Inquiry regarding the phenomenon where BLE TX power varies depending on the lithium-ion battery voltage

[sangyun.kim]

Device : wearable IOT device

Battery capacity : Max 4.2V (Typ. 3.7V)

BLE Tx Power : Tx power is highest when the battery capacity is 4.2V (measured in air, not conduction)

I wonder if this phenomenon is natural, and if so, what the reason is. 

[Ressa]

Hello,

How much variation are you seeing? Also which voltage supply mode are you using (DCDC or LDO mode) ?  Please note that the output power can vary in the range of  max +/-4 dB . 

[sangyun.kim]

I thought it was the nRF52832, but it turns out this phenomenon occured with the nRF5340 MCU. 
Please double-check based on the nRF5340
VDD input voltage is 3.3V(for nRF5340)
RF Tx power is +3dBm

[Ressa]

Hi,

Can you provide more details about power variation? how much it varies? and what is the voltage mode you are using ? For High voltage mode you need at least 2.5 V for reliable operation. IS  the voltage dropping below 2.5 V when you see lower TX power? 

[sangyun.kim]

There is similar content in the nRF5340 datasheet, so I am sending you a screenshot. 
voltage mode is normal voltage mode and dcdc mode. 
3.3V is input to the VDD pin of all nRF5340. 3.3V dose not change.

[Ressa]

Hi ,

If the battery voltage is regulated before reaching to nRF5340 then you should not get any TX power variations. If you want I can have a look into your schematic / layout files to see if something obvious stands out.
Also you did not answer about how much power variation you are observing ? And how are you observing this variation in radiated mode ? is it in a controlled radiated chamber or free space? Please provide more quantitative details about your observation. 

Best regards,
Ressa