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LM3477: Inverting Buck Boost Current Sense Positioning

Dario Antonio Quintero Dominguez
Prodigy 20 points
Part Number: LM3477
Hi,
In an LM3477 Inverting Buck Boost configuration where the IC ground is negative voltage referenced and sinking current, what is the correct current sense configuration? In the buck configuration, the sense resistor is between VIN and the source of the nmos, and the designed voltage drop across the sense resistor is inputted into the "current limiting internal circuitry" as seen below. 
 
Looks like the internal "short circuit detect" and "PWM" comparators are both referenced to a proportional gain-VIN hence a nominally higher voltage than the ISEN pin after the sense resistor voltage drop. There are some IC-ground-referenced current sources  which would be negative-voltage referenced in an IBB. Hence, as current would still be sunk into the lower voltage node, system ground, and the fact that the IC is sinking current is independent of the positioning of the current sense resistor in this configuration as the current control circuitry operates assuming still a current drain from its perspective? In summary, nothing changes in terms of the current sense resistor topology in the LM3477 IBB.
  • 0
    TI__Expert 5310 points

    Hello,

    The current sensing function of the LM3477 will read the voltage drop between the VIN and ISEN pin across a sense resistor when the controller turns on the N-Channel MOSFET. So long as the GND pin is referenced to the same system ground that the input (Vin) is referenced to, the device will sense current as it normally would by sinking current from a higher voltage node to a lower voltage node through the high-side MOSFET. As you mentioned, this would not change anything in terms of the current sense resistor topology.

    Regards,

    Alec Biesterfeld

  • 0 in reply to Alec Biesterfeld
    Prodigy 20 points

    Hi Alec, thanks for the thoughts. In an inverting buck-boost configuration the GND pin is tied to the the negative output rail. The VIN ground would be different to the GND IC system ground. In this case the controlled-current sources in the ICs would be referenced to this negative rail output and I slightly wonder whether the current sense measurement accuracy would be functionally different?

  • +1 in reply to Dario Antonio Quintero Dominguez
    TI__Expert 5310 points

    Hi Dario,

    I have confirmed that the LM3477 will work in an inverting buck-boost topology. In the case of the LM3477, the current will flow through a sense resistor from a higher potential to a lower potential, and the ground reference to the negative output rail will not affect the current sense circuitry. This current sense ramp will create a  positive voltage internally that is compared against an error voltage generated by the error amplifier (plus some slope compensation). This will generate a voltage that the comparator will be able to recognize and supply the logic to the internal gate-drive circuitry.

    Regards,

    Alec Biesterfeld

  • 0 in reply to Alec Biesterfeld
    Prodigy 20 points

    Great thanks! I was slightly concerned about any change in accuracy of the internal circuitry mainly as well given the reference of the error voltages and internal references as well if you have any details. However, thanks for the confirmation on it functionally working well