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LMC555: LMC555 accuracy

Part Number: LMC555
Other Parts Discussed in Thread: TPL5111, TPL5010, CD4040B, TPL5110-Q1

Dear team,

Customer use LMC555 want to set the time delay to 12s, and want to control the accuracy at about +/-0.1s. But during their test, the accuracy nearly +/-1s. Any suggestion to the R and C? They wonder if they choose R,C bad accuracy?

Thanks.

Best regards,

Sammi

  • Hello Sammy,

    For best accuracy, the supply voltage should remain constant during the timing interval and the recommended supply bypass capacitor placed near the timer. 

    Accuracy near +/-1s would be an accuracy of +/-8.3%. What was the resistor tolerance and the capacitor type and tolerance? Many capacitor types have really poor, voltage modulation and temperature effects. To get +/-0.1s , that is a +/-0.8% tolerance which is too tight even if the external components were perfect.

    Here is a quote from the 555 timer monostable FAQ, "For full temperature range final accuracy better than 4%, the cost of improving the timing capacitor and resistor may be higher than using a crystal oscillator plus logic counter solution."

    Let me know the parameters of R and C that was tested and if you would like help with a crystal solution.

  • Hi Ronald,

    Thanks for your reply. I think the accuracy of customer design is not very good are mainly caused by 2 points: 1. VCC is not stable. They use a 100uF capacitor to supply power, trigger point is 2/3 VCC; 2. The R: 2M, 1% tolerance, 0402;  C: 10uF*2, 20% tolerance, X5R 0402. The board size for customer is very small, so it will limit the selection of capacitor and resistor.

    The goal of accuracy for customer is 100ms. If they want to set the time delay to 8s, could LMC555 satisfy their requirement?

    Also I see we have other kindly of timer, TPL5111. The function that customer need is to set a fixed time delay, only need trigger one time. I am not sure if TPL5111 is OK for such application.

    Thanks.

    Best regards,

    Sammi

  • Sammi,

    I believe the TPL5111 could work. The part is supported by a different team, so start a related post for that part number and they will be able help you with usage and accuracy.

    For the LMC555 they will need to get NP0/G0G ceramic capacitors with better accuracy. The X5R with have voltage modulation for a slightly different pulse time than expected; these caps also change with time (aging). at 10uF NP0/C0G cap will be hard to find and expensive. 

    Try a new TPL5111 post and if that doesn't pan out then come back and I'll give you a crystal solution. 

  • Hi Ronald,

    Thanks for your suggestion before. I checked with TPL5111 product line and they recommend TPL5010, but customer think TPL5010 could not work for their application because they wanted to reckon by time when initially power up. Currently TPL5010 will communicate with MCU when initial power up, customer only want MCU send commend when watch dog timer expires.

    So I am back here, would you kindly share your crystal solution?

    Thanks.

    Best regards,

    Sammi

  • Welcome back Sammi,

    Here is a block diagram for 12 second solution with crystal accuracy. A low cost useful crystal is 32,768 Hz. The X04 will turn the crystal into an oscillator. The first CD404B divides clock by 2^12. The last CD4040B will count to 96 (32+64) then reset the both CD4040B back to zero.  32768Hz/2^12/96 = 12 sec. The set/reset or flip flop latch will allow starting the 12 second pulse and automatically stop it. If needed, the logic team can help you pick a RS latch and AND gate.    

  • Hi Ronald,

    Many thanks for your suggestion. As customer PCB size is small and only a capacitor to supply power, so the crystal solution is not appropriate for them.

    I checked with customer again, and if they need 5% accuracy, is current R/C value support?

    The limitation for customer's board is that the board is one-time, and temperature and aging will have less influence of the capacitor. Would you help double confirm if LMC555 could support their application?

    Thanks.

    Best regards,

    Sammi

  • Sammi,

    Accurate large value capacitors are bulky and expensive for the LMC555 solution. The TPL will work, but the usage has to be creative.

    If the output can be a falling edge signal then just keep DONE pin low and use falling edge or RST to indicate the 12 second expiration. There will also be a falling edge 24 seconds and so on falling edge.

    I'm I correct that 12 second timer starts at power up? For the end of interval, does the output need to be low / high and does it need to stay that way or could edges be used?

     

  • Hi Ronald,

    Many thanks for your suggestion, even I know TPL5010 are not product you support. Customer need one bit that keep one status from VIN power up until time done, then turn to opposite status to trigger MCU. Both falling edge or rising edge are ok for them.

    I see the figure 1 TPL5010 timing in datasheet that when power up, RSTn will turn from low to high, and this may not suitable to the application.

    I really hope LMC555 could be used in the application, as it is easier for customer to use, so I always want to find some way to make the accuracy better.

    Thanks.

    Best regards,

    Sammi

  • Sammi,

    Here is a 12 second LMC555 solution. Accuracy is based on capacitor accuracy and supply voltage stability 

    LMC555 12 second Monostable.TSC

  • Sammi,

    For TPL it seems that TPL5110-Q1 has a better one shot mode. This could be used directly or a to make DRV low sooner could be done with a D latch that start in low state.