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UCC3818: VRSENSE range and current amplifier noninverting input

Marzieh Karami
Prodigy 190 points
Part Number: UCC3818

In the app note of SLUS395K, it is mentioned that VRSENSE is selected to give a dynamic operating range of 1.25 V, what are the downsides of going way higher or way lower than this value for example going to 14 V or 0.05 V?

Why is it need to use a resistor between pin 4 and ground? Can it just be grounded?

  • +1
    TI__Mastermind 34260 points

    Hello Marzieh, 

    VRSENSE is the voltage developed across the current-sense resistor Rsense (R14 in Figure 9 of the datasheet).  The current in this resistor is the same as the current in the inductor, so it has a high value at full power and low input line. RMS current through Rsense generates resistive power loss. 

    If Rsense value is chosen to provide a high VRSENSE value (much > 1.25V), then losses in Rsense will be huge.  For most PFC applications, this is unacceptable.
    If Rsense value is chosen to provide a low VRSENSE value (much < 1.25V), then losses in Rsense will be tiny but the signal level to the current amplifier (CA) will be very small and subject to distortion by offsets and noise. Input current distortion will be high. 

    As a rough "rule of thumb", a VRSENSE level of about 1~1.25V results in a reasonably high signal level for the CA and reasonably low power loss. 

    For your other question, the CA has offset currents from both inputs (which is usual for op-amps).  The offset current from the non-inverting input (CAI pin 4) is about the same the offset current from the inverting input (at MOUT pin 5).  It is common practice in op-amp design to insert the same impedance at the (+) input as at the (-) input so as to minimize the effect of input bias currents on the signal. 

    When R10 matches R9 (in Figure 9), the effects of the CA input bias currents is minimized (if not completely cancelled out).  
    The CA can work with pin 4 connected directly to GND, but then there will be an offset voltage generated on R9 that appears between (-) input and (+) inputs which will increase THDi.

    Regards,
    Ulrich