UC3846: Output Frequency Issue

Vicky Siraswar

Part Number: UC3846

1. As per frequency calculation formula for uc3846 our circuit output should be 20KHz but we were getting 16.5KHz.

2. Now, considering 16.5KHz we were using this circuit but within last year we observed that with same value of Rt and Ct, for some ICs frequency is reduced more than 16.5KHz like 15.5KHz to 14.5KHz

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Hello Vicky,
What is the value of the timing resistor and timing capacitor that were used in your application ?
What is the component tolerance of these parts?

Regards
John

0 Vicky Siraswar over 5 years ago in reply to John Griffin1

Prodigy 205 points

Value of Capacitors:
15nF and 3.3nF in Parallel combination with tolerance of +/-5 %

Value of Resistors:
3.3K and 30K in Parallel combination with tolerance of +/- 1%

0 John Griffin1 over 5 years ago in reply to Vicky Siraswar

TI__Mastermind 34465 points

Hi Vicky,
With R = 3k and C = 18.3nF you should expect to get an oscillator frequency of about 40kHz.
However I believe the equation is only valid for resistor values of 5k or greater.
I suggest you use R = 10k since this is a value used in the test parameter tables in the data sheet.
This will give a C = 5.5nF for a 40kHz oscillator.
Please try using a parallel combination of 4.7nF and 860pF.
Let me know how this works out.

Regards
John

0 Vicky Siraswar over 5 years ago in reply to John Griffin1

Prodigy 205 points

As per your instruction, for frequency of 40KHz I assembled R=9.5K resistor as there was shortage of 10 K.
therefore, as per calculation capacitor C=5.8 nF i.e. parallel combination of 4.7 nF and 1 nF.
But instead of 40KHz we are getting 33.6 KHz.