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TMS320F28386D: TMS320F28386D: Overdriving DSP with +5V RS485 Transceivers SN65LBC170DB

Marcin Brylak1
Prodigy 50 points
Part Number: TMS320F28386D

The specs of DSP are not exactly clear; it may be interpreted that the input clamping (protection) circuits on the digital inputs, for the positive input voltage, are actually activated at 4.6 V.

At that voltage each input is able to take 20 mA momentary (and total for all inputs), and 2 mA continuous. Together with the series resistors that you have -- this may explain why it works without blowing up.

 

On the other hand, a more traditional understanding is that the protection diodes will kick in at VDDIO+0.3V , especially considering the note (2) in the spec below.

 

Second issue:

If I use SN65LBC170DB with +5V supply can I go with 22 Ohm in series on RX and 15 Ohm in series on DSP side without any issue?

Can be the lifetime reduced?

  • 0
    Prodigy 50 points

    More details: Use case 2 and use case 3:

    USE_CASE.pdf

  • 0 in reply to Marcin Brylak1
    TI__Guru 60865 points

    Hi Marcin,

    The easiest way to design is to assume that the clamping diodes switch fully on at VDDIO + 0.3V or VSS - 0.3V,  and then control for clamping current using series resistance. 

    For instance, if you could have 5.0V applied with VDDIO = 3.3V, then you'd need (5V - (3.3V + 0.3V)) / 2mA = 1.4V / 2mA = 700 ohms series resistance to reduce the clamping current to 2mA.

    I'm honestly not sure how useful the 4.6V specification is for the IOs since I think we have clamping diodes on all IOs and analog pins: if you can get the actual pin to this voltage you'll already be violating the clamping current specification. (It is a useful specification for the supply rails)

    I'm not sure I follow your second question.  Is this about the lifetime of the SN65LBC170DB or the F2838x?

       

        

  • 0 in reply to Devin Cottier
    Prodigy 50 points

    Hi Devin,

    In 2nd question I am asking about lifetime of F2838x which will be permanently overdriven by +5V supplied RS-485 lines ( from SN65LBC170DB)? With 22 Ohm on RS-485 and 15 Ohm on DSP  we reduce this voltage but DSP is 3V3 and we reduce it to around ~ 4.4V. Do you recommend some redesign?

  • 0 in reply to Marcin Brylak1
    TI__Guru 60865 points

    Hi Marcin,

    Per my previous post, I'm not sure that 22 or 15 ohms is enough to meet the clamping current absolute maximum specification? 

    Either way, the clamping current specification is an absolute maximum, so meeting this only ensures that the device isn't permanently damaged.  Functionally correct operation while exceeding the normal operating conditions is not implied or expected, so you definitely don't want an overvoltage condition to be continuously present while the application is operating.  The datasheet actually cautions about this specifically: