Coin cell

Rechargeable coin cell/supercap

VCC_LICELL is the input power line of an always-on regulator of the PMIC (VSNVS) that feeds the low-power mode circuitry of the CPU. By keeping this CPU power domain powered, the consumption of the SOM increases significantly in this low-power mode. See Power supply architecture.

However, this connection is not required at all since the low-power mode functionality of the SOM is supported by default by the on-module MCA. This means that the coin cell should only power the VCC_MCA power domain (through the diode configuration explained in this chapter). By doing this, the coin-cell charger functionality is lost but the power consumption is drastically reduced.

If coin-cell charger functionality is mandatory, place a reverse diode between VCC_LICELL and the coin cell to keep the low-power mode power consumption of the SOM under desirable values. This allows current to flow from the SOM to the coin cell (charging) and blocks current going into VCC_LICELL line.

Note You must take into account the forward voltage drop of the diode for charging the coin cell.

Digi has validated this hardware configuration with the Panasonic DB2J31400L diode. It's low reverse leakage current (300 nA) is ideal for this low-power mode application.

Non-rechargeable coin cell

For non-rechargeable coin cell applications, add the following RC circuit at the input of VCC_LICELL pad (B1):

Electrical values of the passive components of the filter: