Disconnection after a voltage drop

[far987]

Hi,

I'm using nrf51822 with a custom board, where I should turn on a buzzer sometimes. I configure a timer and ppi to make a bip alarm. each bip lasts about 1sec; and the buzzer uses about 80mA to bip at his nominal frequency.

After the first bip, the battery (cr2032) voltage drops to ~2.5v and it causes a "timeout" disconnection (I checked it in debug mode; code "08"). even continues to bip after the disconnection without any problem until the asked time.

Would you please help me to find out the disconnection reason and prevent it? Because I feel 2.5v is still enough keep the connection alive!

Thanks

[AmbystomaLabs]

You should review the content on CR batteries.  Just search in the devzone you will find a lot.  Also there is a Nordic whitepaper on the subject.

This is a wholly inappropriate application for a CR battery. Lithium manganese button cells cannot supply that much current.

A CR2032 is rated for 0.2mA continuous load and according to the panasonic spec has a pulsed rating of 3.3mA. The folks at Energizer show a pulsed rating of 6.8mA.

If you monitor the voltage with an O-scope during radio operation I'm sure you will see it dropping significantly below 2.5v.

[far987]

Thanks for your reply.
we use a capacitor to supply this amount of current.
I understand this voltage drop, but we have always enough voltage to keep nrf51822 and buzzer ON.
As I mentioned, the buzzer keeps beeping even after disconnection.

Is there any solution to maximize connection probability? I mean stay connected on the worst case of voltage drop. is there any predefined minimum threshold of voltage to stay connected ?

[bjorn-spockeli]

Could you provide a plot of VDD of the nRF51822 during an active connection when the buzzer is turned on?

As long as VDD is within the recommended operating conditions stated in the PS(see below), then you should not see the connection drop. 

Symbol     Parameter Notes                                 Min.   Typ.   Max. Units
VDD      Supply voltage, internal LDO setup       1.8      3.0     3.6    V
VDD Supply voltage, DC/DC converter setup     2.1      3.0     3.6    V
VDD Supply voltage, low voltage mode             1.75     1.8     1.95  V

[AmbystomaLabs]

How big of a capacitor do you have?  

According to what you wrote above, the buzzer uses 80mA and operates for 1 sec. Assuming that the nRF is using a modicum of current compared to your buzzer. The capacitor would need to be i=Cdv/dt

(0.080A / 0.5V (voltage drop) x 1sec) equals 0.16Farads or 160,000uF.