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OPA818: Must OPA818 thermal pad be connected to GND?

Kazuya Nakai59
Guru 20550 points
Part Number: OPA818
Other Parts Discussed in Thread: OPA657,

Tool/software:

Hello guys,

One of my customers uses OPA657 for their current products.

Also they are considering replacing this OPA657 with OPA818 for their next products.
At this moment, they have the following questions.
Could you please give me your reply?

Q1.
OPA818 has thermal pad.
Must OPA818 thermal pad be connected to GND? 
In other words, is the thermal pad GND connection mandatory?

Q2.
OPA818 has FB pin.
Is this FB pin connected to VOUT directly in the device?
If the answer is yes, can FB pin be left open when the feedback resistor is connected to OUT pin?

Your reply would be much appreciated.

Best regards,
Kazuya. 

  • 0
    TI__Genius 9290 points

    Hi Kazuya,

    The thermal pad is electronically isolated from the die, but we typically recommend grounding the pad as your ground plane is usually the best heat spreading plane in most designs. However, any good heat spreading plane can be connected to the thermal pad. Layout guidelines are highlighted in section 11 of the datasheet. The FB pin is connected internally to the Vout pin. I did not understand your second question. Are you asking if the feedback resistor can be used and routed around the device and not use the FB pin? If so, I don't see why that can't be done, however performance will degrade as you will be adding parasitic capacitance to the design that the FB design is meant to reduce. Figure 58 in the datasheet gives an example of how the FB pin should be used for most designs.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Guru 20550 points

    Hi Ignacio,

    Thank you very much for your reply.

    Could i ask you one additional question.
    OPA818 datasheet say about Thermal pad that Electrically isolated from the die substrate but ESD diodes down-bonded to the thermal pad.

    Does this mean that ESD performance would be degraded when thermal pad is not connected?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Genius 9290 points

    Hi Kazuya,

    Let me get some information on this and get back to you shortly. 

    Best Regards,

    Ignacio

  • 0 in reply to Kazuya Nakai59
    TI__Genius 9290 points

    Hi Kazuya,

    I was able to get some clarity on your question and was assured ESD performance will not be degraded by leaving it floating or not connecting it to a specific voltage. Although it is strongly recommended to use the thermal pad as this will help keep the device in the optimal operating temperature.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Guru 20550 points

    Hi Ignacio,

    Thank you very much for your strong supports.

    Could I ask you one additional question?

    The customer has PCB size limitation problem.
    So they would like to know whether FB pin can be used as VOUT because the FB pin is connected internally to the OUT pin.
    I think it is not recommended because some voltage error would be generated by current flowing to the internal connection between FB pin and OUT pin.

    Is my thought correct?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    Guru 265070 points

    Section 8.3.2 of the datasheet says:

    Internally the FB pin is connected to VOUT through metal routing on the silicon. Due to the fixed metal sizing of this connection, FB pin has limited current carrying capability.

    And the connection between FB and VOUT is specified to have a resistance of about 1.2 Ω. You can use FB as output if the output current is small and you can live with the voltage drop over this resistance.

  • 0 in reply to Kazuya Nakai59
    TI__Genius 9290 points

    Hi Kazuya,

    Clemens highlights a major concern with trying to use FB as Vout. The pin was likely not characterized as a potential Vout pin so it is hard to say how well it can drive a load out of this pin but for these reasons it would probably not be advised to do this in their application.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Guru 20550 points

    Hi Ignacio and Clemens,

    Thank you very much for your supports.

    I understood well what you said. It is very helpful for the customer and me.

    Thank you again and best regards,
    Kazuya.