This board takes in up to three power supplies – USB micro, single-cell LiPo battery, and a coin cell battery – and outputs 3.3V. The supplies are hot-swappable and the ORing circuitry will connect the load to the largest supply automatically. A voltage supervisor and load switch disconnect the LiPo when it reaches 3.5V and reconnects it when the voltage rises to 3.7V. A charging IC allows the USB supply charge the LiPo at 100mA (configurable by resistor). There was no stand-alone v1.0 for this board. Instead, the v1.1 comes from a portion of the Sensor Node board albeit it with a few modifications.
LiPo Undervoltage Protection
A protection circuit is needed to prevent the LiPo cell from damage due to over-discharge. It is implemented by a voltage detector (U2) and a load switch (U3). When the LiPo voltage is above 3.5V, U2’s output floats. Once the voltage drops below 3.5V, however, U2 grounds its VOUT pin. This opens the internal switch of U3 and disconnects the LiPo cell from the rest of the board. U2 has hysteresis and will continue to ground VOUT until the LiPo cell voltage is above about 3.605V – 3.78V (3.675V typical). Note than even when U3 has disconnected the LiPo cell, there will still be a discharge of 0.35mA through R3.
Assuming the undervoltage protection isn’t activated, the LiPo cell voltage will pass through U3 and reach U4 – an automatic battery back-up switch. U4 will output whichever input voltage is higher and indicate which source is being used by the SBAR and PBAR open drain outputs. Since USB power is 5V and higher than a fully-charged LiPo cell, U4 will always switch to USB power when it is present. This allows the LiPo cell to be charged by USB without also having to power a load.
In the first revision there were pullup resistors for SBAR and PBAR. This resulted in a continuous 0.33mA current draw and was not negligible based on the overall power budget. Removing the resistors allows the final application to decide if 0.33mA is acceptable or if a solution based on a microcontroller’s internal pullups or GPIO would be better (see this application note for an example).
LDO and Coin Cell Switch
An LDO (U6) is used to step either the LiPo cell or USB voltage down to 3.3V. The NOR gate U5 is used to disable the LDO to save power when neither the USB nor LiPo cell are present. Finally, the LDO output joins with the coin cell at the power switch U7. U7 selects which input to use based on the EN pin and the voltages at the two inputs. By connecting EN to IN1, U7 will use the LDO output when present and otherwise switch to the coin cell. The full truth table is found on page 3 of the datasheet.
LiPo Charge Circuit
A LiPo charge circuit almost straight from the datasheet is present so that the battery may charge when USB is connected. The charge current can be modified by populating a different resistor for R1 (e.g. 500mA with a 2k). The STAT pin is broken out to a external pin so the final application can decide whether it wants to connect an LED or microcontroller GPIO for determining charge status.