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I am powering a Charlieplexed LED array with this current source as shown in the first picture attached:

enter image description here

One requirement was to be able to control the current so I decided to insert an array of MOSFETS that allowed me to control the current through a PWM signal following the datasheet of the LED driver manufacturer.

enter image description here

As can be seen in the second image the circuit actually works, what I don't understand is why.

Ignoring the current variation given different loads (resistor + LED) my intuition would say that the PMOS would control the voltage level but not the current due to the AL5810 that should keep it constant.

Is that so or are there other variables which I'm not considering?

JRE
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Smart_Celery
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1 Answers1

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The PMOS, which is in series with the IN terminal of the current source IC, operates as an on-off switch: When it's on, the constant-current driver IC sees the input voltage, turns itself on, and sets the output current. Then this current flows through the LED. When it's off, no current flows through the LED.

If you apply this input voltage as PWM (i.e. on-off signal with variable on-time), the variable duty cycle will reflect the LED as variable brightness because the average value varies with the duty cycle: \$\mathrm{I_{LED-avg}=I_{set}\ \frac{T_{on}}{T_{on}+T_{off}}}\$.

Rohat Kılıç
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  • Thanks for your answer, with that I was able to connect the dots with figure 4 of the datasheet, now it all makes sense! – Smart_Celery Jul 02 '21 at 09:15