LMC555: How to handle the RESET pin

Taito Takemura

Part Number: LMC555

Hi experts,

My customer will use LMC555 with 50% Duty Cycle Oscillator (in the section of 9.6) and the LMC555 will control an LED connected to LMC555's Output terminal. They are planning to make the LED light up when the RESET pin is L and blink when the RESET pin is H. Could you please answer the following questions about how to handle the RESET pin in this case?

Questions about Reset pin

(1) If Reset is kept L all the time after the power is turned on, will the Output be L?

(2) Or is it necessary to set Reset to H when the power is turned on and then change Reset to L?

(3) If Reset is changed from L to H, will the Output operate as an Oscillator?

Question about current output

(4) Does the Current Output Max 100 mA in "6.1 Absolute Maximum Ratings" also apply when sinking current?

Thank you for your help.

Best Regards,

Taito Takemura

over 2 years ago

0 Chris Featherstone over 2 years ago

TI__Mastermind 23590 points

Hello,

If Reset is kept L all the time after the power is turned on, will the Output be L?
- Please refer to this post for startup behavior: https://e2e.ti.com/support/clock-timing-group/clock-timing-internal/f/clock-timing---internal-forum/1118153/lmc555-unexpected-output-pulse/4160347?tisearch=e2e-sitesearch&keymatch=%25252520user%2525253A77528#4160347
If Reset is changed from L to H, will the Output operate as an Oscillator?
- The output will remain low if the trigger pin is held low. A negative pulse applied to this pin will disable the timer if it is held low and will reset the timer when this pin is brought back high. Once reset the output will oscillate according to the RC values designed into the application.
Does the Current Output Max 100 mA in "6.1 Absolute Maximum Ratings" also apply when sinking current?
- Correct, this is for both sourcing and sinking.

Best Regards,

Chris Featherstone

0 Taito Takemura over 2 years ago in reply to Chris Featherstone

TI__Intellectual 2540 points

Hi Chris,

Thank you for your answer. Let me check my understanding about (1).

(1) If Reset is kept L all the time after the power is turned on, will the Output be L.

=> It is correct, but they should Note that Output is HiZ until Vcc reaches 1.5V.

Is my understanding correct?

Best Regards,

Taito Takemura

0 Chris Featherstone over 2 years ago in reply to Taito Takemura

TI__Mastermind 23590 points

Hello Taito,

The output will remain in high z until the device is fully biased on with the minimum supply. Anything under minimum supply is not guaranteed to keep the output in a defined state. Between 0V and minimum supply it is possible to see a pulse on the output in any configuration the device is in since there is no power on reset circuitry. The only possible work around is shown here:

https://www.ti.com/lit/an/slaae33/slaae33.pdf?ts=1659045301743&ref_url=https%253A%252F%252Fwww.google.com%252F

Best Regards,

Chris Featherstone

0 Taito Takemura over 2 years ago in reply to Chris Featherstone

TI__Intellectual 2540 points

Hi Chris,

Thank you for reply.

Could you please explain the answer below a little more?　 I don't quite understand this answer. I think "the trigger pin" and "this pin" represent RESET pin, is this correct?

If Reset is changed from L to H, will the Output operate as an Oscillator?
- The output will remain low if the trigger pin is held low. A negative pulse applied to this pin will disable the timer if it is held low and will reset the timer when this pin is brought back high. Once reset the output will oscillate according to the RC values designed into the application.

Best Regards,

Taito Takemura

+1 Chris Featherstone over 2 years ago in reply to Taito Takemura

TI__Mastermind 23590 points

Taito,

I made a typo. My apologies. I meant to type reset instead of trigger. This pin also refers to reset as you mentioned.

Best Regards,

Chris Featherstone

0 Taito Takemura over 2 years ago in reply to Chris Featherstone

TI__Intellectual 2540 points

Hi Chris,

Thank you for your answer and I have one thing to check.

There are words of "Unfortunately there isn't a way to control the behavior of the device output during startup" in E2E "LMC555: Unexpected output pulse" as you told.

Does this means that after all the LMC555 alone cannot control the output at startup regardless of the state of the RESET pin (because unexpected pulse may occur)? The only way to avoid is https://www.ti.com/lit/an/slaae33/slaae33.pdf?ts=1659045301743&ref_url=https%253A%252F%252Fwww.google.com%252F, right?

Best Regards,

Taito Takemura

0 Chris Featherstone over 2 years ago in reply to Taito Takemura

TI__Mastermind 23590 points

Hello Taito,

"Does this means that after all the LMC555 alone cannot control the output at startup regardless of the state of the RESET pin (because unexpected pulse may occur)?"

Yes this is correct. There is not a POR circuit in the designs of 555 timers that control the state of the output when the devices are powered on fully. This is very similar to supervisor chips shown in the link below. Michael has an external solution to show how to control the state of the output of the supervisor device and may be considered with the 555 timers as well as a possible solution.

https://e2e.ti.com/blogs_/b/powerhouse/posts/tips-and-tricks-for-optimizing-your-voltage-supervisor

Best Regards,

Chris Featherstone

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LMC555: How to handle the RESET pin