Turn on a SPMS with low signal from button, and keep it low with MCU - Page 1
Forum section: electronics / projects

author: eTobey
author_rank: Super Contributor
timestamp: March 14, 2023, 10:32:14 pm
content: Hi there,i have made my own circuit for the task. Still i wonder if there is a way to do it with less parts.Button on the left acts as real button and MCU signal for simplicity. Unconnected line on the right is converter enable.My circuitThe buckconverter starts when there is a level at about 1.5V (right side). The cap holds the FET in the conducting state, until the MCU holds the gate low.Another thing i think of, is: will the circuit stay off, if there is some EMI?

author: CountChocula
author_rank: Unknown
timestamp: March 15, 2023, 12:15:38 pm
content: Hi! Not sure that I really understand what you're trying to do, so apologies if this isn't what you weren't expecting, buthere's a solutionthat is a bit simpler. You may have to play around with the resistor values a bit to make it work (never trust a simulator), but I think it should be okay. I hope this helps!—CC

author: eTobey
author_rank: Super Contributor
timestamp: March 15, 2023, 12:55:19 pm
content: This works almost like my circuit. Just without the diodes, not protecting the Vgs voltage limits and no capacitor, that keeps it high for a bit when things start up.

author: eTobey
author_rank: Super Contributor
timestamp: March 15, 2023, 01:51:35 pm
content: I found a better solution (i think).Using a npn transistor, i dont have a gate that would be charged outside of the limit, so i can get rid of the zeners.New circuit

author: MathWizard
author_rank: Super Contributor
timestamp: March 17, 2023, 12:21:20 pm
content: Even with 42Vcc, using 40/50Meg R seems bad, only 1/2uA. IF it works IRL ok, but I'd go for 10uA at least, so 4M/5Meg, they are at least commonly available. Most sellers won't even have over 10M, and then you have to use a few, and it's not necessary. Assuming your 40Vcc has tons of power, I'd let the enable pin have a few 100uA if it wants it. The enable pin probably want a high or low signal, or a path to one.What buck IC are you using ? Some tie the inverting pin of their error amp, directly to the outside world, and if it's held low, the converter goes into STBY/shutdown. I just made a SW that would work for some stuff, but not other stuff, and not that case.So here's a open collector switch (1st pic). If the sw is open, Q2 is on, and will pull the pin low. But it's open collector, and will allow a lot of current, if that pin would allow it. So where's the datasheet ?Thats a 5.1V zener

author: eTobey
author_rank: Super Contributor
timestamp: March 18, 2023, 06:34:11 pm
content: The buck is a AP66300https://www.diodes.com/assets/Datasheets/AP66300.pdfThe 40M is the internal resistance of the enable pin of the buck regulator. Figured out, that i would need the 50M, so that i have only a little current going out through the base (does not need a big cap), and to have the proper levels for the eneable pin.I have made a new circuit, and placed the cap of the base to the Vin, so the system would not turn on, when the battery is put in.I only can use a low signal to turn on the system.Digikey has some G-Ohms resistors.Find attached my solution in ltspice.

author: MathWizard
author_rank: Super Contributor
timestamp: March 20, 2023, 04:03:53 am
content: Yeah my second circuit is like anything you would find on a motherboard or PSU, when the sw is on, Q2 is off and it R4 is pulled high. When then sw is open, Q2 is on, and R4 is pulled low, for off. That would be much more reliable than trying to measure input impedance to some mosfet's in some input buffer circuit or something, and using resistors a lot of DMM's can't read either, either by measuring resistance, or in circuit with voltage or current.