PMP5123: Inquire PMP5123 max duty cycle concern

user4773172

Part Number: PMP5123
Other Parts Discussed in Thread: UCC2897A

Hi Sir,

According to PMP5123 design, if the Vin=8V, the duty cycle will be Duty=ton/T=4.1us/5.6us=73.2%, it's very close to the maximum duty cycle value 74% in the datasheet.

1. How could we make sure the the UCC2897A can work properly if the minimum duty cycle is 66% but the design condition is 73.2%?

2. Can UCC2897A be operated in 80% duty cycle design? What modification that we need to do on PMP5123?

Thank you.

Edward Chen

over 6 years ago

0 Ryan Manack over 6 years ago

TI__Intellectual 2610 points

Hi Edward,

The chart you have sent is at one condition, Ron = Roff = 75kohm with Rdel = 10kohm. You can set the maximum duty cycle higher by adjusting these parameters. However, if you allow a higher maximum duty cycle, the reset voltage will go higher to balance the V*s on the transformer in both directions.

I am looping in the PMP5123 designer to comment further.

Thanks,
Ryan

0 Brian King over 6 years ago in reply to Ryan Manack

TI__Intellectual 2885 points

Hi Edward,

Ryan is correct. The UCC2897A maximum duty cycle is programmed with the RON and ROFF resistors. The information in the electrical characteristics table is at a specific condition of RON=ROFF=75k.

thank you,

Brian

0 Kai Hsiao over 6 years ago in reply to Brian King

Guru 12675 points

Hi Brian

May I know the max duty is 66% for UCC2897A and can't support 80% is it correct ?

0 Brian King over 6 years ago in reply to Kai Hsiao

TI__Intellectual 2885 points

Hi Kai,

The max duty cycle is programmed by the RON and ROFF values. Please see section 8.3.1.3 of the datasheet for the equations. The UCC2897A can support 80% duty cycle.

thank you,

Brian

0 Kai Hsiao over 6 years ago in reply to Brian King

Guru 12675 points

Hi Brian

How much tolerance I need to consider for 80% duty?

I saw +/-4% in datasheet ,is it the same for 80% duty ?

0 Brian King over 6 years ago in reply to Kai Hsiao

TI__Intellectual 2885 points

Hi Kai,

I am looping in someone from the product line to help answer your question about the controller.

thanks,

Brian

0 John Griffin1 over 6 years ago in reply to Brian King

TI__Mastermind 34465 points

Hello Kai,

Yes you can expect the same tolerance (about 4%) with an 80% maximum duty cycle.

Regards

John

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PMP5123: Inquire PMP5123 max duty cycle concern