setting USB to UART input/output/VCC levels for general situations
You could use the target MCU's VCC as a voltage source for the data lines. In this approach the CH340 uses mosfets to push/pull the target mcu's VCC and feed it back to the MCU. The same thing applies for signals sent from the MCU to the CH340: the MCU's signal connects the CH340's supply voltage to the input pin via mosfets.