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UC1843AMDREP

Shihab
Expert 1205 points
Other Parts Discussed in Thread: UC1843

Hi,

I am doing a tolerance analysis for a current driver circuit using UC1843.

Can I get the specification/tolerance of the diodes and resistors (-55degC to +100degC) in the following figure?

Thanks,

Shihab.

  • 0
    Prodigy 40 points
    I have the same question but am getting no direct answer. If these are truly silicon diodes, we should expect a variation of about -2mV / deg C. If we start with an assumption of 25deg giving 0.7V per diode, and a delta T of 170 degrees ( 100 degrees hotter and 70 degrees cooler), the delta Vf of each diode would be spanning 340mV (140mV more at cold and 200mV less at hot). If the 25 degree Vf = 0.7, we should approximately vary from 0.84V at -55deg to 0.500 at 125 deg C. Of course these drops would be 2X. On top of that, the forward current and Is would be a factor in the -2mV/deg C approximation so this would not be exact. Also there would be process variation to stack on top of that. Unless the UC1843 uses some form of temperature compensation strategy, this could be a pretty hairy error component to your driver.
  • 0 in reply to Jeff Wright55
    TI__Intellectual 1615 points

    Why is this accuracy important? The error on the diode and resistor divider will be compensated by the voltage level on COMP, which is normally generated based on the integrator. Unless you are talking about the headroom tolerance. As long as you get enough headroom, the tolerance on these devices should not affect your operation.

  • 0 in reply to Bing -PSC Lu
    Prodigy 40 points

    Not using the integrated feedback loop and therefore the diodes are not inside a loop for my application.   Therefore in this application, the variation is relevant.

  • 0 in reply to Jeff Wright55
    TI__Expert 7445 points

    I will follow up on your querry and provide you input on it.

  • 0 in reply to Ramesh Khanna
    TI__Genius 15910 points

    According to the test group, statistics show that the diodes have a mean of 0.633 V and a standard deviation of 0.006.  We do not measure at temperature so we don't have any data on the temperature coefficient but the -2 mV/C is typically a good rule of thumb.  As for the resistors, the absolute value isn't imprtant but the ratio is and that is very consistent and as shown in the block diagram.