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AM1808 Power Supply Advisory 2.0.18

Inderjit Bains
Expert 2745 points
Other Parts Discussed in Thread: AM1808

In the AM1808 Silicon Errata (SPRZ313B), Advisory 2.0.18, the following statement is made:

"The 3.3V DVDD3318_x supplies must not be driven to 0V during zones A – E or high current capable of damaging the device may occur. The 3.3V DVDD3318_x supplies should not be driven during this time."

This conflicts with the AM1808 datasheet (SPRS653A), section 6.3.1:

"There is no specific required voltage ramp rate for any of the supplies as long as the LVCMOS supplies operated at 3.3V (DVDD3318_A, DVDD3318_B, or DVDD3318_C) never exceed the STATIC 1.8V supplies by more than 2 volts."

I don't see any way to guarantee that the 3.3V supply will be less than 2V above the 1.8V supply, other than to shunt it to ground when the 1.8V supply is below a certain value.  Of course, one could turn off the shunt when the 3.3V supply is below 2.0V, but that would be a more complex circuit.

Any recommendations?

  • 0
    TI__Guru* 84005 points

    Hi Inderjit

    We don't see the statements as conflict. The errata note is highlighting that you should not drive the 3.3V supply to ground. You can float it , you just can't tie it to ground , while other supplies are on.

    There is no connection of that statement with the note in the datasheet. The datasheet not is only about ramp rate and highlights that there are no specific requirements as long as voltage limitation is met.
    The simplest solution is still a zener or multiple diode stack as recommended by Tommy in some of your previous posts on this topic.

    If you have a circuit in mind and like to get to get it reviewed, feel free to share.

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    Regards

    Mukul

  • 0 in reply to Mukul Bhatnagar
    Expert 2745 points

    Floating won't work:  there are too many factors, such as load and capacitance on the power supplies, to predict which supply will decay faster.  There was no recommendation that worked in the previous topic - if you have one, can you provide it?  Zeners and diode strings have too many variables, such as forward voltage vs. current to provide a precise solution.  How would they be connected?  Once supply connected to the other through diodes would potentially make the leakage issue worse.

  • 0 in reply to Inderjit Bains
    TI__Guru 62975 points

    Float should be ok during power up because the 3.3V logic will provide a small load.

  • 0 in reply to tlee
    Expert 2745 points

    Power-up is not the issue.  The 3.3V supply can be enabled by the 1.8V supply reaching a certain voltage.  The problem is power-down:  if the 3.3V supply is allowed to float down and is lightly loaded, a heavy load on the 1.8V supply will allow the 3.3V supply to be more than 2V above the 1.8V supply.  The only way to keep the 3.3V supply from being more than 2V above the 1.8V supply on power-down is to shunt it through a low-value resistor to ground.  If this shunt cannot be active at power-up, it will greatly complicate the circuit.

    If there is a way to keep the 3.3V supply less than the 1.8V supply using just diodes, please post the circuit.  My concerns with a diode approach again are that the 3.3V supply could leak into the 1.8V supply, and that there will be no precision with this approach.  The margin is only 0.5V:  normally the 3.3V supply is 1.5V above the 1.8V supply.

  • 0 in reply to Inderjit Bains
    Expert 2745 points

    Is the condition that the 3.3V supply not be driven to 0V only applicable during power-up?  If it doesn't apply during power-down, then it's not as difficult a design problem.