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TMS320F28377D: Power Sequencing Requirements Question

Peter Conklin
Prodigy 50 points

Part Number: TMS320F28377D

For the TMS320f28377D microcontroller, the datsheet clearly states the power sequencing requirements per section 4.7.1. In one of our designs we have found a condition where a few analog voltage inputs will be out of sequence per the requirements stated in section 4.7.1. during a power down sequence. Some of the analog input voltages will exceed the 0.3V difference requirement from the VDDA supply voltage.

To resolve this issue we plan to add series current limniting resistors to each of the analog inputs to assure the Input clamp current per table 4.7, +/-20mA will not be exceeded during the power down sequence.

Does TI agree that this is an acceptable approach to eliminate this issue during a power down sequence? We have confirmed that this is not an issue during a power up sequence, only a power down sequence. The intent is to use the series current limiting resistors at each of the analog inputs of concern.

  • 0
    TI__Expert 7320 points

    Hi,

    I will take a look and reply in timely manner.

  • 0
    Prodigy 50 points

    Thanks Steven. One thing to note, our design does utilize a TPS3820-33DBVR supervisor and will drive the XRS pin low once the +3.3V supply is out of regulation. This alone may cover the power down case. However, for a belt and suspenders condition, we may still want to add series current limiting resistors. Maybe TI can share some thoughts on adding the resistors?

  • 0 in reply to Peter Conklin
    TI__Expert 7320 points

    Hi Peter,

    Thank you for sharing more information. I will keep you posted.

  • +1 in reply to Stevan Duraskovic
    TI__Expert 7320 points

    Peter, 

    I could not find the outlined sections and tables. Are you sure you put correct table numbers: https://www.ti.com/lit/ds/symlink/tms320f28377d.pdf

    When it comes to using current limit resistor, it will be helpful in your case. Since your primary concern is staying within the ±20mA input clamp current limit, the resistors will directly limit the current flowing through the internal ESD/clamping diodes.

    When selecting the resistor value, ensure you calculate it based on the worst-case scenario. Rmin = (Vinmax - Vddamin) / I_clamp(max). Additionally, consider the impact of these resistors on your signal integrity, specifically regarding impedance/sampling time and noise injection (thermal noise), especially for high-speed or high-precision ADC conversions.