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Switching power supply sequencing - Power down

Tamas Somhegyi , 11-04-2015, 07:42 AM

Suppose that you have two switching power supply: one supplies the IO (3.3V) and the other the Core (1.2V) voltage of an MCU. According to the datasheet the 3.3V must turn on earlier and turn off later than 1.2V. It can be achieved by PowerGood and Enable pins for the turn on. However at turn off if the 3.3V fails it disables the 1.2V supply but the capacitor remains charged after that, similar happens with the capacitor of 3.3V. Unfortunately the power down sequence is determined by the load currents which might be hard to determine. Can this cause improper sequence or damage to the device?

Best regards,
Tamas Somhegyi

robertferanec , 11-04-2015, 08:34 AM
What chip are you trying to power on? In simple boards you can not really influence turn off sequence - especially in cases when power is lost or when you use a design with only one power state. If you are not sure, have a look at the reference design and check how they did that.
Tamas Somhegyi , 11-04-2015, 09:11 AM
ATSAME70Q21A microcontroller (http://www.atmel.com/Images/Atmel-11..._Datasheet.pdf)
Unfortunately the reference design uses the internal regulator where the power sequencing is not an issue. This is my first board where I don't use the internal regulator and I have no experience whether how serious this power down sequence is. I found on the web a paper which describes a discharge circuit for fpga power rails: http://www.diodes.com/downloads/9490. I know fpga is much more complex in power sequencing and consumption point of view but I was not sure which are the circumstances where it becomes an issue.
I use ~10-100uF so I think this should not be an issue in this capacitance range.
robertferanec , 11-05-2015, 12:36 AM
Hmm, interesting requirements for a microcontroller.

I know about discharge circuits. I have used them before, but they are usually used in boards with more power states (e.g. x86, where part of the board is still powered so you can control the discharge circuit and power down sequencing). Also, be aware, many modern regulators have the discharge circuit build in.

I am not sure how it is going to be in your board, but I guess, your microcotnroller board is not going to have more power states, or?

I would probably still consider to use the recommended circuit from reference design - internal regulator.
Tamas Somhegyi , 11-05-2015, 11:42 PM
Yes, I have one power state only. I have checked and my regulators unfortunately does't have this feature. I haven't seen any design in the MCU world where these circuits (discharge) are used. I think I will put the pads there for the discharge circuit and I will feed them if it seems necessary. Thank you for your help.
robertferanec , 11-06-2015, 01:46 AM
I didn't help you much But it is interesting to know, that even microcontrollers may require power down sequencing
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