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UCC29002: Common mode range of current amplifier in low side sensing applications

Andrew Fendrich
Prodigy 70 points
Part Number: UCC29002
Other Parts Discussed in Thread: UC2902, UC2907, UC3902

I'm designing a load sharing supply with the 29002.  I've read through many posts and looked at lots of app notes and diagrams.  What I don't understand is how the 29002 can operate with the ground referenced to the load through a sense line type of connection.  I'm operating with a low side current sense and if you were to connect the IC ground to the connection at the load, wouldn't this imply that the CS+ and CS- pins are then naturally biased below ground?  In the 29002 datasheet it states that the CS inputs have a common mode range that 'includes' ground, but in a remote sensed application those pins would be forced below ground.  I see in the datasheet that the pins have a minimum absolute rating to 0.3V below ground but this just seems to be a rating to prevent damage to the IC. 

In my prototype circuit each power module is referenced locally to the "-" side of the sense resistor.  The power modules when sharing a LS bus are referenced each to their individule ground.  I understand that offsets between grounds will lead to sharing errors, but I have had no success in referencing each ground circuit at a remote point at the load.  The loadshare bus no longer seems to respond to a changing load current when referenced remotely to the physical load.  I have come to the conclusion that both the cabling drops and the sense resistor drops place the CS pin voltages below the IC ground and can no longer properly sense the current sense voltages.

I have read through a powerpoint presentation where towards the final slides it is pointed out that the range of CS pins is -0.3V and in the next slide that the absolute limit is -0.3V.  

In my design I have 28V outputs carrying up to 40 amps each module.  The sense resistor is a low side 0.002 ohm resistor with kelvin connections.  I can provide more detail as to the connections that I'm trying to experiment with.

Just to reiterate the real question:

What is the common mode range of the CS pins?  

  • 0
    Prodigy 70 points

    This is the connection method I originally had envisioned with each module's UCC29002 IC connected to supply ground on the 'inside' of the current sense resistor.  My thought was that I needed to maintain a positive common mode voltage at the CS input pins of the IC.

  • 0 in reply to Andrew Fendrich
    TI__Genius 13660 points

    Hi Andrew,

    The attached picture is broken, could you upload it again?

    And I am out of office today, please allow me a delay reply.

    Thank you.
    Teng

  • 0 in reply to Teng Feng
    Prodigy 70 points

    I'll try again to insert the picture

  • 0 in reply to Andrew Fendrich
    Prodigy 70 points

    To further clarify what I'm asking about using the remote sense here is an image below of the remote sense connection.  I only show a single 'module'.  I believe I've seen in other posts or app notes that this scheme is intended.  I've attempted this type of setup with no meaningful results.  What concerns me is the fact that in my system I see easily 100-300mV of cable drop between the load and the sense resistor.  Would this violate the common mode range of the UCC29002?  I also have to wonder if in this type of setup would my output voltage adjustment range need to include the possible cabling drop?  

  • 0 in reply to Andrew Fendrich
    Prodigy 70 points

    I was hoping to get some feedback on this topic by this point.  I have been doing some more research scanning through more forum posts and I've seen various places where it is recommended to tie the ground remotely to the point of load.  I still have concerns that this potentially violates the common mode range of the IC, but since it seems that this is a comman way to connect and use the IC I will investigate further this connection method.

    Furthermore I have come across the UC2902/UC3902 controller which looks to be a bit more promising given the differential load share bus.  We have also used the UC2907 controller which has a larger pinout and has a voltage loop which I'm still trying to understand its purpose.  This also seems to be a controller which seems to be suited for remote sensing and seems intended to deal with ground level offsets.  

    Most posts and responses that I see which talk about these other IC's all point to the UCC29002 as the preferred controller due to its simplicity and I believe because it is a newer and more supported device.  I'm still hoping to use the UCC29002 due to its lower cost as well. 

    I am now leaning more towards using the UC2902/UC3902 family due to the differential share bus which seems to aid in paralleling modules which do NOT share a common ground.  This device seems to handle referencing the load share bus at the point of load. 

    I'm still waiting for any feedback on this topic.  Please advise and thank you

  • 0 in reply to Andrew Fendrich
    TI__Genius 13660 points

    Hi Andrew,

    Current sense can works for both low and high output voltage applications, normally low side is a better choice.
    The current can be expressed by below formula, usually the errors are small enough to ignore, the gain AM and As are fairly high and the imbalance is dominated by difference in Rcs rather than Rext.

    yes the Vcs+ and Vcs- minimal voltage is -0.3V per the datasheet, too large voltage drop in current sensing (Rcs) and external PCB resistances (Rext) can cause the pins exceeding its range.
    But the cable voltage drop of 100-300mV looks very large, theoretically the Rext can be designed very small and in ignored.
    I suggest to optimize the layout and reduce the PCB resistances(Rext).

    Thank you.
    Teng

    •the errors are small enough to ignore

  • 0 in reply to Teng Feng
    Prodigy 70 points

    Thank you for the response.  The Rext that I am implying are cable drops associated with the connection between my power supply unit and the load.  This drop is indeed more than would realistically be expected and can be reduced.  

    I just need a little bit further clarification on the operation.  By placing the gnd connection 'ahead of' the current sense resistor inlcluding any external drops seems to place the CS+ and CS- pins inherently below gnd.  Will the CS opamp operate properly biased slightly below ground?  I can only assume from everything that I've read that the circuit will operate below ground, and correctly amplify the CS signal developed across the sense resistor.  Assuming that this external drop is less than the 0.3V limit of the IC, does the performance degrade as it gets closer and closer to this -0.3V absolute limit?

    I have marked this post as resolving my issue as my original question revolved around the limits of the CS pin voltages.  However, what I'm really after is whether the current sense amplifier (CS+ and CS-) will properly reflect and 'measure' the current sense voltage if these pins are biased below ground, but less than the limit of -0.3V.