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LM628/LM629 - PWM / 8 bit binary output relationship query

Steve Hobbs1
Prodigy 40 points
Other Parts Discussed in Thread: LM629

I have a legacy design using the LM628 (to drive a DAC which drives a motor).  Due to the LM628 going EOL, I need to come up with a suitable replacement.  This is an older design with lots of code specifically written to carefully tune the PID in the LM628.  Also, for reasons beyond the scope of my project, I cannot change the DAC or motor.  One possible solution I am exploring is replacing the LM628 with an LM629 and then using a fast microcontroller (>64MHz - or 8 x the clock of the LM628/9) to convert the PWM output of the LM629 back into an 8 bit binary value.

What is the relationship between the LM629 PWM and the LM628 binary value?  Is it purely linear?  Meaning:

0% DC --> 00000000

100% DC --> 11111111

And NN% DC --> (0.NN x 255)?

Thanks in advance for any feedback.

  • 0
    TI__Mastermind 23421 points

    Hey Steve, 

    See this post for a similar question answer: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/698467/lm629-what-is-the-connection-between-lm629-s-pid-output-and-lm629-s-pwm-output

    The LM628 DAC output port can be configured to provide either a latched eight-bit parallel output or a multiplexed 12-bit output. The 8-bit output can be directly connected to a flow-through (non-input-latching) D/A converter; the 12-bit output can be easily demultiplexed using an external 6-bit latch and an input-latching 12-bit D/A converter. The DAC output data is offset-binary coded; the 8-bit code for zero is 80 hex and the 12-bit code for zero is 800 hex. Values less than these cause a negative torque to be applied to the motor and, conversely, larger values cause positive motor torque. The LM628, when configured for 12-bit output, provides signals which control the demultiplexing process.

    You can find that in the motor outputs section of the datasheet.

    Here's a detailed application note on the LM628/629 if you haven't seen it already:  www.ti.com/.../snoa184d.pdf

    I don't see any modern motor drivers with a DAC output, though many of our modern motor drivers have smart-tune and other features that might be able to replace some of your system depending on the application.  

    Cheers,

    Jacob Thompson

  • 0 in reply to Jacob Thompson
    Prodigy 40 points

    Thanks for the response.  However, either I don't understand the other thread or it isn't asking the same question at all. 

    My question is: what is the relationship (equation) between the single wire PWM output (not counting sign) on the LM629 and the 8 bit output on the LM628?

    Essentially, I want to "undo" the 8-bit PWM box in Figure 1 of the document you linked to, and convert the 8-bit PWM back to the PID Filter output.

  • 0 in reply to Steve Hobbs1
    TI__Mastermind 23421 points

    Ahh my bad, I misunderstood your question the first time.  

    Yes, it appears that the PWM output signal is linear like you wrote it out according to Figure 12 in the datasheet.  Do keep in mind that the DAC output data on the LM628 is offset-binary coded, so you might have to account for that.   

    Regards,

    Jacob