• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 366 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS54425 max duty cycle

JosefMieslinger
Intellectual 2170 points
Other Parts Discussed in Thread: TPS54425

My customer wants to use a TPS54425 to generate 3.3V from a 5V rail.
He simulated the design in Webench and got the following warning:
---
Webench has calculated a duty cycle of 73.9% for the minimum Vin case. The maximum supported duty cycle for Webench designs using the TPS54425 is 65%. Operation with duty cycles above 65% and below 84.4% is possible with some loss of line regulation performance. Load step transient response may also be impaired.
See the TPS54425 datasheet for component recommendations.
73.9% is the Webench calculated duty cycle and 84.4% is the max duty cycle for the device based on min controllable on time.
---
It seems the device can work well up to over 80% duty cycle, but what will be the limitations, if the device is operating at a duty cycle beyond 65%? 

  • 0
    TI__Guru**** 191445 points

    In general, the maximum duty cycle is governed by the minimum off time specification.  Ideally, the operation would fall into two distinct areas with the duty cycle increasing linearly as the input voltage falls towards the output voltage then hitting a hard limit when the required off time is below the set minimum off time.  TPS54425 uses DCAP2 control with adaptive on time controller and semi-hysteretic off time comparator.  There is no internal oscillator.  The timing relationship between on time and off time (duty cycle) is not linear as the the minimum off time limit is approached.  This is seen by the user as a loss of line regulation at low input voltages.  The transition between the linear region (below about 65% duty cycle) and the hard limit (84.4% in your case) is "soft".  If you can tolerate a slight loss in regulation it is acceptable to operate in that zone (but we want you to know about it).  The other issue is transient response.  When the load current increases suddenly, the adaptive on time controller instantly increases the on time to allow the inductor current to ramp up rapidly in response.  If you are operating near the maximum duty cycle already, there is not much additional headroom.  Your output voltage may decrease more than usual in response to the load transient.