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I am working with a micro controller which has this written in its data sheet:

"A step increase in supply voltage of 300 mV or more, with rise time of 300 ms or less, within the valid supply range, may result in a system reset."

In my application, the supply voltage may in the worst case scenario rise from 2v to 3v, how can I prolong this rise time?

I have looked at an RC circuit which seems like it may do the job, however I don't want to add too much series resistance to the supply voltage.

If anyone has any suggestions it would be greatly appreciated!

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    \$\begingroup\$ Why would your supply voltage rise from 2 to 3V faster than 300ms? Is that a transient or a true rise? But to answer your question, use an inductor instead of a resistor...but that introduces its own peculiarities (resonant peaking). Or use a linear regulator. \$\endgroup\$
    – DKNguyen
    Commented May 1, 2019 at 15:57
  • \$\begingroup\$ It would increase faster than 300ms because when the battery is being charged at a constant voltage (3V) as per its data sheet the supply voltage for the micro-controller will jump fairly quickly from as low as 2V to 3V. \$\endgroup\$
    – jm212121
    Commented May 1, 2019 at 16:02
  • \$\begingroup\$ I will have a look at using an inductor instead of a resistor. \$\endgroup\$
    – jm212121
    Commented May 1, 2019 at 16:03
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    \$\begingroup\$ Check your charging method. Charging at a constant voltage does not sound right. \$\endgroup\$
    – DKNguyen
    Commented May 1, 2019 at 16:04
  • \$\begingroup\$ Look at the charging specifications: industrial.panasonic.com/cdbs/www-data/pdf/AAF4000/… \$\endgroup\$
    – jm212121
    Commented May 1, 2019 at 16:10

2 Answers 2

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You could run the MCU from an external LDO (Low Voltage Dropout) voltage regulator, design the output voltage to be a bit less than the minimum input voltage. Or, as Tim suggested in the comments, you could use something like a buck-boost converter to maintain a constant voltage regardless of input voltage.

If you, for some reason, need the voltage to vary, you could use an LDO with a "soft start" circuit (see below example) but it will be considerably more compex most likely, unless you can find a part with that functionality built-in. See, for example, this application note.

enter image description here

As yet another alternative, you may be able to disable the internal reset circuit in the MCU configuration and replace it with an external circuit that doesn't have that nasty characteristic. Often the internal ones are not all that good anyway (in particular they may not reset when they should, but this one has the opposite characteristic). Really rapid changes in Vcc can cause the MCU to malfunction so care is called for if you follow this path.

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The supply voltage needs to be regulated to the value specified on the datasheet. If you are referencing the initial rise time of the supply voltage then this should trigger a power-on reset.

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  • \$\begingroup\$ Yes but the chip can be supplied at anything form 1.8V-3.3V as it has an internal LDO, however my issue arises when my power supply is plugged in to charge I don't want a system reset. \$\endgroup\$
    – jm212121
    Commented May 1, 2019 at 16:05
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    \$\begingroup\$ If your minimum battery voltage is 2V and you want simplicity, regulate it down to 1.8V with a linear ("LDO") regulator. If you want efficiency and have the circuit design chops, then use a switching regulator that delivers the voltage of your choice from the voltage supplied by the battery. \$\endgroup\$
    – TimWescott
    Commented May 1, 2019 at 16:15

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