I would like to understand how the oscillator of this PWM controller works, and what would happen if I do not apply an RC network but directly a PWM signal generated from a microcontroller for example.
It is indicated into the datasheet (https://www.ti.com/lit/ds/symlink/uc3845.pdf?ts=1590914073928) that the capacitor is charged from the reference voltage Vref until the voltage across the capacitor reach an upper threshold and then the capacitor is discharged via an internal current source. Nevertheless the oscillator as showned above generates pulses.
When the capacitor is discharging, we can think that a pulse is generated. It could be the inverse... I would like to know exactly how it works... ie when the pulse is generated ? Is it when the capacitor is discharging or when it is when the capacitor is charging.
Now, if I directly inject a PWM signal into the RT/CT pin, like this :
When the PWM signal which rise (very rapidly as it is a rectangular waveform) to the upper threshold, the controller will draw current (for discharging the capacitor), it will be provided by the microcontroller, then the PWM will be low and the PWM controller will stop to draw current and will wait the the PWM controller rise again to draw again current. So it seems necessary to not insert a resistance as it could insert a voltage drop when the pwm controller draws current and the PWM controller could not able to see the different voltage level of the PWM.
Can anyone know how it is built internally and if what I want to do is possible ?
Thank you very much and have a nice day.