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TMAG5124: Rsense value

Yuta Nishie
Genius 5005 points
Part Number: TMAG5124

Hi,

Customer would like to design with a Low-Side Sensing Diagram.


Is it acceptable if the resistance value of Rsense is higher (ex. 150kOhm) or lower (ex. 100Ohm) than 220Ohm stated in the data sheet?
If unacceptable, what are the implications?
Do you have problems with device operation, especially during Start-up?

Also, I would like you to check the customer's circuit diagram and waveform.
So could you please tell me your account or email address?

Best Regards,

Nishie

  • 0
    TI__Mastermind 21120 points

    Nishie,

    When selecting the resistor for the output, it is important to consider the corresponding voltage drop which will be observed on the resistor.  The Vcc current may vary as shown here: 

    In the case where Icc = 17 mA, there would be 3.74V drop across Rsense = 220 Ohm.  As long as you can accurately measure the voltage on Vsense, then there is little concern with making the resistance lower.  For very small resistance values, you might consider a current shunt monitor to provide a larger output signal for your measurement.

    The second option you asked about, where the sense resistor is 150kOhm would not be appropriate.  In this case the device would not have enough head room to properly force the output current required and the device would not function normally while it attempts to force more current through this resistance than is possible.  This would prevent the device from a normal start up sequence since the output does initialize in the high state.

    Thanks,

    Scott

  • 0 in reply to Scott Bryson
    Genius 5005 points

    Hi Scott-san,

    Thank you for your support.

    "In this case the device would not have enough head room to properly force the output current required and the device would not function normally while it attempts to force more current through this resistance than is possible."

    -> 1.  What is the head room required for the device to boot normally?
    　      I checked the internal circuit, is it about 1.8V?

    2.  Also, the data sheet states that 2.5V is required to start up the power supply.
         Is there a limit to the rise time until the power supply voltage reaches 2.5V?

    3. Is there a time limit to reach the headroom ((1) below) required for the device to start normally after applying the power supply voltage?

       

    4.  I would like you to check the customer's circuit diagram and waveform.
         So could you please tell me your email address?

    Best Regards,

    Nishie

  • 0 in reply to Yuta Nishie
    TI__Mastermind 21120 points

    Nishie-san,

    The datasheet doesn't have any published data regarding the ramp rate of the supply nor any minimum operating voltage below 2.5V.  I wouldn't recommend attempting to set the resistance so high that the resulting supply range for the sensor is less than this.

    I have sent a request to connect as friends, and I will be able to send you my direct email information to discuss reviewing the customer's diagram and waveforms. I will mark this request closed and we can continue discussions further via email.

    Thank you,

    Scott