Is the smooth ramp up voltage important on power-up? - Page 1
Forum section: electronics / projects

author: mrkev
author_rank: Regular Contributor
timestamp: September 01, 2017, 08:12:36 am
content: I have a simple mosfet switch that disconnects power from the main circuitry (microcontroller with some HCT logic) while the device is in RESET. I had to do some changes and prolong the time the RESET is active because the VCC didn't sink below 0,5V.Unfortunatelly, my switch is not fast enough (and is connected directly to a slow RC network). The ramp up of VCC looks like this:Repearing this means either adding more stuff to the circuit and/or doing a complete rework of PCB and I am not really keen on either. So the question is, am I just paranoid or could this be an issue...

author: Ian.M
author_rank: Super Contributor
timestamp: September 01, 2017, 08:42:29 am
content: It could be a major issue. What MCU are you using? Does it have internal BOR, with a threshold above the messy part of your power trace?

author: mrkev
author_rank: Regular Contributor
timestamp: September 01, 2017, 08:54:32 am
content: It's a PIC32MX370. The RESET is active for another 500ms. And the BOR is just barely above "the messy part" (but I am not sure if that would do anything since the device is already in reset).

author: Ian.M
author_rank: Super Contributor
timestamp: September 01, 2017, 09:07:35 am
content: Microchip MCUs generally aren't bad at handling erratic power sanely. As long as you don't mind the risk of some unexpected I/O pin toggling before Vdd gets into the operating voltage range you should be OK

author: mrkev
author_rank: Regular Contributor
timestamp: September 01, 2017, 12:49:35 pm
content: Well, thank you. I had to change it, because I wasn't confident it won't be a problem in the future. By adding single transistor (that I will be hopefully able to fit on PCB) I made a shmitt trigger and it looks like this (if anything, it's too quick now):

author: suicidaleggroll
author_rank: Super Contributor
timestamp: September 01, 2017, 03:23:44 pm
content: Having a slow turn on generally isn't an issue that I'm aware of (unless it's REALLY slow), but having a power supply that does not constantly increase in voltage on power-on can be. I'm not sure what your circuit looks like, but with a Pch mosfet switch you can get a pretty clean ramp by adding a capacitor from gate to drain and a resistor from gate to whatever is driving the gate, eg:

author: mrkev
author_rank: Regular Contributor
timestamp: September 03, 2017, 12:13:38 am
content: It looks like that... The problem is that purpouse of this switch is to disconnect the logic circuitry from VCC while the microprocessor is on RESET. There is a derivative trigger that temporarily deactivates the switch. But because the time was too short (and the VCC didn't went below 0,5V), i had to change the RC constant. The switch is not driven from any logic, but from other Nch mosfet, which was connected directly to RC network. And both of them together didn't have the amplification to make slow RC ramp up switch VCC quick enough (there wasn't a constant VCC ramp up but rather constant decrease of switch impedance). At the end I added one more Nfet, making shmitt trigger.