TLV3501: Relaxation Oscillator frequency calculation

Intellectual 625 points

Part Number: TLV3501
Other Parts Discussed in Thread: TLV3201

In the datasheet of the TLV3501 there is section 8.2.1 about Relaxation Oscillator.

I don't understand the frequency equation in this section and I need more details.

over 6 years ago

0 Jaskaran Atwal over 6 years ago

TI__Intellectual 1740 points

Hi,

Thanks for reaching out to us. Could you provide some more clarification on which part of the equation you needed more details on.

Regards,
Jaskaran

0 as over 6 years ago in reply to Jaskaran Atwal

Intellectual 625 points

"Therefore, the period is 1.38 × R1C. For 62 pF and 1 kΩ as shown in Figure 18, the output is calculated to 10.9 MHz. An implementation of this circuit oscillated at 9.6 MHz."

please read the datasheet as I said above.

1/(1.38*1000*62*10^-12) don't make 10.9x10^6.

I need proper equation for determining theorical frequency.

0 Chuck Sins over 6 years ago in reply to as

TI__Mastermind 24530 points

I see what you are referring to. Depending on what frequency you are trying to create, the parasitics that are eluded to in the datasheet may or may not affect you. We will need a couple days to look into this and see if an appropriate equation or model of the parasitics can be created. In the meantime, if you can share what frequency you are trying to create we can try to help you find a solution.

Chuck

0 as over 6 years ago in reply to Chuck Sins

Intellectual 625 points

Ok but I need calculated, theorical output frequency. There are wrong or inadequate sentences in the datasheet. I will produce square waves in 1,10,20 and 40 KHz and I will change resistors with analog mux for selecting frequency.

0 as over 6 years ago in reply to Chuck Sins

Intellectual 625 points

I'm still waiting for your answer

0 as over 6 years ago in reply to as

Intellectual 625 points

The datasheet is wrong

0 Jaskaran Atwal over 6 years ago in reply to as

TI__Intellectual 1740 points

Hi,

Sorry for the delay in getting back to you. The parasitics eluded to in the TLV3501 datasheet typically apply to oscillators at much larger frequencies than the target frequencies you have listed. The best device for your application and frequency range is the TLV3201. I have used the TLV3201 to create the oscillator circuit described in the TLV3501 datasheet for one of your target frequencies of 40khz. I both simulated and tested this circuit in the lab to verify the equations and set up. A 40khz oscillation would have a period of 25us and a half period of 12.5us. Based on this you would have 12.5us = 0.69RC. Using this equation you can select your resistor and capacitor values. In my example for 40khz I set C = 1nF and R = 18K to get the oscillation period I would like. In your case you would change the resistor and capacitor accordingly for each frequency. I created this circuit in TINA and simulated it to get the target oscillation period of 25us as shown in the screenshot. I have also attached the TINA file to this circuit if you would like to simulate it. I then verified the simulation results in the lab. You should be able to use this device and set up to create the oscillator circuit for your frequency ranges.

Regards,

Jaskaran

0 as over 6 years ago in reply to Jaskaran Atwal

Intellectual 625 points

Thank you for kindly help. If I use dual supply I don't need the R2, is this right?
Also I still don't understand why the eq. is 0.69*RC . 0.69 is ln(1.99) . B=R2/(R2+R1)=0.5 . 1+B/(1-B)=3 . So it need to be ln(3).

0 Jaskaran Atwal over 6 years ago in reply to as

TI__Intellectual 1740 points

Hi,

Even with a dual supply set up you will need R2. The R2 resistor network is used to set the trip threshold at 1/3 and 2/3 of the supply regardless if you are using single supply or dual supply. This equation comes from the discharging and charging equation of an RC circuit. The oscillator circuit is designed to trip at 1/3 and 2/3 of the supply which is 1.67 and 3.33. In the case of discharging the equation would be Vc=Vs * e^(-t/(RC)). This would result in 0.5 = e^(-t/(RC)). With this equation you would get the t = 0.69RC and hence where 0.69 comes from. You will get the same value with similar derivation with the RC circuit charging equation.

Regards,
Jaskaran