Problem with DRV8833

Support Motor controllers/drivers and motors
1

Hello,

I have a DRV8833 set up the following way:

Both GND - connected to 0V
VIN - connected to 5V
OUT B1 - connected to small electrical motor
OUT B2 - connected to small electrical motor
OUT A2 - connected to laser +5V (laser GND connected to 0V)
OUT A1 - connected to 0V
AISEN - not connected
BISEN - not connected
VMM - not connected
IN B1 - connected to 0V
IN B2 - connected to motor PWM
IN A2 - connected to laser power
IN A1 - connected to 0V
SLP - not connected
FLT - not connected

The problem I’m having is with the laser. Whenever the DRV8833 starts with IN A2 at +5V everything works fine, the laser starts and I get 5V at OUT A2. However, if I set IN A2 to 0V and then +5V again a couple of seconds later the DRV8833 never outputs more than 1.6V on OUT A2 and the laser won’t start. The laser uses <100mA according to manual so shouldn’t be a problem, and I haven’t activated current limiting on the chip anyway.

I have noticed that the FLT pin is pulled low, but I don’t know why it would be that. The power to the chip is at a stable 5.12V, the chip is not warm and as stated above, the laser should not draw more than 100mA and does according to my measurements seem to draw around 80mA in this scenario.

What could be wrong? A problem with the chip or my wiring?

2

Sounds to me like the laser has a big capacitor that draws more current that supported very briefly.
Try adding a 10 Ohm resistor before the laser?

3

Thanks for the tip, this did indeed help!
10 ohms was way too high though, even at 2.5 ohms which is the smallest I have the voltage drop is high enough to put the power to the laser right at the lowest allowed input. When I connected my testing board it even started working without a resistor, so it’s probably enough with even < 1 ohm.

4

Hello.

I noticed that you specified that OUTA1 is connected to 0V in your system. If that is connected to your system ground, you should remove the connection. That pin can be driven high by the DRV8833, so it is not good practice to connect it to system ground even if you do not ever expect it to drive high in your particular setup. If you meant that it is connected to ground on the laser (which is completely separate from the grounds of the rest of the system), it might be okay.

If you have an inductor, you might consider using that in place of the 2.5ohm resistor, since they generally have less resistance, and motor drivers like the DRV8833 are designed to handle inductive loads well.

-Claire

5

Thanks for the tip about the inductor.

As for OUT A1 being connected to 0V I wanted to be sure the GND of the laser was the same 0V as the rest of the system (to make sure all signals to it work correctly). I was unsure if the output from the DRV8833 was floating in relation to the rest of the system. But having looked up H-bridges I realize that is not the case. Hopefully it shouldn’t be a problem given the input is hard soldered to 0V as well.