LM5026: can't reach to maximum duty cycle

JAP

Part Number: LM5026
Other Parts Discussed in Thread: LM5025C

Dear Experts,

I'm working with a LM5026-design. LM5026 is programmed to allow maximum duty cycle, 80%. For some reason LM5026 is not increasing duty cycle beyond ~72%.

DCL is connected to RT.

Voltage in UVLO is 1.3...1.4V.

Free running frequency set by RT-resistor is 250kHz. Design is sync'd externally with 300kHz frequency. Duty cycle of ext sync signal is 50%.

Do you have any ideas what could cause such behavior?

Thanks!

Rgds,

Juha

over 7 years ago

0 Peter Meaney over 7 years ago

TI__Mastermind 31580 points

Hi Juha,

Have you tried testing the design without the sync signal?

Regards

Peter

0 JAP over 7 years ago in reply to Peter Meaney

TI__Intellectual 2410 points

Hi Peter,

Yes, without external sync max measured duty cycle is ~78%. It is higher, but still below 80%. For some reason duty cycle is restricted to lower values when ext sync is used.

There is no min or max values for maximum duty cycle in datasheet, only typical value. Is there any existing tolerance data for maximum duty cycle?

Rgds,
Juha

0 Peter Meaney over 7 years ago in reply to JAP

TI__Mastermind 31580 points

Hi Juha,

A couple of ideas.

1. Program the controller to run at 300kHz with no sync and see what max duty cycle you get
2. Check the UVLO pin voltage as there is a relationship with the programmable duty cycle clamp and the voltage on the UVLO pin, see figure 18.

Regards

Peter

0 Peter Meaney over 7 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi Juha,

Any update here?

Thanks

Peter

0 JAP over 7 years ago in reply to Peter Meaney

TI__Intellectual 2410 points

Hi Peter,

At the moment LM5025C is considered due to higher max duty cycle(91%).

What comes to LM5026, it seems that higher max duty cycle can be achieved without external sync, 78% vs. 72%. Have you seen behavior like this before with LM5026?

Regarding LM502x-series in general, datasheets are only giving typical value for highest possible duty cycle. Is there any existing data for minimum and maximum values?

Rgds,
Juha

0 Peter Meaney over 7 years ago in reply to JAP

TI__Mastermind 31580 points

Hi Juha,

I need to ask the factory for the data on the Dmax spread for the LM5025C and LM5026, please give me a day or two.

Regards

Peter

0 Peter Meaney over 7 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

Hi Juha,

I have a lot of data on bothe devices. I will close this post and send to you by email.

Regards

Peter

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LM5026: can't reach to maximum duty cycle