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How to build the power switch

Former Member
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Hello,
 
I am looking for help how to build the power switch in my controller.
 
Let’s assume there is a 2x 3,6V li-Ion battery powering the microcontroller with sensor on input and graphic LCD TFT display as an output. Also there is a SMPS flyback converter which upconverting the battery voltage (2x3,6V=7,2V) to stabilized +400V dc to power the sensor, battery charger, additional regulator powering microcontroller (3,3V).
 
I would like to add the power switch between battery and rest of the circuit. Pushing the tact button the flyback converter and microcontroller should be on, and on the long push (i.e. 2 second) the power should be off. Of course it is possibility to connect pushbutton to the microcontroller GPIO lines or INT input, but I would to minimize current leakage.
 
I am looking for any design ideas, proposition of the schematic and maybe help how to minimize the current with off state. This is because the meter will be used very ocassionally, assume once per 6 months by 5 minutes.
 
Marvin
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  • 0

    Hi Marvin,

     

    I have just recalculated the current consumed by my cct.  It works out at 0.0000003mA.   No mattter what micro controller options are suggested, I doubt they can achieve a working cct consumption figure this low.  This is 0.0003 microAmps.

     

    Gordon.

  • 0 in reply to Former Member

    GWatPE, can you post a schematic of your power up circuit?  I like the LTC2950 route but I think yours is the cheaper and more power efficient.

     

    Thanks!

  • 0 in reply to jeremyvnc

    Hi Jeremy,

     

    The LTC2950 is a low power regulator that has been around for years.  I have no facilities for publishing ccts at the moment.  The micro option above posted by Dean, does a similar function to my gated oscillator option.  This is used in many laptop power up/down functions, where micros and spare code and IOpins are the norm.

     

    I know how power efficient my cct is.  Micros are pretty common these days and many are simple to program.  The lowest power option may not be the easiest to build.

     

    A micro usually takes control of the switch control function after the cct is energised.  In my cct, The pushbutton controls the ON and OFF function directly.  With the micro cct option, say if the micro is stuck in a loop somewhere and cannot get back to processing the button command, then a manual power OFF switch may still need to be provided.

     

    I will look into maybe taking a photo, and posting this.

     

    Gordon.

  • 0 in reply to jeremyvnc
    Push button power switch.
     
    With the LTC2950-2 you can drive a P-channel enhancement fet like in the following schematic.
    If you use a voltage divider in stead of R2, you can use the undervoltage lockout feature of the IC. (See data sheet) So if the battery is almost empty, the device is switched off. This will enhance the battery life and will prevent unwanted situations at the load side.
    image
     
    The following schematic, I designed some time ago, only has the switch functions.
    On the drawing you see two switches; an ON switch and an OFF switch.
    If you connect the switches in parallel, you can switch the device on and off with the same button.
    A short push will switch the device on, a short or a long push will switch the device off.
    The ON-function is disabled when the 2μF capacitor, C2, is charged.
    For the 2μF capacitor I used two small 1μF ceramic multilayer capacitors in parallel.
    This seems more reliable to me than an electrolytic capacitor.

     

    image

    Powerful enhancement fets with low RDS on and a small footprint are available in SMD.
    Both circuits can be used between 5 Volt and 20 Volt.
    The limitations are a minimum of 5 Volt to drive the P-channel enhancement fet completely in conduction. (RDS on < 0.02 Ohm)
    The maximum gate to source Voltage for most fets is 20 Volt.
    For applications over 20 Volt a zener diode must be placed between the gate and source of each fet and a series resistor must be applied in the circuit driving the gate of the fet.
     
    Johan
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  • 0 in reply to jeremyvnc
    Push button power switch.
     
    With the LTC2950-2 you can drive a P-channel enhancement fet like in the following schematic.
    If you use a voltage divider in stead of R2, you can use the undervoltage lockout feature of the IC. (See data sheet) So if the battery is almost empty, the device is switched off. This will enhance the battery life and will prevent unwanted situations at the load side.
    image
     
    The following schematic, I designed some time ago, only has the switch functions.
    On the drawing you see two switches; an ON switch and an OFF switch.
    If you connect the switches in parallel, you can switch the device on and off with the same button.
    A short push will switch the device on, a short or a long push will switch the device off.
    The ON-function is disabled when the 2μF capacitor, C2, is charged.
    For the 2μF capacitor I used two small 1μF ceramic multilayer capacitors in parallel.
    This seems more reliable to me than an electrolytic capacitor.

     

    image

    Powerful enhancement fets with low RDS on and a small footprint are available in SMD.
    Both circuits can be used between 5 Volt and 20 Volt.
    The limitations are a minimum of 5 Volt to drive the P-channel enhancement fet completely in conduction. (RDS on < 0.02 Ohm)
    The maximum gate to source Voltage for most fets is 20 Volt.
    For applications over 20 Volt a zener diode must be placed between the gate and source of each fet and a series resistor must be applied in the circuit driving the gate of the fet.
     
    Johan
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