Continuous Reset generated by LDO to Microcontroller

Question

I have the below schematic

enter image description here

Datasheet of the LDO - L4995 -SSO24 package

LDO Specification :

Input Voltage : 12-16V Output Voltage : 5V

Load Current : 100mA

Issue :

My LDO is generating RESET pulses continuously.

Objective:

I am trying to program my microcontroller initially. But since, the RESET is continuously generated by this LDO, my debugger is not able to program the microcontroller.

Steps taken :

I removed the R2705 resistor. My microcontroller cannot provide the watchdog pulse since it has not been program yet.

I tried removing the C2704 capacitor, C2703 capacitor each one at a time and tested. But still RESET is generated.

Below is the waveform obtained after removing R2704 and C2704.

Measured at pin 19, 20 and 21.

enter image description here

Questions:

1. What is the reason? Like the time period and the delay are set by the capacitors C2704 and C2703. Without that also, how is the LDO generating a RESET? Can someone please explain.

2. And, what should I do to make the RESET line high so that I can program my microcontroller through the debugger?

• Lundin: The fundamental parameters whenver speaking of any voltage regulator: input voltage, output voltage and load. What are they? Is the regulator getting hot?
• Vance: I suspect I know what's going on here but we could do with a little more information. The datasheet states two causes for reset pulsing low. The first is if Vo is drops below a threshold. Would you be able use your scope to measure Vo. The second is if WI doesn't receive regular pulses. Would you be able to measure WI and see if there are regular pulses there?
• user220456: Thank you. The Regulator is not getting hot. Added some details about the LDO in my question. And I checked the output voltage of the LDO. It is constant 5V (measured in scope). And WI pin is at 0V (measured in scope). I also removed the Resistor R2705 so that no signal is received into the WI pin of the LDO.
• user220456: No oscillations at Vcr pin too (Pins 22, 23 and 24) - Checked after removing the 10nF capacitors connected to those pins.
• Lundin: Seems that you have some strange cicular dependency in your design where the LDO must have a working MCU and the MCU must have a working LDO. How did you plan to break out of that dependency when you boot up cold or when the MCU otherwise isn't present (yet)? This is a highly specialized part, why do you need the watchdog feature? If you are trying to create a voltage supervisor circuit for the MCU supply, then there are far easier ways to do that.

Answer 1

What is the reason? Like the time period and the delay are set by the capacitors C2704 and C2703. Without that also, how is the LDO generating a RESET? Can someone please explain.

The output voltage is lower than 6% below reference (Vo_ref) 5 Volt in this case.
The low time period has a guaranteed minimum, either you are overloading the output or the input/output is unstable.

Or you're having a fight with the watchdog.

And, what should I do to make the RESET line high so that I can program my microcontroller through the debugger?

Fix the regulator operation. Or remove R2704.

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For production, add the option to feed the wdt with your programmer. Inhibit reset to your MCU, or make a bootloader image to flash within the watchdog time.