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UCC28782: FLP-pin and fsw

Yasushi Fujiwara
Genius 12725 points
Part Number: UCC28782

Tool/software:

Hello Experts,

My customer has question as follows. Would you answer it, please?
1. FLT-pin
The datasheet stated that CFLT can be placed to avoid false OTP trigger from noisy environment. I suppose that OTP will trigger when CFLT is large value and VFLT cannot be threshold voltage within  tFLT(NTC), 50us. At this time, do you have a limitation of CFLT value? Customer is adding 100pF to FLT-pin, UCC28782 can be turned-on, but 4700pF case, USS28782 cannot be turned-on.

2. CS-pin
Rcs determine OPP threshold and it does not effect fsw. Is my understanding correct?

3. fsw
fsw will be determined Trans inductance value and duty(turns ratio). Is my understanding correct?

Best Regards,
Fujiwara

  • 0
    TI__Genius 12725 points

    Hello Experts,

    would you please respond to the question?

    Best regards,

    Fujiwara

  • 0 in reply to Yasushi Fujiwara
    TI__Genius 12725 points

    Hello Experts,

    Regarding #1, customer provided schematic around FLT-pin. According to the datasheet, VFLT must increase VNTCTH(0.5V/typ) within  tFLT(NTC)(50us/typ). So CFLT should be less than 1000pF based on my calculation below.
    - I: 50uA
    - V: 0.52V(max)
    - T: 14us(min)
    - C(max) : (T*I)/V = (14us * 50uA) / 0.52V = 1346pF, I suppose that  1000pF is enough to increase VFLT voltage during turned on.



    Best Regards,
    Fujiwara

  • +1 in reply to Yasushi Fujiwara
    TI__Mastermind 24375 points

    Hi Fujiwara-San,

    Thank you for reaching out.

    1. I think their calculation is correct. We need to take 14us as the worst case capacitance for this calculation. Generally a few hundreds of should be fine for this application and 1nf can be used as upper limit.

    2. Yes, it does not determine Fsw. It affects OPP, please the excel calculations.

    3. Yes, Use the equation 0.5 x L x Ipk x Ipk x Fsw = Pout

    For Ipk = (Vin/L) x DTs

    This will give relationship of Lm in proportion to duty cycle and frequency.

    Regards,

    Harish