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AM6442: AM6442BSDGHAALV PDN Targets.

AMARNATH G
Prodigy 245 points
Part Number: AM6442

Tool/software:

Dear Team,

We are using AM6442BSDGHAALV in one of our design and we are calculating the ZTarget for VDD_CORE using the below formula

Ztarget = (Voltage Rail x %Ripple) / 0.5 x Imax

                   Voltage Rail = 0.75V

                   %Ripple = 1.6%

                   Imax = 2A

        %Ripple = 1.6% had arrived based on the below methodology.

1) We are using the PMIC: TPS6522053RHBR whose BUCK-1 Minimum output voltage is 0.73875V (0.75V Typical). Considering the ripple of -1.5% as the worst-case scenario.

2) We are using the Processor: AM6442BSDGHAALV whose Minimum Input Voltage of VDD_CORE (0.75V Typical) is 0.715V. whose minimum ripple is calculated to be -4.66667%.

3) Now ripple margin is calculated to be (-4.66667) - (-1.5) = -3.16667%

4) Furthermore -3.16667 is divided by 2 to have a stringent Ztarget for VDD_CORE i,e.: (-3.16667 / 2) = 1.58333% rounding off as 1.6%.

Now while substituting the above values to the Ztarget formula, we get:

                 Ztarget = (0.75V x 1.6%) / (0.5 x 2A) = 12mΩ.

But when we refer the TI document "SPRAC76G – NOVEMBER 2022 – REVISED FEBRUARY 2024" (SitaraTm Processor Power Distribution Networks: Implementation and Analysis) in Table 7-7 we see the different Ztarget values, below is the snapshot of the same for your reference.

Can you please explain us how did TI arrive to the different Ztarget values as given below?

                       10mΩ for ≤1MHz

                       34mΩ for 1-20 MHz 

                       35mΩ for 20-50MHz

For arriving to the above values, was the input voltage to VDD_CORE considered as an Ideal Voltage as 0.75V?

Thank you.
Regards

Amarnath G

  • 0
    TI__Guru**** 189879 points

    Hello Amarnath, 

    Thank you for the query.

     PDN targets are a result of the PDN simulations / worst case load transient analysis

    Let me check and update.

    Regards,

    Sreenivasa

  • 0 in reply to Kallikuppa Sreenivasa
    TI__Guru**** 189879 points

    Hello Amarnath, 

    Please refer below inputs i received from the expert:

    For AM64x we did not use the load transient approach for calculating target -Z. We extracted the impedance plot for our EVM (which was found to pass characterization) and provided the AC impedance as targets for the different frequency bands.

     

    Also note that a target of 1.6% will be excessively stringent for all frequency bands. Typically we use:

     

    3% for < 1MHZ

    5% for 1 – 20 MHz

    10% for 20-50 MHz

     

    That is because the effectiveness of board decap will be much diminished for higher frequency transients due to the additional pkg/brd inductance seen by the decaps. The higher freq transients will depend on effectiveness of on-die decap to be adequately suppressed.

    Regards,

    Sreenivasa