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UCC28950: Synchronize UCC28950 and UCC28070

Kaji@PAN
Mastermind 7775 points
Part Number: UCC28950
Other Parts Discussed in Thread: UCC28070, ISO7710, UCC28951

Hi,

I am thinking about how to synchronize UCC28950 and UCC28070.
The master is UCC28950. The slave is UCC28070.
The synchronous clock is 100kHz.
I thought of using a digital isolator to connect the clocks of both devices.
The device is ISO7710. Is it possible to use this device?
Also, please let me know if you have any other good products.
If you have a reference design that synchronizes UCC28950 and UCC28070, please let me know.

Best regards,

  • 0
    TI__Mastermind 25570 points

    Hello Kaji

    The ISO7710 device is a good choice for the isolation function - I'd advise you to use the version which defaults to output LO in the absence of an input. This allows the UCC28070 to run unsynchronised in the event of a synchronisation signal failure.

    The other thing you need to consider is that the UCC28070 needs a synchronisation pulse of at least 200ns width. The UCC28950 SYNC signal output is at 50% duty cycle  and runs at twice the power stage frequency. For example the SYNC signal will be at 200kHz when the UCC28950 outputs (OUTx) are running at 100kHz. The UCC28070 also needs a synchronisation signal at twice the PWM frequency. However, you will have to add a circuit to reduce the SYNC signal duty cycle so that the pulse width is 200ns or more. Otherwise the max duty cycle of the UCC28070 will be limited - see note (2) in the Data sheet - below. A simple mono-stable would do that for you.

    Finally,  set the free running frequency of the UCC28070 to at least 10% less than the synchronisation frequency.

    regards
    Colin

  • 0 in reply to Colin Gillmor
    Mastermind 7775 points

    Thank you for your reply.

    When the operating frequency of UCC28950 is 100kHz, the SYNC signal becomes 200kHz, so if the duty is 50%, a 2.5us pulse will be output.
    Since the UCC28070 side requires a pulse of at least 200 ns, I think that the SYNC signal of UCC28950 can be applied to UCC28070 as it is via ISO7710. Is my idea correct?

    Best regards,

  • 0 in reply to Kaji@PAN
    TI__Mastermind 25570 points

    Hello Kaji-san

    I neglected to mention on my last post that I would recommend that the customer choose the UCC28951 device instead of the UCC28950.

    The UCC28951 device is an enhanced version of the UCC28950. It is a fully compatible drop-in replacement for the UCC28950. Refer to application note SLUA853 at http://www.ti.com/lit/an/slua853/slua853.pdf for more details. The UCC28951 is typically the better choice, especially for applications where the system may have to operate simultaneously at a duty cycle> 90% and current limit.

    You are correct but the problem is that the 50% duty cycle pulse from the UCC28951 will severely limit the maximum duty cycle that the UCC28070 can achieve. The magnitude of the effect can be calculated (eq. 9 and eq. 10) as shown in the data sheet. The user will have to determine whether the 50% duty cycle pulses from the UCC28951(0) will cause a significant loss of Dmax but I'd expect that they will. In this case you would need to use some added circuitry to narrow the pulses, a 200ns monostable, triggered on the rising or falling edges of the SYNC signal for example.

    Regards

    Colin