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UCC2803-EP: Current sense measurement

Asha C
Intellectual 385 points
Part Number: UCC2803-EP

Tool/software:

Hello,

1. As per datasheet , overcurrent threshold is 1.55V (typical) , does this mean VCS is 1.55V?

But under maximum input signal its says 1.1V. How does this relate? Also, the gain. Will the fed CS input be multiplied by gain factor and given to the overcurrent comparator

Could you please explain what is to be considered for calculating RCS as VCS threshold?

2. How to calculate the Rramp resistor value? Is there any reference note for this approach of current sense measurement?

  • 0
    TI__Expert 7473 points

    Hello Asha,

    Thanks for reaching out. The below is how you would interpret the protection scheme.

    The 1.55V threshold is the comparator for short circuit condition where the voltage at CS pin would rise above 1.55V but the cycle bycycle comparator checks with 1V as the comparator threshold due to the presence of zener there, the gain is basically the 0.65R and R resistor internally which generate the gain from the 5V comp pin.

    2. Rramp is only needed for slope compensation for more than 50% duty cycle
    Please see the below equation for Rramp calculations:

    For Se please use below equations from datasheet:

    RCSF and CCSF are RC filter chosen to increase the leading edge blanking time.

    Rcs = 1V/Isense

    Hope these helps. Please click the green button if your question is resolved if not feel free to ask more.

    Best,

    Pratik A.

  • 0
    Intellectual 385 points

    Hello,

    1. Will there be no offset added to CS because of adding this slope compensation ?

    2. When exactly would this circuit be needed? There are examples even without transistor as below. Please suggest on what basis we need to have select the external circuit for CS and how would it help us? 

    ...

  • 0 in reply to Asha C
    Intellectual 385 points

    Also , when i simulate the circuit i do not see differences with and without the below circuit , voltage at CS pin is still the same. When exactly does the below circuit come into action and how do we evaluate if it operates in the right way during that condition?

  • 0 in reply to Asha C
    TI__Expert 7473 points

    Hello,

    Slope compensations is needed for duty cycle above 50%.
    There are multiple ways of doing it. You can pick the one sent before, the one mentioned in datasheet where the RT/CT pin is used. These are different way of achieving the same thing.

    Best,

    Pratik A.