• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 271 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC12040: Efficiency Curve at Different Ambient Temperature

PY (Pengyu) Song
Expert 5860 points
Part Number: UCC12040
Other Parts Discussed in Thread: UCC12050

Hi team,

Do we have efficiency curve (or input current) at different Ta as below? This is for VISO = 5.4V and Iout up to 35mA. Thanks. 

  • -25C
  • +70C
  • +85C
  • 0
    TI__Mastermind 33115 points

    HI Py,

    We do not characterize efficiency over temperature but we do have power derating curves vs temperature. For the case that Viso=5.4 V and 35 mA (P=189 mW), the curves below (taken from UCC12040 data sheet, page 10), show at +85C, 189 mW is well within the de-rated power capability even at VINP=4.5V.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    TI__Expert 5860 points

    Hi Steve,

    Thanks. Yes, I understand it is within the operating area. The efficiency curve is required more to estimate the maximum input current which is the load to the previous stage that concerns customer as there is limited current/power budget, not so much on UCC12040 itself. 

    You may email me if there is further clarification needed. Thanks. 

  • +1 in reply to PY (Pengyu) Song
    TI__Mastermind 33115 points

    PY,

    At +85°C, the power is de-rated by ~25% worst case. Let's then say that at +85°C, the efficiency curve is de-rated by 25% worst case. At 35 mA, the non de-rated efficiency is ~45% which we can de-rate by 25% as 34%. Short of putting the customer unit in a thermal chamber and measuring the DC values (VINP, IINP, VISO, IISO) at 85°C, the approach I'm suggesting should give a good approximation.

    Also, since you mentioned a concern about the ability of the pre-regulator to source enough current, we should be more concerned with the pre-regulator sourcing current during UCC12050 start-up. Calculating the worst case input current using de-rated power and efficiency is ok for steady state but when the UCC12050 first turns on, there is an initial inrush current mostly due to the uncharged output capacitor seen by VISO at start-up. I would advise to measure and capture the start-up transient inrush current at room temp and then at +85°C to compare. If the pre-regulator is hitting current limit and unable to source the current needed during UCC12050 start-up, this may be the source of the problem you've introduced in this E2E thread.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    TI__Expert 5860 points

    Hi Steve,

    Thanks for your feedback.

    1. As for start-up/inrush current, what is the rise time for VISO = 5.4V for UCC12040? I find Trise = 750us for VSIO = 5.0V ==> how about 5.4V? 750us*5.4/5? 
    2. With Cout = 10uF, Trise = 750us for 10-90% of VISO = 5V, the calculated charging current would be = C*dV/dt = 10uF*(90%-10%)*5V/750us = 53mA (or 54mA since IISO = 1mA) during start-up ==> is this correct? 
  • +1 in reply to PY (Pengyu) Song
    TI__Mastermind 33115 points

    PY

    1. As for start-up/inrush current, what is the rise time for VISO = 5.4V for UCC12040? I find Trise = 750us for VSIO = 5.0V ==> how about 5.4V? 750us*5.4/5? 
      1. Probably very close to the same but the start-up is given in the data sheet as:

    1. With Cout = 10uF, Trise = 750us for 10-90% of VISO = 5V, the calculated charging current would be = C*dV/dt = 10uF*(90%-10%)*5V/750us = 53mA (or 54mA since IISO = 1mA) during start-up ==> is this correct? 
      1. Yes but I=C*dV/dt gives the average "constant current" needed to charge a fixed capacitor from one initial voltage to another. If the charging current were constant and the cap value were fixed, the start-up voltage would be monotonically linear but you can see from the graphs above this is not the case. Other parameters that need to be considered: UCC12050 input capacitance, pre-reg output capacitance, pre-reg start-up, temp/tol/DC bias effect on ceramic caps. This is why it is easier to measure on the bench rather than try and calculate the transient value. The instantaneous spikes of inrush current can be greater than the steady state DC current - how does the pre-reg respond to this under all corner cases? This needs to be measured.

    Regards,

    Steve M