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LM3481: Current output

Raghu Tumati
Intellectual 720 points
Part Number: LM3481
Other Parts Discussed in Thread: LM3478, LM3488, LM5155

Hi 

I have a system requirement where I need to drive a portable printer from a single cell Li-ion battery (3.6V-4.4V). The printer drives at 12V and has a 3A current requirement. 

I was thinking of using a boost controller for this. 

However when looking at LM3481 datasheet and also the AN-1756 which is its EVM, they only mention a current < 1.5A. Im not sure why this is? Says that on the very first page of the EVM app note. 

If its a boost controller, shouldn't its current output depend on the capability of the external FET. As long as the boost can drive the FET, it should be ok right? Shouldn't I be able to use the EVM to work at 3A output at 12V. 

Also, the datasheet for the efficiency curves for LM3481 only have till 1A? Why is that? 

I've seen similar parts like LM3478, LM3488, LM5155 but they all have the same confusion. 

Hoping someone can clear my confusion or mis-understanding on how the controllers work :) 

  • 0
    TI__Mastermind 21720 points
    Hi Raghu,

    Thanks for reaching out with your questions.

    A boost controller can supply much more current than 1.5A. The power train components just need to be selected correctly. The LM3481 was selected to supply 1.5A of current because this is a common application. This means the MOSFET, current sense resistor, Inductor and diode need to be sized properly.

    You are correct about driving the MOSFET. If the input voltage is low a logic level MOSFET should be selected to ensure the MOSFET is completely enhanced. Assuming that you use the LM5155 the BIAS pin, input voltage and VCC can be tied together. This would allow for the minimum gate drive voltage to be ~3.6V.

    For your application I recommend using the LM5155. I was able to generate a WEBENCH design for you application specs.
    There is also this quick-start calculator that can be used to calculate component values
    www.ti.com/.../snvc224

    Please let me know if you have any questions.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Intellectual 720 points
    Hi Garrett
    Thank you for your quick reply and explanation. This makes complete sense to me.
    But I would like to know why you recommend LM5155 over the other parts for this application. In all honesty I was wondering which boost controller to pick with all the choices. So I am curious?

    Thanks
    Raghu
  • 0 in reply to Raghu Tumati
    TI__Mastermind 21720 points
    Hi Raghu,

    The LM5155 has a number of benefits
    1. Larger switching frequency range (up to 2.2MHz)
    2. Strong gate drivers (1.5A)
    3. Tighter current limit threshold over temperature
    4. Smaller package size.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Intellectual 720 points
    Hi Garret
    I have a follow up question w.r.t LM5155 slope compensation. The data sheet mentions that LM5155 has both fixed and programmable option but does not give a preference on which one to use under certain conditions.
    My duty cycle is definitely greater than 50%.
    Vin(min) = 3.6V
    Vout = 12V

    Should I be using a programmable or fixed compensation for this? How can I decide?
    I read app note SNVA824 - "How to design boost converter using LM5155" and I deduced that if the calculated Rsense is more than the max value of RSense, then external slope compensation can be used. But I am not sure if I understand this right or why this is so? Could you help?
  • 0 in reply to Raghu Tumati
    TI__Mastermind 21720 points
    Hi Raghu,

    Referring to SNVA824, if the value in Eq 7 is larger than the value in Eq 6 then adding external slope compensation is required. To calculated the required external slope compensation resistor (RSL) use Eq 9 and to calculate the Current sense resistor (RSL) use EQ 9. Figure 1 show the proper placement of the RSL resistor

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Intellectual 720 points
    Hi Garret
    Thanks for the reply. Few more follow ups.
    When the data sheet says programmable extra slope compensation, it confuses me. I assume programmable means changing via a register setting . But the LM5155 has no register map or way to do this. The slope compensation is set by external resistor. So shouldn't this be settable extra slope compensation. Just wanted to make sure I didn't miss anything in my understanding here.

    Also looking at figure 5 from data sheet. I see that the maximum current limit cannot exceed 16A for any condition.
    Does this set a lower limit to the sense resistor. It cannot be less than 100mV/16A = 6.25mOhm
    And it also somewhat sets a limit to the max load current and ripple you can drive using the LM5155. Am I correct in my understanding here?
  • 0 in reply to Raghu Tumati
    TI__Mastermind 21720 points
    Hi Raghu,

    The peak current limit can be set higher than 16A. The internal slope compensation is removed from the current limit signal as described in the datasheet.

    Thanks,

    Garrett