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LM2840/LM2841 No Load Condition and Min Vin

FAB63476
Intellectual 530 points
Other Parts Discussed in Thread: LM2840, LM2738

Hi! 

I am designing a Dc Dc converter with LM2840 that needs to have 12 V at the output (0.1A max). 

I prepare a prototype (like figure 4 of the datasheet) with the Y version of the IC for reduce inductor space. I have change the inductor value to 15uH and the Cboost to 0.47uF.

I have 2 problems,

With no load, at startup, the regulatod do not stays in an steady state, I tried to drive CB pin with a diode from the output (as Figure 5 from the LM2738 datasheet) and  I get the regulation. However, I wonder is that is alowable.

The second problem that I have is that, as a requirement, I need to have aroound 12 V (it could be 11V) at the output for 12V at the input. I red in http://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/p/46622/164535.aspx#164535 that when Vin <Vo, the buck converter stops switching, turns on the   FET so that Vo=Vin - Iload*Rds(fet) but the there is some kind of problem because I can't get a steady output.

Any help will be apreciated.

BR.

Franco

  • 0
    TI__Expert 3950 points

    Hello Franco,

    You can drive the CB pin from the output voltage as long as the voltage does not excess the abs max of the part.

    For what concern the maximum output voltage for a given input voltage please consider that the part has a minimum off time therefore a 100% duty cycle is not achievable.

    In particular:

    for the Y version the maximum duty cycle is [1-(1.25M x 200ns)] = 0.75

    for the X version the maximum duty cycle is [1-(550k x 370ns)] = 0.815

    Regards,

    Giuseppe