Verifying a Charger Transition into Battery Powered OTG Output

A charger with OTG capability reverses the usual power direction: the battery supplies an external port through the converter. Enabling the mode is only one part of the transition. The port, input switches, battery state, and system load must all support the intended source operation.

Map power flow in both modes

The TI BQ25798 data sheet specifies conditions for OTG operation and describes how input switch configuration affects it. Battery voltage, port voltage, temperature sensing, and programmed regulation settings are relevant to entry and operation.

Draw charging and sourcing current paths separately, including external input MOSFETs and any port controller. Check how the product establishes the correct port role before applying source voltage. The charger register state does not by itself complete a USB power-role negotiation.

Budget battery and system demand

Estimate battery current from external output power, conversion efficiency, and simultaneous system consumption. At a lower battery voltage, the same external power generally requires more battery current.

For a hypothetical 10 W external load at 90 percent efficiency from a 3.6 V battery, converter input current is about 3.09 A before adding the product’s own load. Compare that demand with the battery, protection circuit, converter, and interconnect limits.

Capture entry and exit sequences

Record battery voltage and current, system rail, port voltage, switch controls, and relevant status. Start with a controlled external load and verify the programmed output voltage before increasing demand.

Test entry from the supported charging or idle states and exit when an external source is connected or the product changes role. Use the manufacturer’s required sequence so two sources are not unintentionally connected against one another.

Exercise limiting and recovery

Increase external load within the test plan and observe which current or power limit becomes active. Check the effect on the internal system rail. A product may need to preserve its own operation while reducing the available external output.

Repeat near the battery voltage and temperature boundaries that permit OTG. Confirm the behavior after a fault, timeout, or host reset, including whether configuration must be restored before another source attempt.

Save the two power-flow diagrams, entry conditions, settings, measured current, and transition captures. Verify the complete port behavior with the relevant controller configuration. A successful OTG design delivers the intended external power without violating battery or system limits and changes direction in a defined sequence. A correct no-load port voltage after setting an enable bit does not demonstrate that the complete product can source its rated load or return cleanly to charging.