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LM5176: Need help about 4V in 12V out 6A

Erman Arslan
Prodigy 135 points
Part Number: LM5176
Other Parts Discussed in Thread: LM3478, TPS43060, TPS61240, TPS60151

Hello Ti Family.

I am trying to find 4V input 12V output with 6A boost converter.

From Webench best solution seems to be LM5176. 

Is there another solution which can support my output?

Thank you.

Erman

  • 0
    TI__Guru* 78170 points
    Hi Erman,

    Thank you for considering TI products in your design. The load current requires a sync boost (sync stands for synchronous rectifier, wrt diode rectifier).
    You may take a look at the LM25122 or TPS43060, however these two devices requires at least 4.5V to guarantee a successful startup. You need to add a small boost converter (integrated FET) like TPS61240 or a charge pump like TPS60151 to generate 5V and supply the VCC. If you don't want the additional small boost or charge pump, you may consider the LM3478/81 to implement a non-synchronous boost which can startup with 2.97V input, but it uses a diode rectifier.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 135 points

    Hi again,

    Thank you for your feedback.

    I designed TPS43060 before and after your feedback i tried it with 5V - 4.5V and 4V inputs. All seems stable.

    My system starts with 5.1V and after load increase Vcc become min 4.5V.

    My demo board frequency set 550kHZ. I will redesign it with 200kHZ which will work more cooler.

    For TPS43060 i just want to ask about mosfets.  

    In data sheet : "The average gate drive current must be less than the 50-mA (minimum) VCC supply current limit."

    And based on equation 25 can i use 2x 95nC mosfet (RDSon less than 1mohm at 7.5 VGS )?  IGD=(95 +95)*200kHZ=38mA

    Thank you for your help.

    Erman

  • 0 in reply to Erman Arslan
    TI__Guru* 78170 points
    Hi Erman,

    Note that the high side FET driver also comes from VCC. The boot diode delivers the current to the boot capacitor. If you use two low side but one high side, you still exceed 50mA. Please consider a FET with smaller gate charge. For you design (12Vout) you can just choose 20V or 30V FET, like TICSD16415Q5 or Vishay SiSA40DN.


    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Prodigy 135 points

    Hi again,

    Thanks for fast replay.

    I attached my 2 schematic.

    Based on TPS43060 i am making two design one is 4.5V - 5V to 12V 6A and other one 9V - 18V to 20V 5A. Both design 200kHZ.

    I made schematics based on TI Webench.

    What you mean with "If you use two low side but one high side" parallel to Q3 or Q5? General design based on 1 low side and 1 high side mosfets.

    Thank you.

    Erman

  • 0 in reply to Erman Arslan
    TI__Guru* 78170 points
    Hi Erman,

    What I meant is your Q3 or Q5 can be parallel FET to share the power dissipation, and your Q2 or Q4 can be single FETs if the if the dead time is proper.

    Please note that if you parallel MOSFETs, please add 1Ohm gate resistor to each of them to avoid gate oscillations between the two.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 135 points
    Hi Youhao,

    Thank you for everything.

    Everything crystal clear now.


    With my best regards.
    Erman Arslan