3V-4.2V boost to 12V2A, the pressure difference is relatively large, it is recommended not to directly boost.
The domestic chip has XL6007 can try. You can use TI's webench tool to find it.
As shown in Figure 1 is a simplified inductive DC-DC converter circuit, closing the switch will cause the current through the inductor to increase. Opening the switch induces current to flow through the diode to the output capacitor. As a result of storing the current from the inductor, the voltage of the output capacitor increases after several switching cycles, resulting in an output voltage higher than the input voltage.
In the actual circuit shown in FIG. 2, an IC with an integrated power MOSFET replaces the mechanical switch, and the MOSFET is turned on and off by a pulse-width modulation (PWM) circuit. The output voltage is always determined by the PWM duty cycle, and at a 50% duty cycle, the output voltage is twice the input voltage. Doubling the voltage doubles the input current size to twice the output current, and for actual lossy circuits, the input current is slightly higher.