LTC4121EUD charge when Vin<12V

Only at an input voltage >12V PWM occurs and the charging current appears (0.14A input current). Otherwise, there are no signals. The charge status and error are read at <12 volts as well as at >12 volts at the input: nCHG = 0, nFLT = 1.

I need Vfloat = 4.0-4.2V, batery voltage which I try to charge V=3.5V. Vin=6-8.4V (two LiPo cells), Vout=4V (single cell LiFePO4).

May be only LTC4121-4.2 can work with <12Vin? But I need Adj Vfloat.

What should I do to start PWM on low voltage with LTC4121EUD?

PS. When Vin < 12V Vfloat avg = 2.74V, voltage ripple with a frequency of 1/280 ms, Vp-p=2.7-2.8V looks like a saw.

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  • Good morning, we also have had similar issue. 

    LTC4121 do not charge battery if input voltage was greater than 10V, our inductor is 68uH and charging current is 220mA for a single LiFePo4 battery. 

    We have solved by inserting a 10 ohm resistor in series with boost capacitor and changed this from 22nF to 100nF. Our layout is 2 layers PCB with standard thickness 1.6mm. 

    Of course this IC remains strongly sensitive to layout and shows some silicon issues.

    Regards, Luca 

  • 0
    •  Analog Employees 
    on Nov 20, 2019 10:32 AM over 1 year ago in reply to LuBel

    Using a 2-layer board for any switch mode supply is unlikely to yield performance or pass EMI.  Thats just a fundamental of DCDC design and I have seen this many many times causing bad results.

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