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UCC2803: Current sense for peak current control mode

Alonso Sanchez
Prodigy 20 points
Part Number: UCC2803
Other Parts Discussed in Thread: UC1842

Hello,

I have to use the UCC2803 for a DC-DC converter, input ranges from 5V - 12V and output must be 3.5V. The load current is fairly low (less than 100mA) and relatively stable, so I don't believe an external power MOSFET to be necessary.

I've been following the considerations in the ICs datasheet, but these are for a flyback converter, while I consider the best option here to be a buck converter. Now, my issue comes with connecting the CS pin to the correct part of the output. If I use a sense resistor, in the buck topology it is in series with the inductor, but the UCC2803 doesn't have a differential input so my resistor would have to be grounded somehow while still measuring the inductor peak current. If I choose to measure the current with an RC filter, the same issue appears.

I'd appreciate some help here, especially because changing the IC for another one is the last option I must consider, and need to present other alternatives first.

Thank you in advance,

Alonso

  • 0
    TI__Mastermind 34465 points

    Hello Alonso,

    I think you are correct about a buck converter being your easiest option.

    However you will still need an external MOSFET.

    If your application can live without current protection then you do not need to sense current and this makes your design a lot easier.

    The UCC2803 is functionally similar to the UC1842 and the datasheet for this part shows a method of implementing voltage mode control by connecting the oscillator to the CS pin.

    I am attaching page 20 of this other parts data sheet which shows a method of doing this.

    You will need to adjust the resistor divider in order to work within the linear range of CS input.

    This is the easiest method of using the UCC2803 in a buck configuration.

    If you require current sensing then you will need a current transformer on the MOSFET and then you may as well use a flyback transformer.

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 20 points

    Hello John,

    Thank you very much. This seems to be the best approach to take. Indeed, I don't believe I need current protection since my source already limits my maximum current, and my load is relatively fixed. I'll try this out. Just a few more Q's: in this case, my osc. resistor would be connected to the PNP emitter, right? So I'd have to choose the voltage divider to the base of the PNP such that the final current through the transistor is high enough for it to get to saturation...

    Regards,

    Alonso

  • 0 in reply to Alonso Sanchez
    TI__Mastermind 34465 points
    Hi Alonso,
    It looks like the voltage divider is set up as a current mirror for the emitter of the PNP.
    This sets a charging current into the timing cap.
    It also set a max voltage on the base of the NPN.
    The resistors on the emitter of the NPN set the peak voltage on the ISENSE pin.
    You will get a ramp signal on ISENSE and you need to ensure the peak value is less than the ISENSE trip level.

    So a constant current into the timing cap gives you a nice ramp signal for the ic and you then adjust the ramp signal from zero to a level below the trip level and use this as an input to the PWM comparator

    Regards
    John