peter,

Some designs added a small capacitor between output with GND to prevent potential EMI issue. It would influence the brightness in low PWM setting (1 or 2 bits). But I hardly see capacitor connected in parallel with the LED. What problem you want to prevent?

Hi Hardy,

Our application would use the LP5012 to drive a light source which we would use to illuminate specialist imaging devices. There is a possibility that the imaging devices will misbehave in certain circumstances as a result of the pulsed light output caused by the PWM. Our intention would be to use a reservoir capacitor in parallel with each LED to provide smoothing of the PWM pulses and reduce light output fluctuations to say 10% (from 100% with raw PPM). We would probably be looking to fit larger capacitors than for the EMI issue you mention. The actual value required is dependent on a number of factors so I can't give a precise figure. As a guide though, it is most likely to between 0.1uF and 10uF, but may even be as high as 100uF.

The question is, would this level of capacitive load cause the LP5012 to misbehave in any way?

Many thanks,

Peter

Hi Peter,

Thanks for the detailed explanation. There should be no misbehavior parallel a capacitor with LED. The output channels are open-drain structure. As long as the voltage dropped on output pins above saturation voltage, they can work normally.