The MMBTA42 is a bipolar transistor and not a FET. Secondly it's NPN, making it unsuitable as a high side switch, because the base needs to be at least at 0.7V higher than the voltage at the emitter for the transistor to turn on. I used a Winsok WST2339 because that was the lowest Rds(on) P-channel FET I could find in a SOT-23 package. However you will no doubt be able to find something similar from Mouser, Digikey, etc. It will be active low, so the gate should be connected to the input voltage rail through a pull-up resistor. Your microcontroller will then have to pull the gate low to enable the power supply. If the supply voltage of the power supply is within the voltage rating of your microcontroller's GPIO pins, you can connect it directly. If not, you should use a N-channel MOSFET or an NPN transistor to pull the gate low. søndag den 6. juni 2021 kl. 22.24.19 UTC+2 skrev flata...@gmail.com:
> That's great to hear as a confirmation. As soon as I get a chance to spend > some time on the bench, I'll give it a try. It should be quick enough. I'm > going to use a standard MMBTA42, please let me know what you have used, if > you can share that piece of information. > > Thanks Christian. > > Il giorno domenica 6 giugno 2021 alle 21:17:00 UTC+1 Christian Riise > Wagner ha scritto: > >> I've successfully used a P-channel MOSFET as a high side load switch for >> a LM3478 based tapped inductor booster. IIRC the off current was >> immeasurable, so sub 100nA. >> søndag den 6. juni 2021 kl. 19.33.50 UTC+2 skrev flata...@gmail.com: >> >>> Thanks David, indeed you are absolutely right. The power supply with >>> LT1308B is very reliable and not as sensitive to layout design as other >>> ones that I have tried. >>> >>> LT1308B has in Iq of 0.01uA in stand-by, as you say. I've built your >>> nixie watch (picture attached) and in my design it seems to use 2.3uA. If I >>> build the HVPS on a breadboard, I can measure 0.1uA or less, which is great >>> (I guess the resolution of my multimeter Keysight U1241B cannot measure >>> less than 0.1uA accurately but that's great at 0.1uA or less). Of course I >>> have tried to troubleshoot my LT1308B HVPS with no success. In my quest for >>> a more efficient HVPS, I have designed a power supply using the MAX668 >>> chip (as per datasheet and similar to many commercial HVPS), this was >>> tricky to design with no noise, it works well now (after 6 different >>> designs) however the stand-by power is 2.4uA, as per datasheet. So I can >>> have higher efficiency than with LT1308B but also higher stand-by current >>> (I believe because MAX668 has an internal LDO). >>> >>> Which is why I was thinking whether I can use a high load switch to >>> truly bring shutdown current to zero using a power supply built around >>> MAX668. Of course, this is a bit of a (pointless) extreme challenge to >>> reduce stand-by current as much as possible to break even with solar >>> panels, but that's where the fun is for me. And of course I could keep on >>> troubleshooting the LT1308B design, it actually works perfectly other than >>> it gives me higher stand-by current than on the datasheet, not sure if that >>> depends on the design layout. But then again, I'm only a hobbyist and not >>> an engineer, so I may be missing something. >>> >>> So any suggestions on why my LT1308B draws more than 1uA with SHTN=0V >>> welcome (I have tried 4 different chips, same thing on the PCB, fine on the >>> breadboard). Or any other suggestions to use any other power supply with >>> perhaps a high load switch to bring standby current to less than 1uA, which >>> I worry is unnecessary additional components and introduces a potential >>> start-up delay. >>> >>> >>> Il giorno domenica 6 giugno 2021 alle 18:12:46 UTC+1 nixiebunny ha >>> scritto: >>> >>>> Max, >>>> The LT1308B has a .01 uA typical, 1 uA maximum standby current when >>>> shut down, according to the data sheet, shown on page 2 as Iq (the >>>> third line with Vshdn = 0V). >>>> >>>> I don't think you have a problem here. >>>> >>>> >>>> On 6/6/21, Max DN <flata...@gmail.com> wrote: >>>> > Hello, >>>> > >>>> > I'm working on a solar powered nixie watch (similar to the Kopriso >>>> Nixie >>>> > watch). >>>> > >>>> > It's crucial that I dramatically reduce stand-by power as much as >>>> possible >>>> > and within 2.5uA, which is where the solar panels will balance off >>>> the >>>> > stand-by current. >>>> > >>>> > Regardless of which power supply I use, even using the shutdown pin, >>>> the >>>> > power supply still takes 2.5uA. This is perfectly in line with the >>>> > electrical specifications of the datasheets. >>>> > >>>> > So, my question is, can I 'brute force' a shutdown, effectively >>>> removing >>>> > the power from the VIN pin via a PNP high speed mosfet such as >>>> MMBTA42 or >>>> > it will introduce too much of a delay in starting the power supply? >>>> > >>>> > I'm going to try it on the fly but if anyone has any suggestions on >>>> how to >>>> > bring to 0 or to 0.01uA the stand-by current that would be great. >>>> > >>>> > As an example, I'm using the LT1308B DC/DC converter as on page 17 of >>>> the >>>> > datasheet >>>> > >>>> https://www.analog.com/media/en/technical-documentation/data-sheets/1308abfb.pdf >>>> >>>> > >>>> > Any suggestions will be much appreciated. >>>> > >>>> > Thank you, >>>> > Max >>>> > >>>> > -- >>>> > You received this message because you are subscribed to the Google >>>> Groups >>>> > "neonixie-l" group. >>>> > To unsubscribe from this group and stop receiving emails from it, >>>> send an >>>> > email to neonixie-l+...@googlegroups.com. >>>> > To view this discussion on the web, visit >>>> > >>>> https://groups.google.com/d/msgid/neonixie-l/d45edcd8-0176-4205-95f1-b04eb34a32f1n%40googlegroups.com. >>>> >>>> >>>> > >>>> >>> -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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