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LMR33630: DIP switch to change rail from +12V to +15V

Jim Smith1
Prodigy 160 points
Part Number: LMR33630

Hello,

Below is a rail design for inputting +18-22V DC and outputting +12V DC, and a nearly identical design for outputting +15V DC.

There is a resistor "Rfbb" and a capacitor "Cff" with significantly different values, and an inductor with a small difference in value.

I'm considering installing a pcb DIP switch to have the resistor and capacitor changed to different values so as to be able to flip a switch to choose between 12 and 15 output voltage. The current would of course flow through the switch terminals. 

My question is... how much current is flowing through that resistor and cap whenever the load is drawing a lot of current? Would it be safe to use a small DIP switch inline with those points? Or would i need to use heavier duty switches that can handle a full 3A?

I would guess that the resistor's pathway is pretty low current activity since there's a 100K resistor before it, but am unsure about the current flowing through the capacitor Cff. 

Any advice would be appreciated, thanks!

As for the inductor, i might be able to select a tighter tolerance inductor and have the value be split between the two designed values so that it's always in range of both.

  • 0
    TI__Mastermind 34625 points

    Hi Jim,

    We suggest to change output voltage by parallel resistor on Rfbb:

    Rfbb=9.09K for 12V

    Parallel 33.6K on Rfbb by DIP switch to adjust output to 15V.

    Rfbt=100K

    Cff=6.8pF. 

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 160 points

    Hi Andy, thank you! 

    A few questions..

    1. What will be the approximate maximum current passing through the parallel resistor that gets added for the 15V operation? I need to make sure the DIP switch can handle the current. I checked the datasheet and the FB pin only draws a max of 50 nA, so perhaps there's nothing to worry about since even the smallest switches can handle 25mA.

    2. If I interpret your message correctly, the 6.8pF cap will be ok for either 12V or 15V operation?

    3. What about the inductor? Would it be safe to choose one of 3.3uH and 3.5uH and stay with just one for both 12V and 15v operation?

    4. Does this Rfbb resistor value adjustment work for a similar Negative rail too? I had tried entering a desired "-15V" output in WEBENCH using the Negative -12V 1A (not 3A) design below but it won't allow me to enter anything greater than "-12V" in the field. In the real world, can I just use a 33.6K parallel in the same manner as positive and get a negative "-15V 1A" output? Or can the LMR33630 not handle it? Here is the -12V 1A design...

    Andy Chen said:

    Hi Jim,

    We suggest to change output voltage by parallel resistor on Rfbb:

    Rfbb=9.09K for 12V

    Parallel 33.6K on Rfbb by DIP switch to adjust output to 15V.

    Rfbt=100K

    Cff=6.8pF. 

    B R

    Andy

  • 0 in reply to Jim Smith1
    TI__Mastermind 34625 points

    Hi Jim,

    1. The FB pin voltage is 1V, you can calculate the current by voltage across the resistor. DIP switch is definitely can be used in this case.

    2. Yes, 6.8pF cap is OK for 12V and 15V output.

    3. use more popular 3.3uH for 12V and 15V rail.

    4. LMR33630 operating voltage is Vin+[Vout]=22+15=37 which is higher than max 36V input range, Vout can be adjust by same way in negative conversion.

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 160 points

    Thank you Andy!

    One last question..

    How about the 10uH inductor in the Negative 12V design? When i use the DIP switch to change from -12V to -15V, can the 10uH remain the same or should i choose a different value that works for both -12V and -15V?

    On a side note, i will make sure to specify an 18V input limitation, not 18V-22V, so as to never exceed the IC's total capabilities. 

  • 0 in reply to Jim Smith1
    TI__Mastermind 34625 points

    Hi Jim,

    Better to use 22uH inductor for 12V--18Vin to -16V/1A at Fsw=400KHz, 10uH is OK to use but inductor ripple current is high as 1A peak to peak, you need to design proper Cout to meet output ripple requirement.

    22uH (or 10uH) is Ok for -12V or -15V output, no need to change inductor in this case.

    B R

    Andy