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I am using a PCA9685 (outputs 12bit PWM) to control an LED driver, AP8802. The LED driver datasheet states (on page 11, paragraph 2):

A Pulse Width Modulated (PWM) signal with a max resolution of 8-bit, can be applied to the CTRL pin to change the output current to a value above or below the nominal average value set by resistor RSET.

What I would like to know is if it is possible to use the ICs together or will that not be possible, in which case why?

Schematic

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    \$\begingroup\$ Can you clarify what the problem is? How will they be connected? What problem are you trying to solve? \$\endgroup\$
    – Transistor
    Jul 23, 2016 at 12:19
  • \$\begingroup\$ They will be connected using a transistor. So an output from the PCA9685 will go to the base of an NPN transistor. The collector going to pin 8 of the AP8802 (Control pin) and the emitter to GND. And just to reword the problem, it is basically asking if the 12bit pwm will be read by the AP8802 or if 8bit is vital to it working \$\endgroup\$
    – Nikita K
    Jul 23, 2016 at 12:23
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    \$\begingroup\$ Why would you want to use two PWM together at the same transistor? I don't understand what do you want to achieve \$\endgroup\$ Jul 23, 2016 at 12:24
  • \$\begingroup\$ Maybe it will help if i explain what the AP8802 actually does. So it is a constant current LED driver which requires a PWM input in order to set the current flow through its output. In the data sheet for the AP8802 it says that it requires 8bit pwm but the PCA9685 chip outputs 12bit pwm and that is why I am wondering if it will work as I am using the PCA9685 to driver 16 of the AP8802s \$\endgroup\$
    – Nikita K
    Jul 23, 2016 at 12:27
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    \$\begingroup\$ @NikitaK please make a schematic of your system, I think you, and we, are confused about what you mean with PWM signal here. A PWM signal doesn't have "bittiness". \$\endgroup\$ Jul 23, 2016 at 12:31

2 Answers 2

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I am using two chips with two different PWM resolutions. The AP8802 (needs 8bit) ...

As per your comment below, this is stated on page 11, paragraph 2:

A Pulse Width Modulated (PWM) signal with a max resolution of 8-bit, can be applied to the CTRL pin to change the output current to a value above or below the nominal average value set by resistor RSET.

enter image description here

The datasheet is very short on details of the inner workings other than Figure 1. Normally the PWM input would be passed through to the power output stage but the highlighted block suggests an internal low-pass filter (the line through the high and medium waves signifying that they are removed). The control input also has to cope with PWM and regular dimming so there's probably some funky stuff going on inside - but it could be as simple as an R-C filter to average the PWM input into an analog voltage.

The best suggestion I have to offer is that the subsequent block has an 8-bit A/D converter and that there are only 256 discrete output levels available.

For your application you don't have to worry about this. Your 12-bit PWM will be filtered and read by the AP8802 ADC and converted to an 8-bit value internally, effectively losing some of the resolution of the external controller.

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  • \$\begingroup\$ I understand that but it seems to want it. Please forgive how crude the image is but its the best I can do on my phone. The text between the lines is what is causing the confusion. dropbox.com/s/fb9qeipe5lphhdm/… \$\endgroup\$
    – Nikita K
    Jul 23, 2016 at 13:06
  • \$\begingroup\$ Where did you read "The AP8802 (needs 8bit)"? \$\endgroup\$
    – Transistor
    Jul 23, 2016 at 13:10
  • \$\begingroup\$ that was the data sheet surprisingly! \$\endgroup\$
    – Nikita K
    Jul 23, 2016 at 13:22
  • \$\begingroup\$ Please help us to help you. You are making this very difficult. The datasheet is 19 pages long. Where in the datasheet is this stated. \$\endgroup\$
    – Transistor
    Jul 23, 2016 at 13:23
  • \$\begingroup\$ oh it was in the picture I sent in the last message. But is is page 11 second paragraph \$\endgroup\$
    – Nikita K
    Jul 23, 2016 at 13:26
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When talking about bits of pwm resolution, my assumption would be that for a system with N bits of resolution, the processor is capale of breaking up the cycle time into 2^N-1 slices, and can thus identify or output that many different values.

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