Hi,
When I examined the UC3525A IC datasheet, I noticed that the oscillator frequency, resistance and capacitance values in sections 6.3 and 6.5 are different from each other. What values should I refer to? I'm designing a half bridge circuit. How should I determine my oscillator frequency? My switching frequency will be 100kHz.
Thank you,
Yarennur.
Hi Yarennur,
Section 6.3 simply shows a recommended range for RT (2k to 150k) and CT (100pF to 0.01uF). Section 6.5 shows how the parts are tested in Production, and gives RT and CT values for minimum and maximum operating frequency (120 Hz and 400 kHz).
In the oscillator section, notice the accuracy is specified as 2% typ, 6% maximum and has Notes 1 and 2. Note 1 says it was characterized at 40 kHz with RT=3.6k, CT=0.01mF (10nF), and RD (deadtime)=0. Note 2 provides a formula for frequency, 1/[CT*(0.7*RT+3RD)].
So, to have accuracy near 2%, I would choose RT and CT values "near" the 3.6k/10nF used during characterization. For 100 kHz, let's make RT and CT each about 37% smaller - standard values are 2.55k and 5.6nF. This will provide exactly 100 kHz.
Regards,
Eric
