Large cap connected to VDD plane

[eriksen]

Hi,

As you can see in the picture attached I have coin cell application. The positive terminal of the battery is connected directly to the VDD plane and then there is a large cap(100u_1210) connected to the plane aswell further away.

My question/concern is if the circuit might draw high currents from the battery instead of getting the current from the cap. Should the positive terminal be connected to the cap, and then the cap should be the only source of VDD to the VDD plane or isn't this necessary?

My thoughts when designing was that: When the radio goes into TX and draws approx. 20-30mA, then the battery voltage would drop and the cap would start to discharge to compensate for the drop regardless if the VDD was forced "through" the cap.

Any thoughts would be greatly appreciated.

Thanks.

-Fredrik

 

[KlausST]

Hi,

i don´t see any problem.
The connection to the cap should be low impedance, because the cap is used for pulsed / high frequency currents.
The connection to the coin cell should be in a way to carry the average load current.
No need for super low impedance, because the coin cell itself is relatively high impedance.

But on the other side i don´t see a problem if it is directely connected to the power planes.

Klaus

 

[Audioguru]

The capacitor value is so small that it can supply 20-30mA only for a very short duration and its voltage is dropping during the duration.
Since you will be overloading the coin cell then it might take a fair amount of time for its voltage (and the capacitor voltage) to return to normal.

 

[eriksen]

Thanks for your answers.

@Audioguru
I agree, but that's the idea; to pulse as short as possible and wait for the cap to load up again before I pulse again to avoid the coin cell ever "seeing" the high current draw.