Duty cycle of UCC28251

Hi Takahiro san

Thank you for your interest in UCC28251. The maximum duty cycle listed is 100%.

Best Regards,
Ben Lough

Thank you for your answer!

I understood that maximum duty cycle of UCC28251 is 100%.

Please tell us the method that to keep the duty cycle to 50% or less.
We are concerned about the following things.

　　When exceeding duty cycle50% at short-circuit etc., the possibility that the Hi-side and Lo-side FETs turn ON at the same time.



Best regards,


Takahiro Nishizawa

Hi Takahiro san

I would recommend using cycle by cycle current limiting to prevent this case. If the voltage at the ILIM pin exceeds 0.5V, the controller immediately terminates the OUTA/OUTB pulse. Take a look at section 8.3.13 of the datasheet:www.ti.com/.../ucc28251.pdf

At steady state, a higher COMP voltage results in higher duty cycle. You can also implement a voltage clamp on the COMP pin if necessary. There is an example in section 8.3.10 of the datasheet.

Best Regards,
Ben Lough

Thank you for your answer!


The maximum duty cycle listed is 100%. This is your answer.

But UCC28251 has dead time of FETs in datasheet p.8.
Is duty cycle of UCC28251 really 100%?

Best regards,


Takahiro Nishizawa

Hi Takahiro-san,

This device has programmable dead time and it is possible to configure the device to have no dead time at all. Sections 8.3.5 and 9.1 of the datasheet mentions this subject briefly.

Best Regards,
Ben Lough

Thank you for your answer!

Maximum duty cycle is 100% about your answer.
But our customer says that duty seems to be limited by 50%.
Please tell us the cause.

Best regards,


Takahiro Nishizawa

Hi Takahiro,

Are you able to share a schematic? You can email it to me at ben.lough@ti.com if you do not want to post it publicly.

Best Regards,
Ben Lough

Thank you for your answer!

We check our costomer.

We suggested method to adjust duty cycle by Vcomp.
Do you know other cause to be limited duty cycle in a way other than Vcomp?

Best regards,


Takahiro Nishizawa

Hi Takahiro-san,

Duty cycle can be limited by the power stage (assuming the device is operating with closed loop feedback). Do you know how your customer is configuring the feedback to operate at different duty cycles? If they are hitting the cycle by cycle current limit, it will immediately terminate the current pulse. I would check the voltage at the ILIM pin to see if it is exceeding the 500mV threshold.

Best Regards,

Ben Lough

Thank you for your answer!

I have a question about Duty 100%.

Is Duty 100% the sum of High-side FET duty(50%) + Low-side FET duty(50%)?
I think that High-side FET and Low-side FET does not turn on at the same time in Half-bridge.

Best regards,


Takahiro Nishizawa

Hi Takahiro-san,

Correct, the high side and low side MOSFET's should not be on at the same time. If this occurs, you would have shoot through.

Best Regards,
Ben Lough

Thank you for your answer!

I have a question about max duty cycle described in the datasheet.

Max duty cycle of UCC28251 is 100%.
It means 50% of High side and 50% of Low side.
You said this is correct.

Max duty of LM5039 is 50% in datasheet.
Does it mean High side 50%(max) and Low side 50%(max)?

Best regards,


Takahiro Nishizawa