EasyDriver A3967 current regulation

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jameslo
Posts: 3
Joined: Mon Jan 08, 2018 10:51 am

EasyDriver A3967 current regulation

Post by jameslo » Tue Mar 13, 2018 11:46 am

Hi, can anyone help me understand how the A3967 controls the current through its internal H-bridge? Here's what I think I know from poking around the EasyDriver with a scope and looking through the A3967 datasheet--please correct me if I'm wrong:
  • The H-bridge is pulsed at 32kHz
  • The output current is proportional to the duty cycle
  • The A3967 measures the voltage across the current sensing resistor, compares it with the DAC output representing the target voltage, and adjusts the duty cycle accordingly.
If that's all true, here's what I don't get:
  • why isn't there insane amounts of inductive kickback at the output pins due to all that switching? I expected to see peaks at a diode drop above the load supply and/or below ground.
  • the signals at the sense resistor pins 8 and 17 just look like noisy square waves. Are they being continuously compared with their corresponding DAC output, or being sampled at some point?
  • I originally assumed that the inductance of the motor coils, together with the sense resistor, would form a low pass filter and smooth out the current. The fact that I'm seeing square waves at the sense resistor pins implies that's not true, right?
Just curious. Thanks in advance for your help.

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