• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 268 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.

LMR33630: How to calculate Vin(min) to have 4.5 V of Vout when fsw=1400kHz

Hung Ching Hsu
Expert 3086 points
Part Number: LMR33630

Hi Team

I would like to ask,

How to calculate Vin(min) to have 4.5 V of Vout when fsw=1400kHz.

The device is "LMR33630BDDAR". 

Thanks.

Jo

  • 0
    TI__Mastermind 27470 points

    Hi 

    you need use Toff min to caculate the Vin_min for 4.5V Vo.

    when the Vin decrease, the Ton will increase until the toff=Toff_min which is 50ns, then the converter will extend ton until it reach Ton_max=7u.

    then you can use (Vin(min)-(Rdson_H+Rdc_L)*Io-Vo)*7u=(Vo+(Rdson_L+Rdc_L)*Io)*50n to caculated the Vin(min)

    Thanks

  • 0 in reply to Daniel Li14
    TI__Expert 3086 points

    Hi Danial

    Thank you for the reply.

    I think the equation below comes from volt-second balance of the inductance. 

     (Vin(min)-(Rdson_H+Rdc_L)*Io-Vo)*7u=(Vo+(Rdson_L+Rdc_L)*Io)*50n to caculated the Vin(min)

    Let me ask some more questions.

    Can I always use the value of Ton_max=7u, Toff_min=50ns in the equation regardless of the frequency?

    Since I want to calculate when operating at fsw=1400kHz.

    To have fsw=1400kHz, which means the tsw=1/1400kHz=714ns.

    I thought that tsw=Ton+Toff, but 714ns doesn't equals to 7us+50ns here.

    Did I just understand sth. wrong?

    I still tried to calculate Vin from the equation above from using typ. and max. value of Rdson_H and Rdson_L respectively. 

    And I use the value of Rdc_L recommended by webench.

    By Rdson_H(typ.)= 95m ohm, Rdson_L(typ.)= 66m ohm, Rdc_L=21m ohm, Iout=2.2A 

    (Vin-(95m+21m)*2.2-4.5)*7u=(4.5+(66m+21m)*2.2)*50n

    Vin(min)=4.788V 

    -

    By Rdson_H(max.)= 160m ohm, Rdson_L(max.)= 110m ohm, Rdc_L=21m ohm, Iout=2.2A

    (Vin-(160m+21m)*2.2-4.5)*7u=(4.5+(110m+21m)*2.2)*50n

    Vin(min)=4.97V

    But after I entered the Vin value for both 4.788V and 4.97V on the webench, the simulation couldn't get started maybe because of the low value.

    So I tried to figure out Vin from two different ways. Could you also kindly take a look at it to see if it's correct?

    ①Calculate from datasheet(the device itself)

    fsw=1400kHz → tsw=714ns

    Vout=Vin*(tsw-toff_min(max))/tsw

    Vin=Vou*tsw/(tsw-toff_min(max))

         =4.5V*714ns/(714ns-85ns)

         =5.108V

    ②Get from webench simulation(take the discrete components into account)

    Conditions : Vout=4.5V, Io=2.2A

    I got Vin=5.2V (I tried to decrease the value until the simulation was not possible to be done)

    The conditions are shown in the link below.

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=53D23866152FFC9E

    Maybe the value from webench would be the most accurate, could you recommend me which value to use? 

    Thanks again.

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 27470 points

    Hi

    You can check Datasheet Fig 15, which shows what's the min value for Vout=4.5V,  for 2A, it is around 4.7V-4.8V

    the Vinmin cacualted by using fsw is the min input votlage that the converer can maintain 1.4Mhz, after Vin smaller than this value, the frequency will decrease to keep tonmin constant until ton=7us.

    you can also simulate by Pspice , it will shows the result too.

    Thanks