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XTR111 MOSFET and BOM selection (low cost)

George Photopoulos
Prodigy 120 points
Other Parts Discussed in Thread: XTR111

Hi,

I am designing a PCB based on XTR111 for the DIY universe.
From a previous post I found that the suggested specs for a MOSFET are:

Vgs         >=  +/-20V
Vds         >=  -60V
Id             >=  1A
C-Gate   <=  500pF

My question is:

Can I use a lower Id Mosfet?
What is the purpose of the 1A criteria? meaning, what problems may occur by using a -300mA MOSFET (or even lower)?

I understand that the C-Gate is about response speed.

Is there anything else that I should have in mind choosing a low cost BOM? (ie Rset is going to be a 0.1% with 50 ppm TC)
Is it a good practice to use a trim POT as an Rset?

  • 0
    TI__Guru 64215 points
    Hi George,

    Good questions. We typically recommend using larger PMOS devices because they do a better job of dissipating power and therefore don't get as hot during operation. With a +36V supply and a 0Ohm load the PMOS needs to dissipate roughly 0.72W with a 20mA output. Smaller devices will get much hotter and will heat up the nearby XTR111 and RSET resistors causing thermal drift issues. Since the heat is based on the output current the system will suffer from greater drift issues at 20mA than at 4mA.

    The C-gate value is based on response speed and stability of the OPA inside the XTR111.

    RSET is the only component which requires precision in the design. The gain error and gain error drift will largely be set by the specifications of RSET. The VREG resistors don't have to be as low tolerance or TC as long as the circuitry powered by the VREG voltage has good power-supply rejection (PSRR).

    Trim-pots provide a useful way to determine a proper value for RSET in the design but low-cost trim pots tend to suffer from drift, noise, and resistance value stability issues.
  • 0 in reply to Collin Wells
    Prodigy 120 points

    Hi Collins.

    Thank you for your analytical answer.
    I have one two last questions.
    If I use an array of resistors with a dip-switch for value selection, or a digital pot is it more safe?
    I presume the dip-switch will also add some noise but not that much as the trim-pot.

    Can I add a Cap to SET pin to filter some of the noise?

    Best regards,
    George Photopoulos

  • 0 in reply to George Photopoulos
    TI__Guru 64215 points
    Hi George,

    The array of resistors with DIP switch is probably the better option. Digital pots tend to have large stray capacitances that may negatively interact with the circuit.

    That leads into your next question, I don't recommend adding a capacitor to that node. That will cause a delay in the feedback path which may cause stability issues.
  • 0 in reply to Collin Wells
    Prodigy 120 points

    Hi Collin,


    Thank you very much.

    Best Regards,

    George Photopoulos