TI DRV8876 question - Page 1
Forum section: electronics / projects

author: Kjelt
author_rank: Super Contributor
timestamp: October 05, 2020, 10:00:47 pm
content: I am playing around with a TI DRV8876 evsluation board driving a brushed dc motor with 24V.This ic has an mirror current output connected to an ADC the uC can measure the current through the low side fets.The eval board works nicely a 27kHz PWM generated by the onboard uC controls the motor.Now I attached a scope to the PWM input and the Ipropi output and I don't get it.I see the current rising when the PWM goes high which is correct.Then when the PWM goes off the current decreases also understandable but then while the PWM is still in off the current increases again.I don't get this, is the driver ic doing this or is the Ipropi output only "valid" when the PWM signal is high?

author: Kjelt
author_rank: Super Contributor
timestamp: October 06, 2020, 09:44:50 am
content: Ah it is more clear now I connected a resistor on the ouput (see picture).When the Hbridge is idle the induction of the motor causes a negative current to flow, the current mirror hardware in the drv8876 makes this a positive current Ipropi which over the Rpropi shows as a positive voltage.Solved.

author: gnuarm
author_rank: Super Contributor
timestamp: March 19, 2021, 10:44:04 pm
content: Hi, I found this thread and would like to ask some questions about the IPROPI signal. I've been banging on the data sheet hard the last few days and running the spice simulation. In the simulation the current mirror does not seem to work correctly. Did you use the spice model at all?In your waveforms below, I see two signals, the magenta waveform would seem to be the IPROPI voltage. Which input or output is the greenish signal? It would seem to be the wrong polarity for the input if you are switching between forward and brake mode. If you are switching between forward and coast mode then it would be right. Or it could be an output which would also be the correct polarity depending on the two input modes you are using.

author: Kjelt
author_rank: Super Contributor
timestamp: March 20, 2021, 02:19:58 pm
content: Hi gnuarm,No I did not use any simulation.The yellow/greenish signal is the input PWM to the 8876.This remembers me to continue this project it has been laying dorment since

author: gnuarm
author_rank: Super Contributor
timestamp: March 21, 2021, 12:19:32 am
content: If you are getting back to this, I would appreciate any info you can give on the IPROPI signal. My simulation is working pretty well except for the IPROPI signal. This is important in my application so I want to simulate it before I bother building a board. This project has a lot going on and it is best not to muck up a detail like this.I've tried to glean some insight into how the simulation is operating thinking it has something to do with the way I am using it, but it seems broken to me.I am running in PWM mode with the two inputs high putting the outputs in "brake" mode where both are grounded through the FETs. The one input is toggled low which brings the other output high. So IN2 is pulled low to drive OUT1 high. I have a high inductive load with some capacitance, so the current into OUT2 does not vary much as it is always grounded. The current out of OUT1 does vary some which is what shows up on IPROPI although not the correct current values, just the general shape of the waveform.This is not correct according to the data sheet. Only the low side FETs are supposed to be used for the current mirror. In driving mode only one low side FET is conducting and when in brake mode the FET that is conducting in reverse is not supposed to be counted in the current mirror. They don't apply theI'd like to be able to separate the defects in the simulation from how the chip actually works. I will be driving the PWM at a high rate and there are a number of timing specs that will impact the operation. I can't see any of that working at the moment.