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TPS544B20: Current Sense Accuracy

Behzad Assar1
Prodigy 60 points
Part Number: TPS544B20
Other Parts Discussed in Thread: TPS544C20,

Hi,

We are seeing very inaccurate output current readings from the device across multiple samples. We have two similar designs, one based on TPS544C20, and another based on TPS544B20. Both exhibit a wide range of readings across multiple assemblies.

Design details:

1: TPS544C20, 5V input, 3.3V output, 15A max output current

2: TPS544B20, 5V input, 0.8V output, 11A max output current

With the application board running in an "idle" mode, we typically expect about 6-7 Amps for the first design. However, the readings (depending on the assembly measured on) range from 1A to 9A!

We'd like to understand:

a. what we can expect in terms of accuracy for the output current reading

b. what may be the source of error/variation from board to board.

Thanks for your support.

Regards,

B. Assar

  • 0
    TI__Genius 16457 points

    Hi Assar,

    Our US team will check this and get back to you soon.

  • 0
    TI__Guru 55485 points

     

    The accuracy of the READ_IOUT function of the TPS544B20 and TPS544C20 are listed in their datasheet on page 10 in the electrical characterization table.  The specification is +/- 3A of the actual loading current.

    There are system level items that can affect the current sense accuracy in addition to this tolerance.

    1) Stability of the output voltage

    The TPS544x20 family uses a PLL controlled mid-point sampling on the low-side FET to sense average inductor current, but this sensing scheme relies on stable operation of the switching converter.  Output voltage oscillations, especially sub-harmonic operation with alternating switching periods can produce current sense reading errors.

    2) Improper connection from AGND to AGNDSNS.

    The TPS544x20 family uses the AGNDSNS pin as a sense pin for the current sense voltage, if AGND is not kelvin sensed to the AGNDSNS pin without connection to PGND or GND, the TPS544x20 can see a voltage differential between AGND and AGND_SNS, which results in current measurement errors.

    3) VDD bypass currents flowing from AGND to AGNDSNS

    The VDD bypass capacitor can carry a large AC current at the switching frequency as the ripple voltage from VIN to GND is impressed on both VIN and VDD capacitors.  If this current is also impressed across the AGND to AGND to AGNDSNS connection, the resulting voltage differential can impact current sense accuracy.  

    4) Overheating of the TPS544x20 during solder reflow

    The TPS544x20 family of devices use a high-temperature, thermal-reflow die attach between the exposed pad (GND) and low-side FET die, and between the drain of the low-side FET die and SW metallization.  The resistance is part of the current sense path from GND to SW, trimmed at part of production testing.  Reflowing this internal thermal die attach by exposing the IC to temperatures in excess of 300C can alter the current sense element's resistance, and thus the accuracy of the current sense.

    I suggest doubling checking the layout and measuring the SW period.