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UCC3895 limiting max duty cycle

Magnus Pihl
Prodigy 10 points
Other Parts Discussed in Thread: UCC3895, UC3879, TINA-TI

Hi

I'm familiar with the old UC3879 but need some help wiht the UCC3895. I'm using peak-current control. 

With 3879 you can't limit the maximum duty cycle. The linear part can be set but the circuit still jumps up to 100% duty cycle if for example the converter input voltage drops low. Can the 3895 be set to a maximum duty cycle by choosing the right oscillator C and R? I have timing problem above 97% duty cycle and want to avoid this operation point. Is the change from 3879 to 3895 a possible solution to my problem?

 

 

    

  • 0
    TI__Mastermind 22786 points

    Well, first of all pls consider that this part (the UCC3895) is well supported:

    -          Evaluation Module is available

    -          Simulation models (TINA-TI, a freeware SPICE based simulation software that you can download from our web site)

    -          Tons of application notes are available

    Have a look at the link http://www.ti.com/product/ucc3895 for details...

    UCC3895 can only theoretically drive 100% duty cycle as you can see in Figure 6 on page 13 of the device datasheet. But it is made automatically by the control loop: you only set the oscillator frequency and then, depending on the working conditions, the COMP (control signal) will go up or down. If it goes up and stays up, above the ramp, then for sure you should have 100% duty in the resulting PWM. The point is that 100% duty won't last too long since it is not a "normal/steady stable" working condition; furthermore the internal oscillator  generate a reset pulse, so...

    V.