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THS4541: Impedance between FB pin and output pin

haruki okumura
Prodigy 60 points
Part Number: THS4541

Tool/software:

I created a circuit with an input resistor of 200Ω and a FB resistor of 200Ω connected, as shown in the attached diagram. The gain setting is 1x.
However, when I actually measured it, the gain was 1.04x.
The FB pin and output pin should be at the same potential but this is not the case and the voltage of the output pin is 1.04 times that of the FB pin.

When I tried shorting the FB pin and output pin, the gain was 1x as set.

As shown in the attached diagram, it appears that 8Ω is connected to the FB pin and output pin.

This problem was confirmed in multiple circuits.
Is this a part specification or a defective lot?

  • 0
    TI__Mastermind 24955 points

    Hello Okumura,

       We talked to the design engineer, and you are correct in your findings. The impedance value between FB and OUT pin is expected to be within the single-digit range. If this is a concern, you may account for this in your gain/feedback resistors or you can use a different package such as the RUN package, or you can leave the FB pin floating, and connect to OUT pin directly from your feedback resistor.

    Thank you,
    Sima

  • 0 in reply to SimaJalaleddine
    Prodigy 60 points

    Thank you for your reply.

    That means it's not a defective product, but a part specification?

    The data sheet for the THS4541 does not state the impedance between the FB pin and the output pin. (It may be an oversight on my part. If there is any information in the data sheet, please let me know.)

    How many ohms is the impedance between the FB pin and the output pin according to the specifications?
    Also, please tell me about the manufacturing variation of the impedance between the FB pin and the output pin.

  • 0 in reply to haruki okumura
    TI__Mastermind 24955 points

    Hello Okumura, 

      That is correct, it is a part specification that is expected to be within a single digit range, and it is not in the datasheet of the THS4541. I checked our internal files, and this specification was not characterized at the release of the device. We usually spec within 20-30% for max and min to account for manufacturing variation. 

    Thank you,
    Sima 

  • 0 in reply to SimaJalaleddine
    Prodigy 60 points

    Thank you.

    I think I'll continue using the RGT package.

    I plan to use one of the following methods to avoid gain deviations..

    ① Leave the FB pin floating and connect the FB resistor directly to the OUT pin.

    ② Connect the FB resistor to the FB pin and short out the FB and OUT pins.

    Will the performance described in the datasheet be guaranteed with configurations ① and ②? (For example, frequency characteristics, etc.)

  • 0 in reply to haruki okumura
    TI__Mastermind 24955 points

    Hello Okumura,

       You may use either configure 1 or 2 to avoid gain deviations. I confirmed with the team manager that 8 Ohms is the expected value of the internal connection between FB and OUT pin, and there is 20% for manufacturing variation.  

      You can expect performance to match datasheet specs, we characterized the device using internal FB to OUT pin, but we do not expect a change in performance if you use 1 or 2. We would recommend making the trace between IN and OUT pins as short as possible. 

    Thank you,

    Sima