I am reading and trying to understand this Application note from TI on how the dithering circuit works.
I can deduce that the Op-amp acts as a schmitt trigger but there's no details on how the frequency change is effected. Can someone help me with the working of the circuit on how the frequency change is effected with the schmitt trigger?
And also, can someone explain how is the calculation obtained? Why is the 7kHz/65kH multiplied with the 45uA current?
Best Answer
What they are saying is that to make the UC28180 dither its operating frequency there is an external 2 kHz triangle wave generator (below) that is attached to the FREQ pin: -
And, because the operating frequency is set by the current out of the FREQ pin, the dithering oscillator can modulate this by taking, in effect an AC current from the FREQ pin. The main central oscillation frequency (65 kHz) is set by RT but, if a little current is taken and put back through the 100 kΩ resistor then the main chip's oscillation frequency will "dither".
To set the basic 65 kHz operating frequency, RT must draw 45.87 μA and, to make this 7 kHz higher or lower, a current of plus or minus 4.94 μA must be taken through the 100 kΩ resistor. That current is: -
$$ \pm 45.87\text{ } \mu A \times\dfrac{7}{65} = \pm 4.9398\text{ } \mu A$$