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LM5176: Low Vin configuration with Bias to Vout

Jason Tse
Prodigy 10 points
Part Number: LM5176
Other Parts Discussed in Thread: CSD17577Q5A

Hi Ti Experts,

I’m trying to use LM5176 to boost a single 21700’s voltage to a constant 24V.

Referencing the below generated Webench design, a prototype board is already designed and built.

webench.ti.com/.../SDP.cgi

The worst case Vin is assumed to be 2.8V and a lipo battery’s voltage shall never exceed 4.2V. This particular design suggested that I will need a separate voltage source for Vbias to allow the below 4.2V input voltage to work. I somehow misunderstood the datasheet, thought Vbias was optional during designing, so all I did was to include Cbias and anticipate manual rework if needed.

Now after building the PCB, the circuit of course didn’t work with a 3.7V input, where Vin is below the 4.2V input baseline specified in the datasheet. Later, I was able to get the board to output ~24V with an input of 5V. Vin after a ~500mV diode drop still provides sufficient voltage for the IC to run without Bias. It makes sense that the circuit was not functional with a 3.7V battery after knowing that I’ll need external power to Bias pin to get low vin to work. I realized this is a config that will never work for the product I am developing for, as this power stage should be able to be activated without an external power supply.

I then tried to regenerate the design with the “Bias to Vout” option. Referencing the design, I simply reworked the board to have Vout connected to Bias. The circuit was not boosting. As I increased the input voltage to around 5V, the circuit was damaged, likely the SW1 and SW2 pins on the IC. I don’t think the switching FETs are damaged after taking them off and probing their Rds. Later I compared the two designs “Bias to Vout” vs “External Bias Source”, I realized that the switching FETs for “Bias to Vout” have significantly higher voltage rating than the ones for “External Bias Source”

So I think I have two main questions:

  1. Should the “Bias to Vout” config work for my application and design parameters of min Vin voltage being 2.8 from a single 21700 battery, given that my schematic will be referencing the generated Webench design correctly.
  2. Is the FETs with lower voltage rating the main reason why the IC was damaged during my rework? Otherwise I don’t have a theory of why my manual rework didn’t get IC to operate in PWM mode, while Bias is connected to the Vout rail.

Thank you very much for your time!

  • 0
    Prodigy 10 points

    I figured that links are not shared properly so I could't share the link to the webench design, but my design parameters are:
    Vin = 2.8V, Vin Max = 4.2, Vout 24, Iout Max = 1.2A

  • 0 in reply to Jason Tse
    TI__Guru 58471 points

    HI Jason,

    thank you for using the E2E forum

    i am not sure that i understood all of your points but will try to answer:

    Connecting the BIAS to VOUT is the right connection to:

    - start the DCDC with an input voltage of >4.2V 

    - and the be able to operate down to an input voltage of 2.5V 

    What are the parameters (type) of the two used MOSFETs. For sure if the voltage rating of the MOSFET is to low it can get damaged.
    Consider also over and undershoots on the switch node and add enough margin here.

    Best regards,

    Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 10 points

    Hi Stefan,

    Thank you for your response.

    If I understand you correctly, I can infer, since my 21700 battery will never be able to provide more than 4.2V, it will not start the DCDC. As long as the DCDC converter was on and has been functioning till a point that Bias is receiving a stable voltage, it can then operate with my battery voltage which is <4.2V.

    So I'm guessing, with a separate component charge pump/boost, I can kick start the LM5176 with a voltage >4.2 for once, then feed raw battery voltage to Vin and the boost will sustain.

    Now it's becoming a design/cost issue rather than just a technical issue. I will decide if I want to add that extra circuit for getting LM5176 to work, or I will find another part that will work with my battery voltage.

    The MOSFETs doesn't seem to be as crucial as the above problem for now, but for your information, Webench recommended CSD17577Q5A (30 V 60A) with External Bias config, whereas IRFS4227TRLPBF (200 V 62A) with Bias to Vout config.

    Thank you very much for your response again!

    Best,

    Jason