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Testing UCC25706 before implementing it into boost converter

Jesse Walker1
Prodigy 80 points
Other Parts Discussed in Thread: UCC25706

Hi everybody

I have never used this kind of IC before and have never really fully designed a boost converter ethier. I thought this would be a great learning experience. With the power portion of the converter aside I had a few questions about the UCC25706 chip.

Here is a list of parameters for my circuit:

Vin(min): 36V

Vin(max): 49V

Vin(shutdown): 32V

Converter Frequency: roughly 300kHz

My circuit seems to work ok with the exception that when power is first applied to the circuit it takes almost five minutes for a PWM signal to appear on the OUT pin. There is also no output on the RC pin during this time. If I wait long enough the PWM signal, as well as the triangle wave on the RC pin, will appear. Not sure what is causing this. Any ideas? 

VDD on the 25706 is 11.7V. The start up voltage in the spec sheet states: 11.2 to 12.8. So my 11.7V on VDD should be ok.

I also want to know if I can use a 0 to 5V signal on the feedback pin, I have a pot connected to it now, to simulate what the chip will see when the power portion of the boost converter is connected to it?

Attached is the schematic of my current circuit. Please feel free to view and give feedback.

Thanks in advance,

Jesse

UCC25706.pdf
  • 0
    Prodigy 80 points

    It seems that the feedback pin on this chip is intended to be interfaced with an optocoupler.  So do I need to hard set the feedback voltage and only have it come on if the output voltage falls below a set level? I thought that this pin would use something similar to an error amp that would constantly adjust the feedback voltage. I thought this is what maintained the output voltage. That cant be the case if it is connect to an optocoupler....at least I don't think so. Can anyone please help shed some light on how to use the feedback pin on this IC. Thanks,

    Jesse

  • 0 in reply to Jesse Walker1
    TI__Mastermind 25130 points

    Jesse,

    I'm not certain what you mean by "hard set" the feedback voltage.

    The voltage on the feedback pin will be a linearly varying voltage as a result of the feedback through the opt-coupler (look at the block diagram of the inside of the IC (figure 1 in the spec) 2/5 pf the FB voltage will be compared against the voltage on the RC pin (this is a ramp voltage) -0.7 volts and this will adjust the pulse width out of the IC to drive the converter.

    Regards,

    John

  • 0 in reply to John Bottrill
    Prodigy 80 points

    John,

    I see that now, thanks. The optocoupler through me off. I expected the output of an error amp going to the feedback pin.

    In the attached circuit (attached in the original post) there is almost a five minute delay from the time I apply power to the circuit and the time that the PWM signal appears on the OUT pin. During this time the triangle wave that is usually  present on the RC pin is also absent. Any idea what is causing this? 

    Thanks for the time and effort,

    Jesse

  • 0 in reply to Jesse Walker1
    TI__Mastermind 25130 points

    Jesse,

    The max volage to startthe IC is 12.8 V what voltage are you applying?

    JB

  • 0 in reply to John Bottrill
    Prodigy 80 points

    John,

    I have 11.7V on the VDD pin. The start up voltage in the spec sheet states: 11.2 to 12.8. So I  would think the 11.7V on VDD should be ok?  

    Thanks,

    Jesse

  • 0 in reply to Jesse Walker1
    TI__Mastermind 25130 points

    Jesse,

    All parts will start somewhere between 11.2 and 12.8 volts. The one you have may start at 12 volts or not until you get to 12.8. It's a tolerance issue.

    JB

  • 0 in reply to John Bottrill
    Prodigy 80 points

    John,

    Made some small changes and now have  13.5V going to VDD. Works perfect. 

    Thank you for taking time and helping me with such a mundane problem.

    Thanks again,

    Jesse