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Original question:

LMG5200: Max peak current dependent of burst time

LMG5200: Current measured on PGND higher than on SW

Franz Schellhase
Expert 3605 points
Part Number: LMG5200

Hi team,

A customer wants to measure LED current switched by LMG5200. For that they tried using a shunt resistor at PGND. Unfortunately they see higher current on PGND than on SW. For control they need to measure the actual current going to the LED from SW. Measuring on SW is not an option because of time constraints.

Where is the additional current comming from? Is there a pattern to calculate the offset in relation to SW current?

Thank you,

Franz

  • 0
    TI__Genius 16390 points

    Hello Franz,

    Thanks for contacting us. If you are comparing the current of the PGND and inductor current during ON time, then it's possible that the average PGND current measured is higher than the average SW current (assuming you are measuring the inductor current). PGND measurement will see overshoot of current due to capacitor of the PCB board as well as the Coss of the top side FET, while the inductor current won't see such current overshoot.

    If you are comparing the current during the whole switching period, then the PGND current will be much lower than the inductor current since the PGND current will be zero during off time.

    If the bus voltage and dv/dt is the same, the current offset due to current overshoot should not change much.

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    TI__Expert 3605 points

    Hi Yichi,

    They have LEDs as load to achieve high power strobing. So there is no inductor on the output.

    Could ground disturbance also be the reason for higher measured low side current?

    Thanks,

    Franz

  • 0 in reply to Franz Schellhase
    TI__Genius 16390 points

    Hello Franz,

    Is it possible for the customer to get the current waveform at SW and PGND?

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    TI__Expert 3605 points

    Hi Yichi,

    Unfortunately, they can not provide the waveforms because this information is based on tests from a while ago. In the meantime, they found something that might be the cause and will test it. I'll get back to you in case this doesn't solve it

    In the datasheet of the LMG5200 we found a decisive hint, which shows the measuring error of the LED current at the

    PGND pin (see data sheet extract in the file attachment).

    - According to the data sheet, AGND and PGND are connected internally
    - The voltage drop at the measuring resistor in series to the PGND pin is thus short-circuited via AGND.

    We will modify our EVM to this effect and arrange the current measurement low-side in the load circuit.

    Best regards,

    Franz

  • 0 in reply to Franz Schellhase
    TI__Genius 16390 points

    Thanks Franz. Please keep us updated on the results.

    Regards,