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TPS61222: Two-ground symbol in schematic?

Paul Singh
Expert 2750 points
Other Parts Discussed in Thread: TPS61222

Hi,

In TPS61222 DS SLVS776, I see in all three circuit connection diagrams that GND pin is shown connected to what appears as the junction of two grounds, as circled in the following screenshot:

What does the symbol of two grounds mean? Does it refer to "Star Ground Point" or "Multipoint Ground"? I however have checked TPS61220EVM Guide SLVU253, as well as Figure 26. "PCB Layout Suggestion for Adjustable Output Voltage Options" of the datasheet, but they both show only one common ground.

Could someone explain on the two-ground symbol in the datasheet?

 

Paul

  • 0
    TI__Guru*** 142930 points

    In power supplies and most analog circuits, there is a ground for sensitive circuits and a ground for noisy circuits.  In a power supply, the noisy circuits are those that carry the switching currents.  The ground for sensitive circuits is the IC's control GND pin (not present on the TPS6122x), FB resistor ground, SS cap GND, etc.  These should be routed as two separate grounds on the PCB and tied together at one place near the IC.  The D/S figures show this graphically with the different ground symbols joined at the star ground point.

  • 0 in reply to Chris Glaser
    Expert 2750 points

    Chris,

    Thanks for the answer. We need some further confirmations:

    1. Is the "Star ground point" the area marked by the red rectangle in the middle?

    2. Does the left brownish rectangle correctly mark the "noisy ground", because it is connected with the inductor (switching circuit)?

    3. Does the right yellow rectangle correctly mark the "sensitive ground", because it is connected with the FB sensing resistors?

    4. TPS61222's topology looks very different from other converters in that the Inductor Switching Circuit is not connected to Vout at the load, but is at Vin. Could you briefly explain this topolgy?

    5. Does "SS" in "SS cap GND" refer to "Soft Start"?

    6. If we compare the datasheet SLVS776 Fig.26 layout with TPS61222 EVM SLVU253 schematic, we found that R1/R2 designators is incorrected marked in the datasheet layout and is also in conflict with the schematic in datasheet page 1.

     

     

    Paul

  • 0 in reply to Paul Singh
    TI__Guru*** 142930 points

    1.  The single point ground is the GND pin of the device.

    2.  Noisy ground is everything except for the trace from R2 to the GND pin.  This is the quiet ground.

    3.  See #2.

    4.  The TPS61xxx devices are boost converters.  TPS62xxx devices are buck converters where the inductor connects to Vout.  This is a fundamental difference between the topologies.  As a student, it is your job to learn about these fundamental and basic differences.

    5. Yes SS means soft start.

    6.  Correct.  The D/S figure labels those resistors backwards.

  • 0 in reply to Chris Glaser
    Expert 2750 points

    Chris,

    I want to go another level down to the very essential of "Star Ground Point":

    Is it just a simpler and cheaper of the more full-fledged "isolated ground"?

    By "isolated ground" I am referring to the use of an inductor to connect different grounds, typically an analog and a digital, and the inductor effectively filters the noise from the digital ground.

    If the PCB board area does not allow the use of an inductor, or an inductor is considered an extra cost, then if the noise is not severe we might REDUCE the more perfect inductor isolation to a thin "copper neck" which connects analog and digital grounds. If this "neck" is not too wide, its resistance will be relatively large, and this resistance could help to achieve some isolation between the two grounds because it limits the current.

    If there are more than one different ground connected by "neck"s, then a single "neck" becomes a "star", this is what is we now conventionally termed "STAR GROUND POINT".

    So basically:

    inductor isolation -> (simpler and cheaper, reduced)-> thin trace "neck", use its larger resistance -> (several "necks") = STAR GROUND POINT

     

    Does this make sense?

     

    Paul

  • 0 in reply to Paul Singh
    TI__Guru*** 142930 points

    Isolated grounds typically refers to galvanically isolated grounds, as done on each side of a transformer for example.  This is typically used for safety reasons.

    Star grounds refers to the method of connecting the various grounds (analog, power, digital, etc.) at a common point on the PCB.

    For a standard non-isolated power supply, all the grounds need to be connected directly together somewhere.  The place to do this is the star ground point.  It is important to follow the recommendations in each device's datasheet for connecting the grounds.

    I've never heard of inductors being used between grounds on a PCB.  Ferrite beads (which are not inductors, though they sometimes are drawn with the same symbol) might be needed on some analog circuits.  They are not recommended for power supplies.  Just connect the grounds together at one point.