Hi,
This is further to the following thread, which explains how to go about speed control using UC2625.
" EA+ should be somewhere between 1 and 2.2V to set the PWM threshold. This is because PWM_IN is tied to RC-OSC which will see a 1-2.2V sawtooth waveform and PWM_IN's level sets the pulse width of the PWM. To configure the amplifier to filter the feedback coming from the tachometer the cutoff frequency must be set below the PWM frequency. This is calculated using f(cutoff) = 1/(2*pi*C*R2). For example, with a typical 20kHz PWM it might be reasonable to start with a 5kHz cutoff.
Based on the datasheet, the tachometer will go high for 220us every time a hall sensor input changes. For example, if you have a 3000rpm maximum speed motor, at 3000rpms you would be seeing 50 mechanical revolutions per second. You would multiply this times the number of pole pairs, in this example lets say 4, and then by the 6 hall sensor state changes per revolution (50x4x6x220=.264.) Then we can multiply this by the output voltage of tach_out pin which is 5V to get the averaged input voltage of 1.32V (without gain.) To select your gain in this case, you would divide the PWM_IN voltage at maximum duty cycle by the input voltage, i.e. 2.2V/1.32V, or a gain of approximately 1.67V/V. You can then use the gain equation Gain=R2/R1. Also, I would start with a mid-range value for R2 like 10kOhms, and derive the remaining components using the other equations."
We need help understanding how to bias the error amplifier correctly. e.g., for the example mentioned in the older thread, copied above, if we want to set the desired speed to 3000 RPM, then the voltage on EAout should be 1V for max. duty cycle? (so that chopping never happens). For this to happen, what should the voltage at EA+ be? Is the following assumption correct?
EAout = EA+ * (1+G) - EA- *G
