Looking for an IC solution (shorting a line through MOSFET) - Page 1
Forum section: electronics / projects

author: crypton
author_rank: Contributor
timestamp: May 21, 2018, 09:38:23 am
content: Hei,I am giving my best shot to describe what I am doing right now.- I am connecting two inputs together (shorting them) through a high side switch (voltage can be up to 50V, low currents ~mA). I am not powering anything through that line directly. Only the switching circuitry.- This is done by a single tactile push button with a parallel high side switch.- The shorting switching action is latched (currently by 3V3 rail and/or can be disabled by uC).(doesn't need to be in the 'IC solution')- The tactile button also acts as a UI navigation button - thus this needs to be read by a uC.- Tactile button should be able to be isolated affecting the main switching and only give UI input to uC.Rather simplified schematic of already tested circuit:Details:This circuit wakes up a main power module -only then I get power. I'm dealing with a black box. So all I need to do, to get power for my device is to short two lines together.These two lines mostly are somekind of higher impedance inputs and outputs that detect states.That line can and can not have a voltage and the provided current is expected to be low - these parameters vary between those black boxes (from 5V to 50V).When dealing with lower voltages, it's only a matter of not using a resistor divider for the parallel HS f.i.So when I short the lines, 3V3 comes up and I latch the circuit.But depending on the black box,I can remove the short- I don't lose power.For some boxes, I must remove the short asap, otherwise the module goes to sleep again - thus no power. So the current circuit has to be very flexible and currently is also.The button 'isolation mosfet's' idea is NOT to allow short between the linewhile the uC keeps the main HS switch closed. Because previously mentioned, some boxes go to sleep when doing that.I wonder if there's some IC solution that does all or partially these things.Why I'm looking for some IC is that, creating this with dual mosfet packages and having all those bits and pieces is getting very cluttered and starts to take up much board space.I've tried to look for some premade High Side switch ICs, but haven't really found a compact version (SOIC8 is too much).//EDIT:Experimented that tactile 'disable' works better when I put one PMOS in parallel with the low side PMOS (in between M4 and M5).So this complicates the selection of the component even more, because usually discrete HS drivers don't allow to put anything in between the MOSFETs.Uhh, thinking I can't get around simplifying this a lot.

author: jbb
author_rank: Super Contributor
timestamp: May 21, 2018, 08:05:08 pm
content: HmmI have some thoughts on this, but I'm not sure they're exactly in line with what you want to do. I agree you've got quite a few bits and it's probably good to simplify.Is the 3.3V rail always live or switched by this circuit? If your 3.3V rail is always live then you could just go SW2 -> micro -> M3 gate and cut out most of the parts.If your 3.3V rail is ultimately switched by M1, then I have two questions: first, what happens if you hold SW2 down while the uC attempts to turn the power off? Secondly, have you considered using a low Iq buck regulator to make an always-on 3.3V rail? These days you can get ones which consume < 10uA while still producing 3.3V (Texas Instruments, Analog Devices, Diodes Inc and surely others) and can still supply 500 mA or more when required. Then you can deploy your switching controls at 3.3V level. (Modern chipsets often have really good shutdown currents and sleep modes.)I believe Analog Devices (they bought Linear Tech) have some power on switch chips which could implement the push button and latching for you. Maybe one of their Hot Swap or Diode OR controllers could do 50V directly. They also have some such button supervisor chips which I suspect only run off 2 - 5V rails.

author: crypton
author_rank: Contributor
timestamp: May 22, 2018, 09:10:21 am
content: Uff, yeah, probably would be good to give more details:This circuit wakes up a main power module -only then I get power. I'm dealing with a black box. So all I need to do, to get power for my device is to short two lines together.These two lines mostly are somekind of higher impedance inputs and outputs that detect states.That line can and can not have a voltage and the provided current is expected to be low - these parameters vary between those black boxes (from 5V to 50V).When dealing with lower voltages, it's only a matter of not using a resistor divider for the parallel HS f.i.So when I short the lines, 3V3 comes up and I latch the circuit.But depending on the black box,I can remove the short- I don't lose power.For some boxes, I must remove the short asap, otherwise the module goes to sleep again - thus no power. So the current circuit has to be very flexible and currently is also.The button 'isolation mosfet's' idea is NOT to allow short between the linewhile the uC keeps the main HS switch closed. Because previously mentioned, some boxes go to sleep when doing that.I have also thought about having a regulator. I am not powering anything through that line directly. Only the switching circuitry. So having a separate regulator tends to be overkilling it (with inductors, diode, caps etc). No eager to have them there. But haven't seen such low Iq bucks though that won't take much space. But then again, since I'm not supplying from there directly, I don't see the need for regulator.I am also on a lookout for some pushbutton circuit, need more time to look into them, haven't seen at this time anything suitable. Maybe need to think it a a subcircuit fro the main HS switch...(although this tactile button is also inputted into one reset circuitry that I'm using, but that is only 5V capable).