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Hot!BM64 Power issue

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gamli
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2019/08/22 00:54:38 (permalink)
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BM64 Power issue

Hi,
I posted in the 'Bluetooth Audio' and 'Voice and Audio' yesterday, but it seems like my posts have disappeared (and that no posts at all are visible in those forums). So I will try to post again here.
 
I am having some issues in my design that I would really appreciate some support with.
I am using the BM64 designed into my custom board that is battery powered by a single cell Li-Ion battery. The purpose of the BM64 is to pair with a phone and streams audio through Bluetooth (and the charging circuit of the BM64 is not used). All works fine until the battery has drained to about 15% (3.6V), then I start to see the failure. The device does power on at this low battery level, and everything works fine and the BM64 does pair with the phone. However, when I start to stream audio (press play), then the current spike drawn by the BM64 is about 200mA, causing the voltage to drop below 3.2V for a short period of time, causing the BM64 to stop working completely until the device is restarted. The MCU does not even recognize that the connection has been dropped.
To power the BM64, I am using the power relatively straight from the battery, with two parallel ceramic capacitors (100nF and 4.7uF) by the BAT_IN on the BM64, and a ferrite bead placed close to the power pin.
Is this significant current spike of 200mA normal? I have tried with several BM64 resulting in same behavior. Attached is an image of the waveform captured at the moment of fault, where the yellow wave is the current drawn by the BM64, and the green waveform is the BAT_IN voltage.
 
I have some ideas about simply increasing the capacitance or even replacing the ceramic cap with a tantalum cap, but it is not certain that it will comply with the design.
Are there any firmware configurations possible in order to avoid or reduce this problem?
All input really appreciated!
 
/Gamli

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    maxruben
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    Re: BM64 Power issue 2019/08/24 02:57:52 (permalink)
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    Perhaps you need a better power management for your BM64 and battery which requires a buck/boost DC/DC power regulator. Or even a DC/DC boost switch regulator to 5 V and use a 3.3 V LDO from there.
     
    I am not familiar with the BM64 bluetooth module but is it designed to be operated from a single LiIon cell under its entire discharge cycle?
     
    /Ruben
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    gamli
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    Re: BM64 Power issue 2019/08/28 01:38:51 (permalink)
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    Hi Ruben,
     
    Thanks for the response. But yeah, the BM64 is supposed to be able to be driven by a single Li-ion battery, and is specified to be able to run from it down to 3.2V - however it cannot even stream audio when the voltage has drained under 3.6V as the initial current spike is so significant that it drops the voltage too far.
    And currently we can't replace the power source or add any significant regulation circuits to the board.
     
    By adding a 100uF cap on the input I was able to avoid the large power drop and preliminary testing suggests that it avoids the issue completely, but that capacitance is far higher than of the recommended design specs for the BM64 - which worries me. There are probably some drawbacks of this solution that I have not encountered just yet, so I would really like to hear other suggestions.
     
    /Gamli
    post edited by gamli - 2019/08/28 01:41:03
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