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UCC28951-Q1: Trigger to HICCUP operation

Hidekazu Someno
Expert 5130 points
Part Number: UCC28951-Q1
Other Parts Discussed in Thread: UCC28951

Hi Team,

 

Please tell me the conditions to enter HICCUP mode.

 

According to t UCC28951 datasheet, the trigger to enter HICCUP mode seems like that the CS terminal voltage exceeds the threshold voltage of 2V and that actual CS terminal voltage to enter HICCUP is affected by the amount of slope compensation.

 

The questions are;

Q1: Is this understanding above correct?  And If large slope compensation is applied, Does it greatly affects the CS terminal voltage that enters HICCUP(IC enters HICCUP at much lower CS terminal voltage than 2v in lcase of larger slope  compensation)?

Q2: If amout of slope compensation affets it, Please tell me how to set the over current to enter HICCUP considering the amount of slope compensation.

       Or is there a document explaining it?

 

Best regards,

H. Someno

  • 0
    TI__Mastermind 34465 points
    Hello H. Someno,
    The engineer most familiar is travelling at the moment.
    I have asked him to respond to your query as soon as he is available.
    Hope this small delay is ok with you.
    Regards
    John
  • 0 in reply to John Griffin1
    TI__Mastermind 25570 points
    Hello Someno-san

    The Hiccup mode is initiated if the CS+Slope Comp signal reaches 2V. So if the slope compensation ramp adds 200mV to the CS signal then the trip level for hiccup mode will be at 1.8V at the CS pin. Both the CS signal and slope compensation ramp are functions of time.

    There is no document that explains this other than the information in the datasheet. If necessary I can give you a more detailed explanation of how the slope compensation ramp affects the Hiccup mode entry

    Regards
    Colin.
  • 0 in reply to Colin Gillmor
    TI__Expert 5130 points

    Colin-san,

    Thanks for your comments.

    I know that the Equation -(110) exists to estimate current sense resister,RCS on datasheet, P46.

    In the equation-(110), 0.3V is included as slope compensation.  So, I understand that RSUM slope compensation affects peak current detection to enter HICCUP.

    I have a question about the Equation.

    -       What is the reason why 300mV is selected as amount of RSUM slope compensation?  

    -       As an example, in case of RSUM=40k & ton=5us,  Slope(V/us) is 0.125v/us from Figure.37 on datasheet P24 and total slope for RSUM at ton=5us is 0.625v.

           Does it mean the threshold of pulse by pulse reduces to 1.375v(2V-0.625V)?

    Best regards,

    H.Someno

  • 0 in reply to Hidekazu Someno
    TI__Mastermind 25570 points
    Hello Someno-san

    You are correct, the threshold does reduce. Please note that the reduction is by 625mV at ton = 5us. If the system reaches current limit point at a smaller ton (smaller duty cycle) then the reduction will be less, so for example at ton = 2.5us then the reduction would be 312mV and the CS threshold would be 2V - 312mV.

    Why was 300mV chosen ?. Normally the slope compensation ramp is set to be half the downslope of the inductor current during t_off as reflected into the primary of the transformer. The datasheet calculation indicates a required slope compensation of 67mV/us. Part of this ramp is provided by the transformer magnetizing current and the remainder is programmed by the resistor at RSUM. The figure of 300mV is chosen so that the CS signal plus slope comp signal can 'fit' into the 2V window. You can see that this choice is verified later in the design process (eq 172)

    Please note that there is an error in the calculation of m_mag in the datasheet (eq 169) although the process is correct. A corrected version is in the datasheet for the recently released UCC28951 device. The two calculations are very similar although for the UCC28951 we use the minimum input voltage to calculate the magnetizing current contribution. This is done because the system operates at larger duty cycles when Vin is lower and the magnetizing current is correspondingly less.

    Regards
    Colin