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UCC3895: Do you have any application schematic for a external clock input to SYNC terminal?

Kazuya Nakai59
Guru 21115 points
Part Number: UCC3895
Other Parts Discussed in Thread: UCC28950, UCC2895

Hello guys,

One of my customers is considering using UCC3895 for their new products.

Also they want to input a external clock to SYNC terminal. Their external clock frequency is about 50kHz.

They looked for a sample schematic which uses a external clock input to SYNC terminal because they want to know RT/CT and other terminal setting.

But they couldn't any sample schematic?

Do you have any application schematic for a external clock input to SYNC terminal?

Your reply would be much appreciated.

Best regards,

Kazuya.

  • 0
    TI__Guru* 80856 points

    Hello,

    The following link will bring you to an application note showing hot synchronize UCC28950 devices.

    https://www.ti.com/lit/an/slua609/slua609.pdf

    Regards,

  • 0
    Prodigy 210 points

    Please review the data sheet more closely. if the sync input frequency is 50KHZ then bridge will operate at 25Khz since oscillator clock is double FS not sure that is what you want. if the source is pure 50/50 square wave then you can double by creating the short narrow sync pulses from the rise and fall transitions digitally. There are older application notes on the TI website for the older UC chips that can pulse shape from years ago which should work here. I don't know the number of the application note anymore. You need to create a short pulse high enough to trigger the restart threshold of the oscillator added to the ramp to sync the chip. Mike may be able to assist in finding that old note for you but the syncing method is basically the same. 

  • 0 in reply to Peter Repka
    TI__Guru* 80856 points

    Hello,

    I believe that 25 kHz and 50 kHz is to slow of an operating frequency for the UCC2895.  The device is only characterized down to a minimum frequency of roughly 100 kHz.  This would be 50 kHz at the primary.   Please refer to figure 4 of the data sheet.

    So if you wanted to sync the design at 125 kHz you should set the controller frequency at 80% lower at 100 kHz.  This would put the primary switching frequency at 62.5 kHz when synchronized and 50 kHz when not.

    You are correct that the application note shows how to sync multiple UCC28950 and not the UC2895.  However, the block diagram in figure 11 of the data sheet shows how the sync pin works.  Section 7.13.13 describes how the sync pin works as well.

    When the Sync is active it will discharge the CT.  So this means that sync signal should be at least 2% of the oscillator period to ensure it does not affect the duty cycle.  I believe you can use a digital signal to sync the device if you wish.

    Regards,