DRV8811 DECAY PIN

If SRn is low, the DRV8811 always operates in asynchronous fast decay through the external diodes.  So the voltage on the DECAY pin doesn't matter.

Hi.

Thank you for your reply.

Don't you mean internal diodes? According to the slva344b, the pin SRn must be low if external diodes are not populated. I have seen a lot of doubts in the internet regarding this chip. The datasheet is not clear about diodes/SRn/DECAY pins. I think if TI put some circuits examples it would be helpful. Other matters regarding the corretc selection of PWM frequency should me more detailed.

3.5 Operating the Stepper Motor
3.5.1 Turning the Stepper Motor
The Windows Application, in conjunction with the MSP430F1612 microcontroller, utilizes a series of timers
to coordinate the rate of steps sent to the device. Once all the control signals are configured accordingly
(ENABLEn = LO, SLEEPn = HI, RESETn = HI; DIR, USM0 and USM1 can be HI or LO depending on
preferred mode of operation; SRn must be L, if external diodes are not populated), the motor is ready to
be turned.

Best regards,

Marco

Marco -

Sorry, you're right, my response was incorrect!

If SRn is HIGH, the device operates in asynchronous fast decay through the external diodes, regardless of the voltage on the DECAY pin. 

If SRn is low, the device always operates in synchronous fast decay or slow decay, as shown on page 10 of the datasheet.  Current does not flow through the internal diodes (except during the very brief dead time, when the FETs are changing states).  The voltage applied to the DECAY pin sets if the decay mode is fast, slow, or if it is mixed.  In mixed decay, the decay mode starts out as fast, then switches to slow.  This is shown on page 11 of the datasheet.

Unfortunately, the proper selection of PWM frequency (off time in the case of the DRV8811) and decay mode are totally dependent on external factors, namely the motor inductance, supply voltage, and step rate.  This makes recommending ideal component values impossible.  The reality is that the best results come from experimenting with these parameters in the actual application.

Hi Pete.

Thank you for your support and patience.

I ran some tests in my EVM and I confirmed that.

So, I have just one more doubt. I have no external diodes, SRn is low and I am driving my motor in full step. I noticed that the DECAY voltage does not have effect in this case. Just in 1/2, 1/4 and 1/8 step. In full step only VREF has effect. Is thas correct?

Best regards

Marco

Marco -

Yes, you are correct: in full step mode, since we never change the current (though obviously we do reverse the direction), we always stay in slow decay mode.  We really need to add a note that explicitly calls this out in the datasheet.

Hi ,

Im new to TI motor drives, As my friend said there is no much information on the DECAY pin. In the above post U suggested like voltage on DECAY pin doesn't matter.

So i hope this is only when diodes are not populated so if im correct  what would be the voltage on DECAY pin 

Thanks in Advance.