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TLC555 - frequency stability with temperature change

Saqib Bajwa
Prodigy 170 points
Other Parts Discussed in Thread: TLC555, SE555, LMC555

Hi,

I have made a simple circuit for measuring capacitance using a 555 timer (TLC555IDG4 - temp. range: -40 to 85 degrees C). The circuit works good at room temperature but when I put it in freezer (-10 degreec C) and heat it up using a heat gun (upto 70 degrees) the frequency changes a lot. I have attached my circuit and the frequency vs temp curve, please take a look and tell me why there are sudden variations in frequency. 

The whole test was done using a 150pF polystyrene capacitor.

One more thing, if the datasheet says that the temp. range of the 555 timer is from -40 to 85 degree C, does that mean that frequency should remain almost same in the whole range? If not whats the ppm (error) of this chip, couldn't find it on datasheet?

Waiting for your kind replies :) Thanks.

Saqib

  • 0
    TI__Mastermind 24205 points

    Hi Saqib,

    We have contacted the engineer responsible for this device. Someone will get back to you shortly.

    Regards

    Arvind Sridhar

  • 0
    TI__Guru* 93105 points

    Hello Saqib,

    The TLC555 timer is intended for general purpose timing applications and isn't a precise timer. You will note from the data sheet that even though the device - depending on grade - will have a specified operating temperature range, nothing is specified in terms of timing/frequency accuracy (min/max) except under the stated conditions of VDD = 5 V, and TA = 25 C. Even then, the R1, R2 and C values, or ranges, are specified. Therefore, how the frequency moves across temperature is not specified in the electrical specifications.

    The frequency and duty cycle that a TLC555 circuit ultimately attains is not only a function of the timer's electrical characteristics, but also that of the components used with it. The temperature coefficients (TC) of the resistors and the capacitors will cause the frequency and duty cycle to change in response to temperature change. Using low TC components in the circuit may yield better frequency stability from the circuit than you are currently achieving. Also, it is crucial that the supply voltage not change with temperature as well.

    My experience with the TLC555 has indicted that the frequency/ duty cycle is less precise (for near ideal components) as the frequency of operation is increased towards the hundreds of kilohertz and above. That is because internal capacitances play a bigger role in their determination as the external timing capacitance decreases. The internal capacitances are temperature sensitive and don't necessarily have a low TC. You are using a 150 pF timing capacitor which tends to suggest the expected frequency is pretty high relative to the maximum operating frequency. You may want to try increasing the R1 and R2 values to move the astable frequency lower. Whatever resistors you use they should have low TC (< 25 ppm).

    Regards, Thomas

    PA - Linear Applications Engineering

     

  • 0 in reply to Thomas Kuehl
    Prodigy 170 points

    Hi Thomas,

    Thanks a lot for your reply.

    The resistors I have used are both of 1M Ohms and my frequency is ~3.2KHz. I am designing this circuit as a capacitance measuring circuit and our capacitance range is 50pF to 10nF. 

    If TLC555 is not very precise and temp. stable can you recommend any other, e.g. SE555? A little bit variation is fine I can add temperature compensation by placing a temp. sensor on the PCB but it should not go nuts with temp. change.

    Thanks.

  • 0 in reply to Saqib Bajwa
    TI__Guru* 93105 points

    Hi Saqib,

    The SE555 is a bipolar timer produced by another TI group. I don't have direct experience with the device, but because it is bipolar it may behave differently over temperature than the TLC555.

    TI also produces the LMC555 from its the former National Semiconductor. It too may behave differently with temperature than the TLC555. You may want to get free samples of both the SE555 and LMC555 from TI and see if they provide better over temperature stability than the TLC555.

    Regards, Thomas

    PA - Linear Applications Engineering

     

  • 0 in reply to Thomas Kuehl
    Prodigy 170 points

    Hi Thomas,

    Thanks for your quick reply. 

    I will try these timers and will let you know. 

    Thanks again.