Hi!
I will use DRV8840 to drive a electromagnet in a MAGLEV system. I'm thinking to use the current control capacity of this IC to simplify the system, controlling the current by a DAC in Vref pin, because the 32 levels of the I0 ~ I4 are not enough for the control. There will be a cascade control with DRV8840 in a internal current loop and a digital controller in the external loop.
Being the DRV8840 a fixed frequency peak current-mode controller, it presents a sub-harmonic oscillation in output, correct? How can I add a slope compensation circuit to avoid this? MAGLEV systems are so sensitive and this oscillation can destabilize the control.
In my search I found 3 possible problems:
1 - The inductance of the electromagnet varies depending the position of levitated object (610mH ~ 530mH for 0 to 23mm of distance);
2 - The idea is not sense the current in the digital controller, so it can't use a Imin and Imax to internally generate the slope;
3 - I'm using a board from avayanelectronics.com , so a circuit between the Rsense and Isense pin will be not easy to adapt.
I only found techniques of slope compensation for DC-DC converters with external PWM, not for ICs like DRV8840. Any information can be helpfull!
Thanks!
Evandro Rech