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PCM1789: Required clarification on Thermal resistance data

Balaji K R
Prodigy 10 points
Part Number: PCM1789

Tool/software: NA

It has given two different Rthja - thermal resistance value in datasheet 87.8degC and 115degC for TSSOP-24pin package. Please clarify the difference between two value.

And for IC power dissipation calculation, considered (VCC: 5*0.028+ VDD: 3.3*0.03+ Digital control inputs: 3.3*(0.0001*4+0.00001*6)). Is this correct?

  • 0
    TI__Expert 3580 points

    Hi,

    I'm looking into this for you and will get back to you tomorrow. Sorry about the wait.

    Best,

    Mir

  • 0 in reply to Mir Jeffres
    Prodigy 10 points

    Hi Mir, Awaiting for your support

  • 0 in reply to Balaji K R
    TI__Expert 3580 points

    Hi,

    For your thermal resistance value, I see the mismatch too. There is a chance it is the package rating vs the device rating, or maybe the measurement was taken on an EVM vs a testing board. We should assume the worse case, so assume that 115C/W is the closer value for any layout you might try. 

    Also, you are mostly correct about power dissipation. You should be dissipating all the power that is going into the device, which will be Vcc * Icc + Vdd * Idd. This value comes out to 239mW max for fs=48kHz, as described in the datasheet in section 6.7 and verified with a calculator. The addition of "digital control inputs" is not usually necessary as these are orders of magnitude smaller than the power in through Vdd/Vcc, and are heavily time dependent as the clocks are not always on, for example. 

    Sorry about the wait. Let me know if you need more help with this.

    Best,
    Mir