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More Technical Problems with the UCC2894

Daniel Gerber83554
Prodigy 40 points

Thanks Hong,

now i have a pwm signal at OUT and AUX but with changing RON I can´t change the time of my out-signal.

ROFF changes the length of the AUX signal and so does RON too. What is wrong?

Is it possible to get an oscillator frequency of about 30kHz? Because I need 30kHz for my SMPS.

Thanks for answering.

 

Daniel

  • 0
    TI__Guru 73825 points

    It looks 30kHz is achieavable based on the datasheet.

    RON and ROFF are designed for DMAX and switching frequency, as shown in the datasheet page 8, equations (1) to (5).

    The dirve signal duty cycle is controlled by FB-pin which effective control range is 2.5V to 4.5V.

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hello Hong,

    now I have another problem. I got a representation of Dmax an my CS input and with changing RTON and RTOFF I can change DMAX or here 1-DMAX.

    If my voltage on CS is higher then 1.27V then the IC stops operating what is correct. Also I got the ramp at RSLOPE. The LINEOV/UV is ok but my signal at

    OUT lasts for 100ns and the signal at AUX for 30 us. That has nothing to do with my set DMAX. And if I try to increase the voltage on the FB higher than

    1.3V than the voltage on VDD decreases.

    Do u have an idea what is wrong?

     

    Thanks

    Daniel

  • 0 in reply to Daniel Gerber83554
    TI__Guru 73825 points

    CS-pin is also in control of the OUT and AUX pulses which is for current mode control. For your IC test, CS-pin connection is usually made in this way: connect a 100pF cap from CS-pin to GND-pin and parallel a 10k-ohm resistor to that 100pF cap. Then the pulse width should be following FB-pin voltage.

     

  • 0 in reply to Hong Huang
    TI__Guru 73825 points

    Also, when you do this test, do not apply a voltage directly to CS-pin from a voltage source without through a resistor of at least a few kilo ohms. Direct connect a volage source to CS-pin without the resistor mentioned may damage the CS-pin.