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external circuit of UCC2845A(soft-start)

Other Parts Discussed in Thread: UC2845A

We have a question about UC2845A.

Our customer try to use soft-start by external circuit.

Attached document is the circuit that our customer made.

Are external circuit and the formula is correct?

Please check the attached document.

best regards,

Takahiro Nishizawa

UC2845A.pdf

  • How is your situation?

    We hope earlier answer.

    Best regards,

    Takahiro Nishizawa

  • Hello Nishizawa-san

    I think this is the same query as the one at https://e2e.ti.com/support/power-management/f/196/t/818658

    which my colleague has answered. I'm going to close this post. You can open a linked, related post if I am mistaken.

    Regards

    Colin

  • Thank you for your support!

    External soft-start circuit by our customer is different from application note.

    And they use this formula.

       Tss=3600 x Css

    Do you have opinion about circuit and formula of our customer?

    Best regards,

    Takahiro Nishizawa

  • Hello Nishizawa-san

    My apologies - I had thought we had answered this question already, I also pressed the 'TI thinks resolved' button just now by mistake too.

    Anyhow - The SS circuit will work, I don't see any difficulty.

    The SS time is going to be set by the charging characteristic of the RC components, Css and R(1MOhm). The SS ramp will end when Css charges past the COMP pin (minus a diode drop). The voltage at the COMP pin is not fixed, it depends on the operating characteristic. This means that although the charging characteristic is fixed, the SS time can vary a little. You can estimate it from Vc(t) = Vref(1-exp^(-t/tau)) where Tau is the RC time constant. The customer formula is probably accurate enough to get an estimate of the SS time.

    Regards

    Colin 

  • Thank you for your support!

    We understand the relationship of soft-start circuit by Css and R(1MΩ).

    Our customer formula : Tss=3600 x Css(µF)

    What do you think 3600 indicates?

    Is it not necessary to consider 1MΩ in the formula?

    Best regards,

    Takahiro Nishizawa

  • Hello Nishizawa-san

    I think that the formula the customer is using is an empirical estimate for the exponential charging equation, Vc(t) = Vref(1-exp^(-t/tau)) and that they are using it because it works well enough.  I'd expect that the numerical constant 3600 includes the effect of the 1MOhm resistor. If the resistance were different, then the 3600 factor would also be different.

    Regards

    Colin

  • Thank you for your answer!

    We have new questions about external circuit of soft-start about UC2845A.

    Question 1.

    ON duty is controlled by voltage of COMP pin.

    As the voltage of COMP pin increases gradually in soft-start, ON duty is limited.

    Is this understanding correct?

    Question 2.

    You said that soft-start time can vary a little.

    Please tell us the examples that soft-start time can vary a little.

    Question 3.

    Our customer will use this formula.

       Tss=3600 x Css(µF).

    If we use 1000pF, soft-start time is 3.6S.

    Can we set 3.6S in this sofrt-start circuit.

    Does this circuit have the limit of soft-start time?

    For example, maximum time of soft-start.

    Best regards,

    Takahiro Nishizawa

  • Thank you for your support!

    How is your situation?

    We are waiting for your answer.

    Best regards,

    Takahiro Nishizawa

  • Hello Nishizawa-san

    A1/  Your understanding of the way that the soft-start process works is correct. The voltage at the COMP pin sets the duty cycle. The SS circuit forces the voltage at the COMP pin to rise slowly and therefore the duty cycle increases slowly.

    A2/ During the soft start process, the Error Amp output is clamped by the SS capacitor and the diode so the feedback loop is 'open'. That is, the feedback loop is not in control of the output voltage. The soft start process stops when the output voltage reaches its regulation setpoint and the voltage at the FB pin reaches the 2.5V reference value (at the non-inverting input to the Error Amp). At this point, the feedback loop closes, and the COMP pin is no longer clamped by Css and the diode. The external diode becomes reverse biased as Css continues to charge to VREF. The soft start time can vary because the voltage at the COMP pin which is necessary to set the correct duty cycle is not fixed. For example, if Vin is increased by 20% then the duty cycle needed to maintain Vout reduces by 20% and the voltage needed at COMP to set the smaller duty cycle will also reduce (although not by 20% because of the fixed two diode offset). In a similar way, if Vin decreases then the voltage at COMP will have to increase in order to force the larger duty cycle needed to keep Vout fixed. There will be smaller changes in Vout with load but these are not really significant unless the output inductor current becomes discontinuous. The key point is that the voltage at the COMP pin is not constant so the time needed for the Css capacitor to charge to whatever voltage is needed for the system to come into regulation is not fixed.

    A3 There is no maximum soft start time associated with the controller itself so 1000pF for 3.6s is ok although it is one of the longest ss times I've seen.

    Regards
    Colin

     

  • Hello Nishizawa-san

    It's been a while since the last update here so I'm assuming that this issue has been resolved - if it has not then you can open a new linked post.

    Regards

    Colin