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A couple of questions for your ADD5211 LED driver

Dear Sir/Madam,

Our customer is evaluating with your ADD5211 LED driver.

So he just tossed a couple of questions as following

He designed 2 ganged on 2 ch ,and each channels pull 3 LEDs. 

Vf=1.8V LED

Q-1

Can your ADD5211 drive as current dimming control not PWM control?

He needs the dimming fuction as current control.

PWM bring thr flicker for LED on-off.

This flicker make bad for his products.

Q-2

If he put the over 22uF for Cout on your datasheet page-15, then what trouble coming? Growth ripple noise?

Q-3

As for thr PWM frequency,

when 300Hz:dimming=1000:1

So when 25kHz:dimming=?:1, =1000:1 correct?

This is brought from page-1 on your datasheet FEATURES.

Q-4

If he put the frequency of PWM from 300Hz to 20kHz, then he just found that the charge pump voltage become growth high.

Why this result have comin?

Anode voltage of LED(as VLED)

duty 0:900, VLED=4V

duty 1:900, VLED=38.6V

duty 2:900, VLED=37V

duty 3:900, VLED=28.2V

.

.

.

duty 8:900, VLED=9.6V

But when the PWM freq=300Hz, VLED=4V all.

Thanks Kaos

  • Hello,

    Q1:

    Yes, you can dim the ADD5211 with a DC voltage to produce a DC current. I've attached a spreadsheet describing how to do it.  So for this, just tie the PWM pin high.

    Q2:

    Increasing COUT will not help with any PWM dimming variation.  It is only controlled by the current sinks.

    Q3:

    On p. 4 of the datasheet, you can see the "PWM Input" specifications.  Here, the turn on and turn off time is listed.  It is these specs that determine the dimming ratio as the PWM frequency is increased. 

    Q4:

    For measuring the LED voltage when PWM dimming, you must use an oscilloscope.  A volt meter will not properly measure the pulsed output voltage. 


    Best Regards,

    - Jon

  • Hi Jon,

    Thanks for your answer here.

    As for your answer for my question-1,

    Our customer is studying with your ADD5211 as drive current control.

    So from your spread sheet past you enclosed, he has tryed to calcurate as my enclosed.

    Look at the sheet "case of our customer"of "ADD5211 case study.xlsx".

    Min currnet is showed 15mA.

    Question:

    Can we control to drop less than 15mA?(How can we control 0mA to 15mA?)

    Customer have used 3.3V power source.

    Thanks Kaos

  • Hi Jon,

    Thanks I got it.

    I will let him know this 15mA is lower current and 3 models you are recommended also.

    Thanks again and I can still catche our customer.

    Thanks Kaos

  • Hi Kaos,

    No, the minimum is 15mA.  For lower current, I would suggest the ADD5203, ADP8870 (or ADP8866), or the ADP8140.  The ADP8140 has the best DC dimming and DC dimming range. 

    Best Regards,

    - Jon

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