Femtobuck PWM voltage range confusion

Components & Accessories LEDs
1

We have four Femtobucks connected to a [Teknic Clearcore motion control unit, using four of its PWM ports. The ClearCore runs at 24V. With no devices connected to the ClearCore, 100% duty cycle on any of the PWM outputs is 22.9V, basically what we’d expect. With the 4 Femtobuck drivers connected to four of the digital I/O ports, 100% PWM duty cycle on those ports drops to about 11V. Our LEDs are lit in this state (though they seem a bit dimmer than I’d expect). Setting the duty cycle to 50% drops the output of the PWM port to 5.5v and the lights dim by about half. If we keep halving the duty cycle, until we get the output voltage below 0.4V, the lights remain on, dimming each time by about half. They remain on until the duty cycle is 0. (that is, at 0.2V, lights are still on).

The Femtobuck hookup guide says:

Dimming can be done by an analog voltage (20%-100% of max current by varying voltage from .5V-2.5V) or by PWM (so long as PWM minimum voltage is less than .4V and maximum voltage is more than 2.4V) for a full 0-100% range.

So:

  1. Does this mean the only acceptable PWM voltage range for the Femtobuck is 0.4v - 2.4V? That is - 0% should be <0.4V and 100% should be 2.4V or higher?

  2. If so, why does it continue to get brighter when voltage is well above 2.4v and we increase the duty cycle? Similarly, why does it not shut off when the incoming PWM voltage is below 0.4? The description suggests anything below 0.4 is effectively off, and above 2.4 is 100%?

  3. What might account for the extreme drop in voltage (roughly half) when the duty cycle on the PWM outputs of our controller is set to 100% and there are connected femtobucks …but not when they aren’t connected? (that is: 23V with nothing hooked up, 11V with femtobucks connected)

If the weird behavior is because we’re supplying too much voltage to the PWM inputs, what’s the easiest way to knock down the voltage from the clearcore to the femtobuck, maintaining the PWM signal?

Thanks!](https://www.teknic.com/products/io-motion-controller/)

2

Looking at the datasheet for the AL8805 (the active chip on the femtobuck) https://cdn.sparkfun.com/datasheets/Com … AL8805.pdf

ctrl has an absolute maximum rating of 6v. If you have fed a higher voltage to that pin, you likely have damaged the chip. The valid inputs for the control pin are

DC voltage less than 0.4V - output will be off

DC voltage between 0.5 and 2.5v - output will vary between 20% and 100% full scale current

DC PWM square wave with varying duty cycle. Low must be less than 0.4V. High must be between 2.6v and 5.5V. This can be driven with either a 3.3v or 5v logic level signal.

This is a current regulator. The output current will is based on Vctrl and the shunt resistor. The voltage across the LEDs is dependent on the number and type of LEDs.

If your clearcore driver is providing a 23v output level, the best way to reduce it is either a resistor driver or a FET connected as an open-drain output.

/mike