Battery Operation and Buffer Capacitor

When using a small-capacity battery, it may not be able to supply sufficient current. For example, the rated current of a CR2032 is about 0.2mA, and even at maximum, only around 2~3mA is expected.

For instance, when TWELITE transmits a wireless packet, it momentarily consumes about 20mA. At this time, insufficient current supply and voltage drop may occur, causing the system to malfunction.

To avoid this, a buffer capacitor is connected between VCC and GND. (Additionally, it is necessary to control startup using a reset IC.)

Assuming the power is cut off, with a voltage of 3.3V and an average current of 10mA, a 100uF buffer capacitor can keep the system running for about 10ms. From the moment the power is cut off, current is supplied from the buffer capacitor, and the wireless module will operate until the voltage drops near 2V, where the wireless module becomes inoperable.

This is an effective measure for intermittent operation and transmission. A caution is that if retransmissions occur, the time until transmission completion significantly increases. Since one retransmission consumes several milliseconds, a buffer capacitor considering the number of retransmissions is necessary.

However, in cases where the device waits for reception, it continuously consumes a large current (17.2mA), so dealing with this using the above buffer method is difficult. Please use a battery with a higher supply capacity.

Approximate Capacitor Capacity and Operating Time

The current consumption of TWELITE series modules does not depend on the supply voltage. Because the current is constant, we calculate how long the module can operate based on the capacitor’s charge.

If a capacitor with capacitance C stores charge corresponding to voltage v, the time t for the voltage to drop from v to 0 at a constant current i is t = Cv/i.

The time required for the voltage to drop from 3.3V to 2.1V at current i is C(3.3-2.1)/i. For i=10mA and C=100uF, this calculates to 12ms. According to this calculation, if the power to a module operating at 3.3V is suddenly cut, and the average current is 10mA, the operation time can be extended by about 12ms.

The current value of 10mA is only an assumed average. Please refer to actual measured consumption current values.