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LSF0108: Vref_A and Vref_B about LSF0108

Liu D
Prodigy 10 points

Part Number: LSF0108

Our actual application scenario is shown in the figure below, using a single power supply:

VCCB is connected to 5V, with R1 and R2 having resistance values of 1MΩ and 555KΩ respectively.

When we actually measured the voltages at Vref_A and Vref_B, the readings were 3.06V and 4.1V. However, based on theoretical analysis, the expected voltages at these two nodes should be 3.3V and 5V.

Could you please explain what might be causing the discrepancy between the theoretical analysis and the actuPicture.JPGal measurements?

  • 0
    TI__Guru 59711 points

    Hi Liu,

    Section 9.2.3 of the datasheet provides an equation to help solve for the R1/ R2 values to satisfy the specific biasing you need.

    Regards,

    Jack

  • 0
    Prodigy 10 points

    Hi Jack

    I choose 1M for R1,

    and chose 555k for R2,

    I think satisfy the equation.

    But I measured the Vref_A is 3.06V  and Vref_B is 4.1V, 

    I think the Vref_A should be 3.3V  and Vref_B should be 5V, don`t it?

    Best regards

    Liu

  • 0 in reply to Liu D
    TI__Guru 59711 points

    Hi Liu,

    Can you please provide the schematic of your implementation for further review? 

    Regards,

    Jack

  • 0 in reply to Liu D
    Guru 325670 points

    During normal operation, a current flows into Vref_B and out of Vref_A. (It is about 2.3 µA in your circuit.) So the voltage divider at Vref_A should use much stronger resistors to avoid that current influencing the voltage.

    And it appears that R1 and R2 are exchanged.

    Try R1 = 560 Ω, R2 = 1 kΩ.

    (The voltage at Vref_B will be less than 5 V because of the voltage drop over the 200 kΩ resistor.)