TL494: Oscillator Frequency Calculation

Souheil Slim

Part Number: TL494

I'm asked to perform some theoretical calculation and analysis on a design that uses the TL494I programmed to 4kHz for single-ended application .

Question #1

Could you please take a look at the section 3.2.1 of the Application Notes (Designing Switching Voltage Regulators With the TL494) www.ti.com/.../slva001e.pdf

Why, when we calculate the oscillator frequency using the formula fosc=1/(RT*CT) and using the plot of (Fig1 Datasheet TL494) the results are different.

Here are some examples:’
Case #1 RT= 500k CT=10nF
Formula : fosc=200Hz
Plot Fig1. : fosc=300Hz

Case #2 RT= 70k CT=10nF
Formula : fosc=2kHz
Plot Fig1. : fosc=1.4kHz

Case #3 RT= 20k CT=10nF
Formula : fosc=5kHz
Plot Fig1. : fosc=6kHz

The datasheet and the application note didn't mention anything about the formula fosc=1/(RT*CT) accuracy.

Question #2:

I’m using a bleeder resistor (R) across the capacitor (CT) . what will be the fosc equation (refer to Section 4.2.4 of the application note).

FYI, I developed the calculation using the details presented in the datasheet section 9.3.2.

I obtained the following formula: T= R . CT . ln(1 / (1-RT / R)) and fosc= 1/T

The formula is correct because when R is equal to infinity the formula could be simplified using the Taylor Series and we obtain T= RT*CT

The formula proves that there is a mistake in (Figure 22 Fail-Safe Protection), the bleeder resistor shall be 10 x Rt not a fraction of RT "(1/10).RT”. The ln (X) exist when X is >0.

Could you correct Figure 22 and advice what TI recommend to find fosc when a bleeder resistor is used.

Your help with this would be greatly appreciated.

Kind regards

over 2 years ago

0 Eric Reicher over 2 years ago

TI__Expert 7900 points

Hello Souheil,

You've done an excellent job analyzing the TL494 oscillator. I appreciate your efforts and attention to detail!

The question of oscillator accuracy on the TL494 has come up quite often in the E2E forums. Here are a few examples.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/639470/tl494-oscillator-frequency-of-tl494

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/648236/tl494-oscillator-frequency

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1009057/tl494-oscillator-frequency

The first thread recommends using 1.2/(Rt*Ct) to calculate the frequency. The second thread makes the same recommendation and says they will look into changing the datasheet. The third thread created a PCB with 12.2k and 0.1uF, which would be 819 Hz using 1/(Rt*Ct) and 984 Hz using 1.2/(Rt*Ct). Their measured result was 1.15 kHz.

In any case, this is a legacy, low-cost regulator that "gets the job done". TI has an enormous amount of TL494's in the field so there is some reluctance to make changes to the datasheet that aren't critical or divert resources from newer, more accurate regulators.

Choose your initial Rt and Ct based on Figure 1, or 1.2/(Rt*Ct). Then, I would use a functional TL494 and fine-tune your Rt value to obtain the desired frequency based on lab results. If you use 10*Rt for a bleeder resistor then include this when tuning Rt.

As for Figure 22 the bleeder resistor should absolutely be 10*Rt. I will submit a ticket to get the Application Note changed.

Thanks for your help!

Regards,

Eric

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TL494: Oscillator Frequency Calculation