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How to calculate the needed resistance for the VIN_SENSE (Pin 16)

Thread Summary

The user asked how to calculate the resistance for the VIN_SENSE (Pin 16) for the LT3798 in a PFC application with variable input frequency (>400Hz). The final answer specifies that the VIN_SENSE pin requires a current input, with the resistor value calculated as (VMAX/360µA) at the maximum line voltage. For DC or non-PFC AC input, a 25k resistor should be connected from VIN_SENSE to INTVCC.
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Category: Datasheet/Specs
Product Number: LT3798

How do I calculate the needed resistance for the VIN_SENSE (Pin 16) for the LT3798? This is for a PFC, variable input frequency (>400Hz) application. Is this pin looking for a voltage input or a current input? There are different values used on different examples. There is a reference to a LT3799 IC that uses a voltage divider.
Also, are there any thoughts on the input inductor, L2,  to the rectifier on page 17 of the LT3798 data sheet?

Thanks,

Doug

  • Hi Doug,

     

    For the 3798, it's looking for a current input.  See page 11 in the data sheet:

     

    Setting VIN_SENSE Resistor 

    The VIN_SENSE resistor sets the current feeding the internal multiplier that modulates the current limit for power factor correction. At the maximum line voltage, VMAX, the current is set to 360µA. Under this condition, the resistor value is equal to (VMAX/360µA). For DC input or non-PFC AC input applications, connect a 25k resistor from VIN_SENSE to INTVCC instead of the AC line voltage.

  • Thanks,

    Is there also any white papers or simple model of this  LT3798 to help analyze for compensation values and stability? I have been using the LTSpice demo model. Are there any example to generate a bode plot for this part?

    Doug

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