auto switching power IC

Discussion in 'General Electronics Chat' started by adrenalina, Jul 2, 2014.

  1. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
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    Hello everybody. I am looking for an IC that would be able to switch between 2 power supplies automatically. Something similar to the TPS2113. The problem with that IC is that both of my sources are 5V. 5V from the usb port and 5V regulated from a battery.

    Basically what a I want to do is that when I connect the battery the source becomes the 5v from the regulator and when I disconnect the battery the source is the 5v from the usb. Does anybody know of an IC that can get the jobe done?
     
    Last edited: Jul 2, 2014
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There needs to be some measurable difference between the two supplies. So you might be able to set one to 5.2 and the other to 5 if your circuits will be ok with that. Or, you could use a 3rd wire in the battery connection to indicate it is unplugged and switch to the USB.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Use a power jack that has a switch that disconnects the USB power when the battery source is plugged in.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    There doesn't need to be any voltage difference between the two supplies.

    Diode-OR the two supplies together to power a control circuit, and run each supply to a logic-level p-channel power mosfet to do the switching. The two FET drains are tied together to power the load.

    The battery input also goes to two single-transistor inverters in series. The first collector goes to the battery input power mosfet gate, and the second collector goes to the USB input power mosfet gate. Each gate has a high value resistor to the its source.

    When the battery is connected, it acts as a logic high signal to the first inverter, which pulls the battery mosfet gate low. That low goes to the 2nd transistor inverter, which lets the USB mosfet gate be pulled high by its bias resistor. When the battery is removed, both inverters invert and the USB fet comes on.

    2 transistors, 3 resistors, 10 cents.

    ak
     
  5. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
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    @Mrchips I'm using a lipo battery, so I'm not using a standard power barrel connector. I really wouldn't want to cut the connector from the terminal to solder a power barrel.

    @AnalogKid Is this the circuit you are referring to? I attached an image of the circuit. The only problem I found in the sim is that when only the usb is connected the battery mosfet is partially on. The voltage on the load becomes about 2.1V. Current seems to still flow through the base of the first transistor and from the drain to the source of the battery mosfet.

    Working with your idea the second circuit that i attach is what worked, at least in the sim.
     
    Last edited: Jul 4, 2014
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    AK, I was talking about the IC he posted. It needs a difference in voltage to work. Where do you buy those $.02 FETs?:D

    As much as I hate to run down your battery, you need a pull down resistor on the battery side of the battery diode or the leakage current from the 5 volts will turn on the transistor. Use a little signal diode. They have less leakage than schottky diodes.
     
    Last edited: Jul 4, 2014
  7. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
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    Would the circuit that I attach work? Using BU49xx voltage detector IC. According to me it would work. When the battery is connected the IC outputs high and turns of the mosfet when the battery is disconnected the internal circuitry would connect Vout to ground and would turn on the mosfet. the only fear I have is current flowing in the opposite direction through the regulator. I don't know if the would happen. Any theories?

    link to datasheet: http://rohmfs.rohm.com/en/products/databook/datasheet/ic/power/voltage_detector/bu48xxg-e.pdf
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Last edited: Jul 6, 2014
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