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LMR61428 - Minimum on time or Minimum off time

Hirotaka Matsumoto
Guru 27525 points
Other Parts Discussed in Thread: LMR61428

Hi all

Would you mind if we ask LMR61428?

Our customer's condioton is as follows;
-Vin=3.5V
-Vout=4.2V
-Iout=1.2A

Our customer would like to calculate Maximum Fsw.
However we could not find Minimum on time or Minimum off time on the datasheet.
Could you let us know?
(On Webench, the result of Maximum Fsw was 300kHz.)

We appreciate for your help always.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Genius 15810 points
    Hello,
    The LMR61428 is a gated oscillator. The regular mode of operation is that the device switches at a fixed duty cycle of about 70% and has window comparisons to maintain VOUT within a small ripple range. The switching frequency will depend on the Rfq resistor and frequency of the switch bursting will depend on the value and type of the output capacitor.
    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Guru 27525 points

    Akshay san

    Thank you for your support always.
    We are sorry that we can't understand well.

    Please refer to the following file(the result of simulation).
    20160929_LMR61428.pdf

    The switching frequency will depend on the Rfq resistor and frequency of the switch bursting will depend on the value and type of the output capacitor.
    ->On Webench, regardless of Rfq resistor, we can choose Fsw=300kHz. 
      Output capacitors are 47uF x 2.

    And then, we guess that caluculation formula for Fsw as follows; (Is our recognition correct?)
    In this case, we recognize it requires T1(Minimum on time).
    -Duty=(Vo-Vin)/Vo=(5V-3.5V)/5V=0.3
    -T1=Duty×(1/Fsw)
    Is there the definition of Minimum on time on the datasheet? Could you let us know?

    We do need your help.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Akshay Mehta
    Guru 27525 points

    Akshay san

    Could you give us some advice?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Genius 15810 points

    Hello,

    My apologies for the delayed response. The LMR61428 works on the gated oscillator principle. This means that the device will switch at a fixed duty cycle of 70% irrespective of the VIN, VOUT, FSW, etc. This puts limitations on the VOUT to VIN boost ratio. There is no minimum on/off time as such otherwise. The VOUT is regulated by the hysteresis on the feedback voltage. If VOUT goes above the additional hysteresis, then the switching is stopped and the VOUT is allowed to fall back.

    The switching frequency depends on the VDD voltage and the value of the RFQ. As shown on page 10 and figure 15, there are two curves for RFQ for different VDD voltage. Webench sets VDD voltage to 5V always. The value of RFQ will be based on that. I hope this helps answer your questions. Please feel free to ask for more clarification.

    Regards,

    Akshay

  • 0 in reply to Akshay Mehta
    Guru 27525 points

    Akshay san

    Thank you for your reply and your cooperation always!
    The switching frequency depends on the VDD voltage and the value of the RFQ.
    ->We got that there is no minimum on/off time. Despite of Webench simulation, is it possible to choose Fsw=2.0MHz?
       Please refer to the following file.In this file, this simulation result is  Fsw=2.0MHz, however Rt(RFQ) is 499kohm.
       If Fsw=2.0MHz is set,  Rt(RFQ) should be 80kohm on the datasheet page 10 and figure 15.
       Is it possible to set 2.0MHz using Rt(RFQ)=80kohm
    2158.20160929_LMR61428.pdf

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Genius 15810 points
    Hello,
    Webench performs power loss calculations at the max possible switching frequency and then suggests a design that is possible and will not result in overheating the device. At high frequencies of operation the switching losses increase considerably and hence cause the junction temperature to go up. That said, there seems to be some issue with the re-calculation of the operating point values and I will notify the Webench engineer regarding that.

    As such it is definitely possible to choose a FSW of 2MHz. If your VDD is 5V, then according to the chart, a 80kohm resistor should result in 2MHz FSW. But this could potentially cause a lot of heating. I would ask you to bench verify this design.

    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    TI__Intellectual 2155 points

    Hi Matsumoto-san,

    I am following up with correcting the frequency calculation, as there may be a bug in the WEBENCH model. During a re-calculation of the frequency, it looks like it was not using the right value for Rfq.

    As mentioned already, the frequency resistor Rfq (and VDD voltage) will determine the switching frequency operated by the IC. Please follow Figure 15 in the datasheet to deteremine the frequency for now.