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OPA593:About Tj max calculation formula and current limit accuracy

Masato Odaka
Prodigy 70 points
Part Number: OPA593
Other Parts Discussed in Thread: OPA541

Hello!

Tj max calculation formula
It is about the formula described in the section 8.4.1.1 Thermal Considerations of the specification.
I would like to confirm the parameters used to calculate the PDL.
I think that it is substituted into the formula of P=VI=V^2/R
Where did the parameter "4" in PDL=22.5^2V / (4 * 600Ω || 11.47kΩ) come from?

"22.5V" is V+(45V)/2 because the sentence says "the PDL is at a maximum when the output is fixed at a voltage equal to 1/2 of either supply voltage"
"11.47kΩ" is R1(1.47kΩ) + R2(10kΩ)
"600Ω" is RL
I just didn't understand the "4".

Also, in the circuit in Fig. 8, could you tell me how to think about power under the following conditions?
Power supply: ±15V
Gain: 2 times (R1=R2=10kΩ)
Input: AC3Vrms@3kHz
Output: AC6Vrms

  • 0
    TI__Guru 51511 points

    Hi Masato-san,

    Masato Odaka said:
    "22.5V" is V+(45V)/2 because the sentence says "the PDL is at a maximum when the output is fixed at a voltage equal to 1/2 of either supply voltage"

    For DC power dissipation, the maximum power dissipation of the amplifier is occurred at Vcc/2 point for DC. If Vout*Vout/R power dissipation formula is used, then (Vcc/2)^2 is where 4 comes from, where Vout = Vcc/2, see the math derivation and graphs below. 

    Since the OPA593's majority of the heat dissipation is conducted through the thermal pad via the PCB, 17.7C/W Psi thermal resistance figure may be used for power amplifier's Tj calculation. 

    Masato Odaka said:
    Also, in the circuit in Fig. 8, could you tell me how to think about power under the following conditions?

    We have other ways to calculate the power dissipation, but the simulation below may be easier to understand than others. 

    The Iq is simulated in the PSpice model, so the following power should include the quiescent power dissipation in DC. 

    The total power dissipation PD = Icc*(Vdelta+) + Iee*(Vdelta-) as shown in the simulation. This is average power, which the average power dissipation can be extracted from Tina tool as shown below, once the plot is presented. The total average power dissipation for the condition is approx. 820mW.

    OPA593 Power Dissipation 06272023.TSC

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 70 points

    Hi Raymond-san,
    thank you for your reply.

    I understand why the parameter "4".
    Thanks for your thoughts on the AC input.
    Based on the information we received, we will build a model with Psice and examine it.

  • 0 in reply to Masato Odaka
    TI__Guru 51511 points

    Hi Masato-san, 

    Great to hear that you are interested in OPA593. 

    I am going to close this inquiry for now. If you have additional questions, please let me know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 70 points

    hello raymond-san
    I ran a simulation with Spice and a question was born, so I will post it again.
    I understand that the total power dissipation PD = Icc*(Vdelta+) + Iee*(Vdelta-).


    But why are Icc and Iee the same current waveform?
    If the power supply condition is balanced, Iout will change the source-sink at Vout=0V, and I expected that Icc and Iee will be like full-wave rectified waveforms.

    Best regards

    odaka

  • +1 in reply to Masato Odaka
    TI__Guru 51511 points

    Hi Masato-san,

    Masato Odaka said:
    But why are Icc and Iee the same current waveform?

    You are correct. OPA593's Icc and Iee waveforms do not look correct.

    I used OPA541 to plot similar current waveforms, the correct current waveforms look like the following. 

    OPA541 Current 07042023.TSC

    I downloaded the existing OPA593 PSpice model, and discovered that the current waveforms do not behave correctly.  I will ask our modeling engineer to check on it. The image below captured the waveform comparisons.

    When Vout is operating above GND, Icc should follow the Vout, and source the current. When Vout is operating below GND, Iee should sink the current (Iee current arrow defined the sink current, hence is positive). Two current waveforms in OPA593 should be 180 degrees apart, waveform directions do not look correct either.  

    OPA593 Current Waveform Issues 07042023.TSC

    If you have other questions, please let me know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 70 points

    hello raymond-san

    Sorry for the late reply.
    Thank you for your investigation of the Spice model.
    I would appreciate it if you could contact me after completion.

    Thank you for answering my questions so far.

    Best regards
    Masato

  • 0 in reply to Masato Odaka
    TI__Guru 51511 points

    Hi Masato-san, 

    Yes, I will let you know when the issues are fixed. 

    In terms of the average power dissipation, the number should be still pretty close or identical. We can revisit the simulation when the PSpice model is corrected. 

    Best,

    Raymond