UC2844A: Very urgent question on the switching frequency of UC2844A.

Bob,

Thanks for your interest here. I'll have an apps engineer comment soon.

Regards

John

Hello Bob,

This data sheet has been put in for revision.  The frequency equation on page 11 should be.

f = 1.72/(Rt*Ct)

Based on your first scenario the frequency would be roughly 209.8 kHz which agrees with figure 6 RT selection with typical frequency.

f = 1.72/(Rt*Ct) = 1.72/(8.2k*1nF) = 209.8 kHz

The frequency accuracy of the device is +/- 9.6%.  Without taking into account your variation in capacitance and resistance.  This can be calculated based on the oscillator accuracy specification in the data sheet (Typical = 52 kHz, Minimum =  47 kHz, Maximum = 57 kHz).  The frequency range typically would vary from 189.7 kHz to 229.9 kHz roughly.

So with 1% RT and a 5% CT this will add some more variation to your switching frequency on top of +/- 9.6% varation would vary as follows

fmin =  1.72/(1.01*1.05*RT*CT) ≈ 189.7 kHz*(1/(1.01*1.05)) ≈ 178.9 kHz

fmax =  1.72/(0.99*0.95*RT*CT) ≈ 229.9 kHz*(1/(0.99*0.95)) ≈ 244.4 kHz

You should be able to use a similar calculation for your second scenario to estimate the frequency variation.

Regards,

Mike

Dear Mike,

Thank you very much for quickly response.
appreciate.

Best regard.
Bob Lee.

Dear Mike,

May I ask add two question?

1.
Since the maximum duty of UC2844A is 50%,
Would the frequency become 1/2?

f = 1.72/(Rt*Ct) = 1.72/(8.2k*1nF) = 209.8 kHz
⇒104.9kHz?

2.
In frequency become 1/2 that case,
Would the accuracy of the device become +/- 9.6%⇒+/- 4.8%?

Best regard.
Bob Lee.

Dear Mike,

Very urgent.
Please tell me reply?

What time could you reply?

Best regard.
Bob Lee.

Hello Bob,

The UC2844A is a fixed frequency controller and has a maximum duty cycle of 50%.
The switching frequency will not be changed based on duty cycle, it is fixed. The internal duty cycle clamp will not affect frequency accuracy.

Regards,

Mike

Dear Mike,

Thank you very much for reply.

Best regard.
Bob Lee.

Hello Bob,

I talked to one of my colleagues regarding the 44A and 45A devices.  The oscillator frequency is cut in half with a T flip flop internal to these devices.  It is explained well in the UC2844 and UC2845 but is not explained in this version of the data sheet.  However,  the block diagram show the T flip flop with dotted line connections.

Regards,

Mike

Dear Mike,

Thank you very much for reply.

For UC2844A,Is it said that the switching frequency is 104.9 kHz?
Is the accuracy ± 9.6%? Or is it ± 4.8%?

Best regard.
Bob Lee.

Hello Bob,

I believe the error in the frequency will stay the same even though the effective switching frequency is cut in half.

Regards,

Mike

Dear Mike,

Thank you very much for reply.
If there is new update, please reply me.

Best regard.
Bob Lee.