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UCC2806: sync with current ramp?

HAREL LEVY
Prodigy 250 points

Part Number: UCC2806

Hi Everyone,

I want to use 4 UCC2806 with 90degrees phase shifted from each other.

Can you please tell me what will I see in CT pin when I enable external clock operation (connect RT to 5V).

Will I be able to add a voltage slope to the current measurment (using the CT ramp) when operating the UCC2806 with external clock?

  • 0
    TI__Genius 10865 points

    Hello Harel,

    Can you please clarify what you mean by 4 UCC2806 controllers 90deg phase shifted from each other? Are you using a push-pull topology? 

    If you connect RT to 5V then the resistor connected to RT must be at least 50kΩ because the max RT current is 100μA. CT is current mirrored to RT at the beginning of each clock cycle. Thus, charges to CT = 5V at the start of each clock cycle and decreases linearly to zero. The time it takes for the voltage at CT to charge to 5V is the switching frequency and the time it takes to discharge to zero is the dead time. You can change the capacitor at CT and the resistor at RT (>50kΩ) to adjust this switching frequency. I have the formulas to the equations below from the datasheet.

    Let me know if you have any more questions.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 250 points

    Hi Jonthan,

    In the datasheet it says to connect RT (1.25V) to ground through a resistor and that current (out of RT) will be mirrored to CT.

    to enable synchronization the datasheet says to connect RT to voltage higher than 4.4V so if I connect a 50K resistor from 5V to RT then current will from from the 5V into RT. this is an opposite polarity current than stated above.

    Will I see a voltage ramp in CT even then?

  • 0 in reply to HAREL LEVY
    TI__Genius 10865 points

    Hello Harel,

    Yes, you will see a voltage ramp in CT when you connect 5V to RT. These two app notes have the best information on GPPWM SYNC. Even though the devices mentioned are not the UCC2806, the CT pin behaviors will be similar. In particular, pages 3-56, 3-61, 3-62 in slua143 and 3-117, 3-118 in slua110 will be most helpful. 

    I would also like to clarify the original questions.

    • Yes, you can use four UCC2806 90deg phase shifted from each other as long as you have a digital clock capable of generating the four clock outputs.
    • Yes, you can still add slope comp using the divided down CT and summing it with the CS signal. You need to do this for all four PWMs repeated.

    Regards,

    Jonathan Wong

    2061.slua110.pdfslua143.pdf