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TM4C123GH6PM: Internal circuit VBAT pin. Inverse current.

Fabian Toledo
Prodigy 85 points
Part Number: TM4C123GH6PM

I need to know if the internal circuit of Vbat has protection for inverse current, i.e., in case of a failure, how much current can be drained from the VBat pin.

Thank you.

  • 0
    TI__Guru 72500 points

    What type of failure mode are you considering? Section 3.5.6 of the System Design Guidelines for TM4C123x Family shows using a 51 Ohm resistor with a coin cell battery primarily to control the rise time, but it also will limit the maximum current flow.

  • 0 in reply to Bob Crosby
    Prodigy 85 points

    Hello Bob,

    From IEC 61010-1 the battery has to be protected for inverse current in case of failure. Does the pin has any kind of protection for inverse current?

      

  • 0 in reply to Fabian Toledo
    Guru 117855 points

    Fabian Toledo said:
    From IEC 61010-1 the battery has to be protected for inverse current in case of failure.

    Alien (outsider) responding - Fig. 22 (post above) provides (some) current limiting via the 51Ω series resistor.

    Your stated concern - 'Inverse Current' (i.e. Current sourced by the MCU's 'VBAT pin') appears (to an outsider) as unlikely.    

    Yet - while that  'may hold true' under, 'Normal MCU behavior' - under MCU's,  'distress or failure conditions' - inverse current (may then) flow!    And - as the battery, 'Ages & Discharges' the 'vulnerability to such Inverse Current increases!'     (Much to the Battery's detriment - which IEC61010-1 seeks to mitigate.)

    Now - even if there exists (some) 'Directional Current Flow Mechanism' w/in the MCU - that itself is, 'Subject to Failure' - is that not true?

    More positive protection (i.e. Inverse Current Resolving & Regulatory Agency favoring) is afforded by a,'Low Voltage Drop, adequately rated, Schottky Diode' - placed (externally) in series w/the resistor.  (per the above post)   (Diode's anode tied to the Battery's (+) terminal.)    In such case - the values of the R-C Network may have to be 'tweaked.')  

    Optionally - if (even) a (further) reduced voltage drop (than that offered via the diode) is sought - a 'Small Signal P-FET' may present, 'Even lower impedance - yet maintain the SAFE, 'One Way ONLY' current flow path' - between Battery & MCU...

  • 0 in reply to cb1_mobile
    Prodigy 85 points

    Thanks for your very complete answer.

    I was asking if the pin has any kind of protection because I already have the circuit in production and any change is expensive.

  • 0 in reply to Fabian Toledo
    Guru 117855 points

    Thank you - note that our small firm, 'Consults for several 'giants' - our 'deep dive' is well sought (even appreciated) - thus serves as, 'SOP' (Standard Operating Procedure) - even though - especially though  (somewhat) rare here.

    Fabian Toledo said:
    I was asking if the pin has any kind of protection because I already have the circuit in production and any change is expensive.

    If we assume (some) volume production potential - and are 'alerted' by your introduction of, 'IEC 61010-1' -  might you consider:

    • EVEN w/the existence of, 'MCU's internal protection' - can that be positively, 'relied upon'  - especially should the MCU become 'distressed and/or failing?'
    • Indeed 'any change' IS expensive!    Yet - should not the 'IEC 61010-1 concern' have arrived  (at/around) 'Design Time?'    Such 'Changes - Late in the Game' are indeed unfortunate - and costly.    (yet such provides 'opportunity' for 'firms such as mine - willing & able to (regularly) 'deep dive!'    Firms' (potential/hoped/prayed for) 'SAVING' - by 'going it alone' - too often discover, 'Mistakes and/or other Inefficiencies' (very) late - which proves (as you noted) Very Costly, Production De-Railing & Morale Sapping!     Such 'hoped for Savings' - far too often - are (properly) noted as, 'Mirages.'

    With this said - there (may) be a, 'Reduced Cost Means' to, 'Enable you to 'MEET Regulatory Agency Standards' - which my group 'has' developed.   Are those Regulatory Agency Approvals your 'primary concern' - and thus far 'UNMET?'      That (may) be of key/critical importance - yet is presently unknown...

  • 0 in reply to Fabian Toledo
    TI__Guru 72500 points

    I think CB1 did an excellent job answering your question, but in case you want a "vender" to answer it, the TM4C123GH6PM is designed such that it does not source a current out of the Vbat pin even when Vdd is greater than Vbat. However, I cannot interpret IEC 61010-1 for you. If they mean protected by any failure in the microcontroller, then that would imply protection against a failure in the internal circuit that prevents the inverse current. If that is the intent of the requirement, it would preclude the internal circuits from meeting this requirement. You should discuss this with your company safety assessor.

  • 0 in reply to Bob Crosby
    Guru 117855 points

    Bob Crosby said:
    but in case you want a "vender" to answer it

    Such (was) an 'acceptable spelling' - many 'moons past!'     Perhaps note today's (far more 'normal/customary'):

    Deeper w/in your post:

    Bob Crosby said:
    I cannot interpret IEC 61010-1 for you.

    which is excellent - as it provides the appropriate legal 'safeguards' - and casts such responsibility (properly) upon the poster/his firm...

    Bob Crosby said:
    If they mean protected by any failure in the microcontroller

    It is good that, 'We both recognized & presented' this eventuality.     Regulatory Agencies (sometimes) employ 'unusual' logic - and 'Different Agencies' may  enforce 'Different Restrictions/Levels of Compliance!'    How 'Delightful' - yet 'Real SAFETY'  (not 'faked'  (i.e. competition inhibiting) ... does have its place...

  • 0 in reply to cb1_mobile
    TI__Guru 72500 points

    Ooops, I r an inguneer an donna spell so gud.

  • 0 in reply to Bob Crosby
    Guru 117855 points

    Nothing that (even a brief) sojourn - @ UCLA Law - would 'fail to fix.'     (May 'cost you your boat' - at today's prices... and 'what then' - would certain M-Chip 'buddies' do?)

  • 0 in reply to Bob Crosby
    Prodigy 85 points

    Thank you very much both Bob and CB1.

    This was an excellent discussion about safety

    When you say different logic employed by Regulatory Agencies you are very right and this is the case where I am in the middle of two different opinions aboutthe standard where the laboratory where we did our tests in development stages has accepted our circuit without further protection and another regulatory agency has not

    I hope you could send me a schematic with the internal circuit protection of the MCU in order to the testing laboratory could have all the necessary information to approve or not the circuit.

    Best regards and thanks again.

  • 0 in reply to Fabian Toledo
    TI__Guru 72500 points

    I am sorry. I double checked with my management, but I am not able to disclose the internal design of the device. 

  • 0 in reply to Fabian Toledo
    Guru 117855 points

    Mr. Toledo,

    Kindly note that, 'Just as vendor's Bob reported' - my staff too, 'Checked w/2 different 'giants' producing ARM Cortex MCUs.'     And their response was (nearly) identical to that noted by Bob!

    However - while I (both) 'feel & share' your pain/frustration - might the example of a, 'Chain being (ONLY) as strong as its, Weakest Link' - offer 'surprising guidance?'

    By your 'meeting (even exceeding) the highest standard' - you are (then) FREE to 'SELL World-Wide!'    Our firm 'coaches/advises firms' in regards to 'highly similar yet 'Strategically Differing' designs' - which enable the, 'Cost Efficient Means to, 'Meet  Differing, Regional Standards' wherever the product is being Sold...    And note too - our small firm is 'sufficiently PRO-ACTIVE' - that we are able to often, 'Predict & Provide a Design which uniquely, 'Accommodates (EVEN) UPDATED Regulatory Demands' - again SAVING you THOUSANDS...

    Perhaps warrants (some) consideration -  (this/next time) very 'early' - during your next design iteration...

    Staff & I thank you for your 'nice Award' - we do try to offer 'top cabin assistance' - above & beyond - the (usual) 'MCU & ONLY the MCU' (deliberate) Focus!