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TLV521: high-side current sensing

Hugh Yang
Intellectual 900 points
Part Number: TLV521

Dears,

Just curious what's the purpose for adding a transistor in the sensing circuit? Generally, without the transistor, just use an op-amps could monitor the current.

Hugh

  • 0
    Guru 284830 points

    This is an easy way to generate an output voltage relative to ground. A differential amplifier would require precisely matched resistors.

    (For CMOS amplifiers with tiny input bias current, R2 can be dropped.)

  • 0
    TI__Guru**** 152966 points

    Hello Hugh,

    The V+ voltage is 'noise'  which is an unwanted signal that should be eliminated or tolerated. If V+ much larger than Vsense then the 'noise' is much bigger than the signal, 'Vsense' ; therefore very accurate resistors would be needed for a regular difference amplifier setup (to get high CMRR).

    For figure 36, the voltage is converted to a current then back into a voltage referenced to ground. The errors are Q1 base current, so consider Darlington PNP or PMOS transistor. and Vout must not be loaded (needs high impedance destination)

    In all cases the opamp's input voltage offset error is an error source.