A small battery-powered board often combines a charger, a protection circuit, a regulator, and a processor. On the schematic they can look like separate blocks. In operation, they are three tightly coupled systems: energy enters, energy is stored, and energy is converted into a load rail.

Charging current is not system current

If a USB-C input supplies a battery charger while the product is running, the input current may be divided between the load and the battery. A charger that is sized only for the nominal battery charge current can behave badly when the load changes state.

Write the current paths for at least these states:

• USB present, battery charging, load asleep;
• USB present, battery charging, radio transmitting;
• USB removed, battery powering the load;
• battery near empty, regulator at its worst dropout margin.

The board should have an intentional policy for each state. Otherwise, a radio burst can reduce the system rail while the charger is simultaneously trying to regulate the battery.

USB-C starts at the connector

For a basic sink, the CC pins need the correct pull-down behavior. VBUS also needs input protection, current limiting, and a clear path to the charger. Connector shield, high-speed data pins, and power return should not be treated as one anonymous net.

The battery charger and buck-boost schematic example is a useful reminder that the charger, converter, USB-C network, and battery nets must be reviewed together. Use it as an architectural reference, not as a drop-in design.

The converter sees the battery’s entire life

A buck-boost regulator may produce a stable 3.3 V from a battery that moves above and below the output voltage. Its difficult cases are often at the edges: cold-start, low battery, maximum radio burst, and a nearly empty battery with increased internal resistance.

Check the converter’s input ripple current, inductor saturation current, switch-node layout, and feedback routing. The required capacitor value must be evaluated at bias and temperature, not copied from the nominal marking. A practical starting point is the DC-DC converter, followed by the exact regulator data sheet.

For the input and output capacitors, compare dielectric and package options in the TDK component, then verify the chosen part’s DC-bias and ripple-current behavior.

Layout decides whether the blocks cooperate

Keep the charger’s hot loop compact. Keep the converter’s switch node away from the antenna, sensor, and feedback trace. Route battery current through copper that can carry the pulse without excessive drop, and place the voltage-sense connection where it measures the intended node rather than a connector-side approximation.

The PCB impedance-control guidance is relevant when USB high-speed traces or RF lines share the board, but the same layout discipline applies to the power tree: define the return path and do not let an uncontrolled current loop cross a sensitive reference area.

Verify with a state table

During bring-up, log input voltage, battery voltage, system-rail voltage, charge status, and load current. Repeat the test with the battery at several states of charge. A board that passes with a bench supply can still fail with a battery because the source impedance is different.

Use the component derating calculator for a first-pass check on resistors and protection parts, then confirm the manufacturer curves. Inspect the assembled board for capacitor orientation, connector solder coverage, thermal pads, and the correct charger variant through the PCB assembly workflow.

The best battery board is not the one with the most blocks. It is the one whose current paths, transitions, and fault states are explicit.