UCC28070: Concept of external synchronization signal and maximum duty

Kaji@PAN

Part Number: UCC28070

Hi,

I am considering using a UCC28070 device.
Among them, I would like to ask about the content when this device is externally synchronized.
I am now calculating the contents of Equations 9 and 10.
FPWM is 100kHz. Therefore, FSYNC is 200kHz.
Since TSYNC is 2.5us, DSYNC is 0.5 from Equation 9.
Since we intend to set DMAX to 0.95, RDMX is 30kΩ from Equation 10. Please see attached Excel.
Here I have one question.
I believe that setting RDMX to 30kΩ will not result in DMAX of 0.95.
Because, considering the calculation of Equation 4, if RDMX is 30kΩ, DMAX becomes 0.7.
I think that the maximum duty is limited to 70% when RDMX is set to 30kΩ by inputting the external synchronization signal under the above conditions.
is it right?

Best regards,

UCC28070.xlsx

over 5 years ago

0 Teng Feng over 5 years ago

TI__Genius 13660 points

Hi Kaji, an expert will get back to you tomorrow, thank you.—— Teng

0 John Griffin1 over 5 years ago in reply to Teng Feng

TI__Mastermind 34465 points

Hello Kaji@PAN,

If you use a value of 0.5 for DSYNC then you will get a negative value for RDMX in Equation 10
This is a sign that DSYNC is too high.

There is a limitation on the value of Dmax imposed on the UCC28070 because of the pulse width of the SYNC waveform.
This means you need to greatly reduce the DSYNC value

You can use the following method to calculate the correct value of RDMX

You can use equation (2) in order to calculate RDM once you reduce fsync to a lower value
The data sheet shows a minimum SYNC pulse width of 0.2uS.
2.5uS is much too high.

Regards

John

0 Kaji@PAN over 5 years ago in reply to John Griffin1

Mastermind 7805 points

Thank you for your reply.

I calculated Equations (1) and (2) with Fsync = 200kHz and Tsync = 200ns.
The result is as attached Excel.
The result of RDMX in equation (1) is 66150Ω. As a result of substituting 66150Ω for the RDM in equation (2), DMAX (SYNC) was 0.902.
Is this calculation correct?

Best regards,

UCC2807020200319.xlsx