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  1. #1
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    LM3940 (3.3V reg) cannot supply 9V buck-boosk ?

    Hello,

    I use a 3.7V 16850 cell that gets boosted to 9V (board : https://www.aliexpress.com/item/3284...27424c4dSXahMo)

    as I sometime, have to recharge my cell, I thought I'd use 3.3V coming out of a LM3940 but the booster output drops to 4.4V while the output voltage of the LM3940 drops from 4.0V to 2.2V

    I measured the curent while using the battery : 50mA and as I measure the current while using the LM3940, I get 0mA (my voltmeter precision beeing limited)

    can someone explain to me what's going on ? the LM3940 can go up to 1A, I tried another one, same problem, how can I fix this ?

    Click image for larger version. 

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    thanks
    Last edited by andre_teprom; 7th August 2019 at 16:49. Reason: upload picture

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  2. #2
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    Re: LM3940 (3.3V reg) cannot supply 9V buck-boosk ?

    LM3940 needs an output capacitor (e.g. 10 uF) to absorb the boost converter current peaks.



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  3. #3
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    Re: LM3940 (3.3V reg) cannot supply 9V buck-boosk ?

    Ok thx, I'll check this out



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  4. #4
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    Re: LM3940 (3.3V reg) cannot supply 9V buck-boosk ?

    it does not work, I added a 33µF like in the datasheet , yet it's not working



  5. #5
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    Re: LM3940 (3.3V reg) cannot supply 9V buck-boosk ?

    Next step is to make informative measurements, e.g. oscilloscope waveforms.



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  6. #6
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    LM3940 3 3V reg cannot supply 9V buck boosk

    I'm using one... never DIY that fancy though, just clipped on the wires. The voltage never reach 12v but rather slightly just above 11v for mine, but it's sufficient to charge 6s.Ampere about 4A and slightly above.

    mraha, have you measure the voltage output from it?



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