I am attempting to drive some 1.4A 8-wire stepper motors wired in parallel with your A4988 breakout boards (green edition, current setting at 2A as I am full-stepping). What is the switching frequency of the A4988 in this setup?
More generally, how can I calculate the switching frequency for any given setup?
The driver switches off the coil current when the current reaches the limit setting, which depends on a lot of factors including the power supply voltage, inductance and resistance of the coils, wiring configuration, driver configuration, armature rotation, etc. The A4988 data sheet explains much of this.
The frequency is not fixed and without a number of details you cannot even estimate it.
From your description, it sounds like you are trying to use your A4988 boards beyond their specifications. Our green A4988 carrier is only rated to handle about 1A without external cooling, so I suspect it would be difficult to get 2A out of it. If you are having issues with your system and you post more information (like a wiring diagram), I might be able to help.
I fixed my original problem: it turned out that putting a ceramic capacitor across the logic inputs solved the high frequency coupling issues I had been having.
Note that despite the A4988 green ed having a maximum continuous current rating of 1A, by only activating it when I need to run it I can reliably get 15s bursts of 2A operation out of my boards (allowing time in between for me to play with my code and the stepper to cool down).
I would still be interested in anything you could give me about the switching frequency of the A4988.
Like Jim mentioned in his post, the A4988 does not have a fixed switching frequency. The “Functional Description” section of the A4988’s datasheet discusses how the PWM control circuitry works. You can find the datasheet on the Resources tab of the driver’s product page.