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TPSM843B22E: Function and Use of GOSNS Pin

FarronDacus
Intellectual 760 points
Part Number: TPSM843B22E
Other Parts Discussed in Thread: TPS543B22

Tool/software:

Hello: 

The datasheet does not really explain the GOSNS pin, which apparently stands for "ground sense return."  The example on page 1 indicates GOSNS should just be grounded.  The example on page 21 shows it apparently grounded through a jumper or optional jumper that is referred to as a "Net Tie" (name not explained).  Figure 7-5 on page 15 indicates GOSNS drives the plus input of an amplifier named "RSA", while the FB drives the negative input.  It is unclear if RSA is:

1. An op amp with unshown programming resistors to turn it into either an inverting amp or a differential amp that can be used differentially, or by setting GOSNS to ground convert that differential amp into an inverting amp.

2. Some other kind of amplifier.

My question whether some additional explanation can be provided that explains:

1.  Was the grounding of GOSNS intended to be the main or only usage of the pin?

2.  Is the jumper from GOSNS to ground intended to preserve some other use for GOSNS?  If no other use is needed, should GOSNS just be grounded without bothering with the jumper?

3.  Is RSA an op amp or some other kind of amp?

Thanks,

Farron

  • 0
    TI__Mastermind 22180 points

    I see the Figure 7-5 the RSA noninverting and inverting nodes are swapped in drawing.    GOSNS should be negative and FB positive. 

    the GOSNS pin is the inverting input to remote sense amplifier (amplifier gain  of 1). 

    When differential sense is needed on feedback connect GOSNS to the load ground.  Think of the net tie as a 0 Ohm resistor but it is not a component only a trace that allows two nets to connect.  The net tie is a way to give an unique net name so a connection can be made in a specific location on pcb.     A net tie is useful for making kelvin connections or eg connecting a quiet analog ground to power ground at a specific location. 

    If GOSNS does not use a net tie or 0 ohm resistor and connects to ground in a cad tool, the cad tool will connect the GOSNS any place there is ground.  

    If single ended sense is needed, connect GOSNS to AGND at the device

    Connecting GOSNS using a kelvin sense line to load ground improves regulation accuracy by removing the voltage drop across the ground plane.  

  • 0 in reply to David_Daniels
    Intellectual 760 points

    Hi David:

    Thanks for the quick response with good information.

    However, there is one thing still a little puzzling in the response.  For single ended sense, connect GOSN to AGND at the device.  For "differential", connect GOSN to load ground.  Am I correct in thinking this is not differential in the classic sense, but actually a way to avoid ground noise and error with a ground connection that avoids other ground currents?  That does make the +/-0.3V spec on GOSN a sensible range. 

    Of course, using it that way does create a loop inductor that can pick up magnetic noise, so it makes me think that a direct connection to a low Z ground plane system may be superior.  Is there analysis that shows otherwise?

    Thanks,

    Farron

  • 0 in reply to FarronDacus
    TI__Mastermind 22180 points

     The GOSNS is a way to minimize the dc voltage error from the pcb drop from ground currents.   The GOSNS is connected to a specific point (net tie) at the ground plane at the output. 

    Here is an example pcb layout image of a GOSNS connection using the TPS543B22 IC pcb layout example.   GOSNS and vout feedback traces are the dashed lines in the image.   The feedback components are mounted near the device FB and GOSNS pins.    I know of no analysis regarding noise.   The feedback and gosns should be routed as a pair and avoid routing near inductor or switch nodes. 

     

  • 0 in reply to FarronDacus
    Intellectual 760 points

    Hello Again, David:

    OK, thanks. 

    Last questions:

    1.  Is RSA an op amp?

    2.  Are there gain programming resistors on RSA that are not shown in Fig 7-5?

    Best,

    Farron

  • 0 in reply to FarronDacus
    TI__Mastermind 22180 points

    yes, it is an opamp configured as a difference amplifer circuit with a 4 resistor that set the gain of 1.

  • 0 in reply to David_Daniels
    Intellectual 760 points

    OK David, thanks, that clears it up.

    ...Farron