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SN74LV4051A additional supply current

nick43oh
Prodigy 250 points
Other Parts Discussed in Thread: SN74LV4051A, SN74CBTLV3251

Hi,

Can someone clarify a point for me please.

I am looking to use a SN74LV4051A (8-Channel Analog Multiplexer/Demultiplexer). I want to run it from a 3.3V supply and control the selector lines from an MSP430 operating from the same supply. Supply current is critical!

A lot of of similar ICs have a datasheet entry for "additional supply current" when GND < Vi < Vcc.


Supply current is quoted at 20uA (MAX at Vcc = 5.5V, over temperature). There is no supply current quoted for 3.3V operation; would it be more or less?

Can I expect "additional supply current" when using this IC at 3.3V? How high or low do my MSP430 O/Ps need to be to avoid any additional supply current?

Thank you,

Nick.

  • 0
    TI__Mastermind 45575 points

    Nick,

    The supply current into the Vcc pin will be lower with Vcc= 3.3 V than Vcc=5 V.  You can kind of think of it like ohms law V= IR.  If the impedance stays the same and you reduce the voltage the current will decrease.

    To avoid the "additional supply current" you want to place your digital logic input voltages as close to Vcc or GND.  You can read this app note "Implications of slow or floating CMOS inputs" for more details on how keeping your digital logic input voltages at the supply voltage or ground will prevent excess current consumption.

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 250 points

    Hi Adam,

    So if my MSP430 datasheet says:

    Are those output voltages OK to drive the multiplexer without causing additional supply current?

    I can't find anywhere, any information that says current will increase at a specified voltage above GND or below VCC. To me it just says "only absolute GND and VCC is fine". Sorry if I seem obtuse but I'm not very familiar with using logic in critical situations.

    Also, during MSP430 start-up, until its GPIO are configured as O/Ps, will the external logic IC be drawing lots of additional current because the I/Ps are in an undefined state? (The MSP430 GPIO default to inputs on power-up.)


    Thank you,

    Nick.

  • 0 in reply to nick43oh
    TI__Mastermind 45575 points

    Nick,

    No worries nick that's why we are here to help. 

    If you run the SN74LV4051A device with 3V rail the valid logic levels for the device will need to be at least 2.1 V (Vih) for logic high and at most 0.9V (Vil).  If your MSP430 logic out put voltage is 2.7 V for logic high and 0.3 V for logic low you will be able to keep the SN74LV4051A in a defined state.  

    The issue with the extra current consumption is leaving the switch in an undefined state.  This could increase your current consumption from uA to mA.  You will not have this issue since you are placing the switch in defined states. 

    What are you working on that the uA current consumption of the switch is critical?

    Let me know if you have any other questions. 

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 250 points

    Hi Adam,

    Thanks for clearing up my understanding of valid voltage levels.

    Our circuit has a current limited supply (say, 1mA). Draw more than 1mA and the voltage collapses. Can't do anything about that, it's a design requirement.

    Suppose the MSP430 takes 50ms to power-up and configure its O/Ps (during which time those port pins are defaulted to inputs) will that upset the external logic chip?

    If that logic chip starts drawing "additional supply current" because its inputs are connected to micro inputs... our current-limited supply will collapse and nothing will ever start.

    It looks to me, like this will happen. I value your opinion.

    Thanks,

    Nick.

  • 0 in reply to nick43oh
    TI__Guru 61770 points
    Hi Nick ,

    The control lines of the LV4051A are high impedance lines which wont load the MSP430 with 6mA . Look for the input leakage spec on SN74LV4051A datasheet for the loading which is 1uA max.
    With this tiny current , the Voh is most likely going to be at 3.3 which wont cause additional supply current from the lines.
  • 0 in reply to ShreyasRao
    TI__Guru 61770 points
    You could also consider SN74CBTLV3251 which is 1:8 Mux operates at 3.3V and has ioff feature which will prevent unwanted conditions during power down conditions. Please let know what you go with.
  • 0 in reply to ShreyasRao
    Prodigy 250 points

    Hi Shreyas,

    I think you may have misunderstood my point.

    Regards,

    Nick.