Low current latching power switch - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 09, 2016, 03:00:45 am
content: Hello!A latching power switch is not magic and i have seen Daves video about it but i have a problem...The device is powered of two AA Batteries, that feed a 3.3V step up regulator.So i assume a worst case input voltage of 2V and below when they are empty and when it is on i have a 3.3V rail.It should be simple and at that voltage level MosFET are out, i think, please correct me if i am wrong.So i went with a PNP and two NPN transistors:This seems to work in the simulation, my goal is to have one button that turns the thingy on and that the button can be read by the µC.If it is pushed again (a few times, with a pattern) the µC can disable itself and turn of the power supply.But... after i was done i took a look at the current.The soft latching button circuit draws nearly 4mA with is a whole lot when i consider that the µC draws only 6mA and the display just <500µA.And on top of that, since i use a PNP transistor, i have a decent voltage drop.Can someone give me a tip on how i could improve the whole thing a bit?The additional current draw does not impact the whole function and runtime too much, but if i could shave of 1/3 of the power consumption, that would be fairly nice.Greetings,Peter

author: Marco
author_rank: Super Contributor
timestamp: May 09, 2016, 03:26:34 am
content: Clear as mud that schematic ...There's low threshold P-MOSFETs you can use for the power switch (probably specialized ICs as well, but where's the fun in that).R1/2/3 are ridiculously small, try 10K. Add a pull up resistor from the bottom of R3 to the battery voltage to speed up turn off of Q1. V4 makes no sense whatsoever in that simulation (it's continually fighting the main switch).

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 09, 2016, 09:36:42 pm
content: Quote from: Marco on May 09, 2016, 03:26:34 amBoth V3 and V4 are only a transient of a few ms, V4 simulates the "off" command from the µC and V3 the "on" from the mechanical switch.The small resistors are what i end up with, when i calculate the current for the chosen PNP transsitor, highver values do not work.Alas, i completely forgot that my DCDC regulator has a disable input with output disconnect. That will work just fine without voltage drop, high on-current and will only use a few µA in shutdown mode.

author: daveatol
author_rank: Regular Contributor
timestamp: May 09, 2016, 10:07:09 pm
content: You could use a CMOS logic IC, which will have virtually no quiescent current. If you use something like a 74AC04, you should be able to get a decent amount of current out of it.Circuit attached. If you use a mosfet in place of the bjt shown, you don't need the base resistor or the diode (it's built into the mosfet). That will bring your parts down to two resistors, one mosfet and the IC.

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2016, 10:19:47 pm
content: What about a flip-flop such as the 74HC74 or the LVC series which may be able to drive it directly?http://www.nxp.com/documents/data_sheet/74LVC74A.pdfhttp://www.ti.com/lit/ds/symlink/sn74lvc1g175.pdfhttp://www.nxp.com/documents/data_sheet/74HC_HCT74.pdf

author: Marco
author_rank: Super Contributor
timestamp: May 11, 2016, 03:39:15 am
content: Quote from: PeterFW on May 09, 2016, 09:36:42 pmI'm inclined to say you're doing something wrong, here's a slightly tweaked version (simulated with high battery voltage, because I didn't want to import a low threshold MOSFET model like say the NTJS3151P). Simulation with it being turned on by the Switch voltage and turned off by the uC_Off voltage.