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DS16F95: DS16F95 ICC -- Low Toggle Rate, Receiver Only Power

WolfHort
Prodigy 90 points
Part Number: DS16F95

Tool/software:

Hello, 

Considering using the DS16F95 in a receiver only configuration with low input data rates -- but the ICC Max value listed in the datasheet implies >100mW even in this configuration... Is this accurate / correct interpretation?

For further reference to the application -- toggle rate would be very low (once every several milliseconds ... I.E. < 1kHz data rate)

Should we be expecting up to 28mA (140mW) even in this very low toggle rate configuration?

If yes... out of curiosity... what is driving such high currents? The large common mode range biasing the inputs (we expect to be in a very narrow common mode range)? Or just internal biasing?

  • 0
    TI__Guru 52082 points

    HI,

    That current is the idle current - there is no load - so actually your worst case supply is higher than listed here as you would be adding loads and the outputs would be drawing current. 

    Since the toggling rate is slow there probably won't be a lot of AC losses - but you would have to account for any additional current leaving the device as that current will be sourced from VCC. 

    The biggest thing increasing the current is that this part uses BJT's and not entirely CMOS (CMOS is much more common for RS-485 devices and uses less power); RS-485/RS-422 is not necessarily the most power friendly standards to begin with but using bi-polar processes will increase supply current - even though the description on the first page of the datasheet says that is uses lower power - that is in reference to parts from 1998 or earlier- modern parts perform better. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 90 points

    For clarification, per my message -- we are not adding any loads (receiver only configuration).

    The receiver only configuration will have DE de-asserted and RO driving into a CMOS (high impedance) input of a processing unit. As such, I expect almost no sink / source current to the output other than any input bias current the device requires.

    As someone's who designed quite a few BJT circuits -- it is typically possible to have slow transition state power be localized to one or two active branches that draw single digit milliAmps.


    I could see input bias currents getting quite high across the full common mode input range of -7V to 12V -- where the VDD bias is sourcing current into a -7V common mode input on the receiver... But the datasheet implies that is not the case (very low bias currents sinked/source to the input directly).

    If you have an internal circuit diagram of the part I could assess independently.

  • +1 in reply to WolfHort
    TI__Guru 52082 points

    Hi,

    Sorry I accidently clicked the resolved button when I tried to reply. 

    I understand you are using it as receiver only - what I am saying is that even current outside of the "R" pin to the controller needs to be accounted for - because our test has no loading - there is no external current draw - in a receiver only configuration generally speaking the idle ICC is the dominant term. That idle current has nothing to do with common mode input - that isn't how it was defined or tested - this part consumes a lot of power. 

    The biggest difference between this part and most of our portfolio is that is BJT based and most of the other parts aren't. 

    We can't share schematics on a public forum. 

    What matters is that this part can consume a lot of current just sitting there being connected to power and a lot of it has to do with temp. -The max value is only going to approached - generally speaking - near max ambient temperature rating of device

    Why are you focusing in on this device specifically? I am asking because we have a lot of devices that can perform the same tasks with a much smaller current draw - so understanding why you went for this device may be helpful to see if we have a proper replacement that is more modern and has better performance with respect to power usage among other parameters. 

    Best,

    Parker Dodson 

  • 0 in reply to Parker Dodson
    Prodigy 90 points

    Understood -- we will assume the 28mA current corresponds to quiescent power draw irrespective of operational conditions.

    And we understand that driving a load will result in higher currents, especially if the differential driver is enabled and used to drive a terminated bus.

    We are targetting this part for radiation requirements. Likely the QML variant.

    Thanks!