Uc3845 oscillator design

[nishu's]

i went through ON datasheet for UC3845 series and it says operating frequency is decided by timing components referred as Rt and Ct.
"The oscillator frequency is programmed by the values selected for the timing components RT and CT. Capacitor CT is charged from the 5.0 V reference through resistor RT to approximately 2.8 V and discharged to 1.2 V by an internal current sink."

based on above statement i tried to formulate frequency and dead-time based on capacitor charge and discharge theory. Here is what i get
t(c) = 0.82098*RC
t(d) = 0.84729*RC

Fs(Hz) = 1/(tc + td) = 1 / (1.66827)*RC = 0.599423/RC
i took two values from datasheet graph R = 20k and C = 10nF
datasheet gives frequency to be approx F = 8-10Khz

my formula gives 2.99Khz. I havent checked for deadtime yet.
any help would be appreciated.

 

[D.A.(Tony)Stewart]

TI gives a chart of values on page 6, but this part is not recommended for new design.



TI's Recommended alternative parts
UCC28700 - The device has SIMILAR FUNCTIONALITY but is not functionally equivalent to the compared device. Next Generation Current Mode PWM with Primary-Side Regulation for Flyback
UCC28710 - The device has SIMILAR FUNCTIONALITY but is not functionally equivalent to the compared device. Next Generation Current Mode PWM with Primary-Side Regulation for Flyback
LM5021 - The device has SIMILAR FUNCTIONALITY but is not functionally equivalent to the compared device. AC-DC Current Mode PWM Controller