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Original question:

TMS320F28035: Input clamp current

TMS320F28035: Internal clamping diode threshold for ADC pins?

Wayne Huang
Genius 17125 points

Part Number: TMS320F28035

Dear Champs,

I am asking this for our customer.

In order to avoid that the ADC input current is too large, the user wants to design with external clamping diodes. They hope the current are absorbed first by the external clamping diodes and then by the internal clamping diodes (specified in Sec 5.1).

In this way, they should design that the threshold of the external clamping diodes are larger than that of internal clamping diodes.

They wonder if we specify the threshold of the internal clamping diodes, say 0.7V?, so that they can choose proper external clamping diodes.

Wayne

  • 0
    TI__Mastermind 42930 points

    Hi Wayne,

    I'm checking this with our design team if they have the threshold level for the internal clamping diodes.  I'll get back with you soon as i hear back from them.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Mastermind 42930 points

    Hi Wayne,

    Sorry for dropping this.  Working with design to get a better understanding on the clamp requirements.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Guru 60865 points

    Hi Wayne,

    Actually the easiest thing to do is add some series resistance between the external clamping diodes and the ADC input.  

    If you add say 300 ohms and 1mA is flowing into the internal diodes then the voltage delta between internal and external diodes will be 300*.001A = 300mV.  Since diodes have exponential IV characteristic, the external diodes will be taking orders of magnitude more current.  In this way, almost any normal silicon external diodes should be sufficient (the internal ESD diodes are also silicon diodes).   

  • 0 in reply to Devin Cottier
    TI__Genius 17125 points

    Dear Joseph

    The main problem is that the external diode Vf is bigger than internal diode Vf¸ so we are afraid the ESD voltage would flow into internal diode first and damage it!

    Based on this, we need to choose the lower Vf diode to be external diode and protect the MCU.

    So what is the Vf value of internal diode?

    Wayne Huang

  • 0 in reply to Wayne Huang
    TI__Guru 60865 points

    Hi Wayne,

    The diode is a normal silicon diode with forward voltage near 0.7V under nominal conditions.  

    The design process would normally be something along the lines of:

    • Select a max current for your internal diode (e.g. 1mA)
    • Select a series resistor such that you get > 0.3V IR drop under the max current condition
      • e.g. 1mA => 300ohms
      • 300mV is more than enough to account for possible Vf differences among plausible silicon diodes you might use for your external clamping circuit
    • Select a series resistor to limit current into the external diode based on the max expected voltage
      • e.g. VDDA min = 3.0V, Vin max = 5.0V, Imax = 50mA then R should be (5V - 3.0V - 0.7V)/0.050 = 26 ohms
      • Note, you should do this for both + and - input extremes and take the larger resistor   

    Once you do this, you can simulate in TINA with a variety of different silicon diode models to convince yourself that the majority of the current will be steered to the external diode regardless of the specific diode parameters.

    If you select a low Vf diode for the external clamp diode(s) you can reduce the 300mV margin to something lower, thus reducing the series R between external and internal diodes, but this isn't usually necessary.