The MAX232 has a charge pump inside, which can get a voltage of ±9V~±13.2V from the 5V voltage of Vcc. Its function is to ensure that the MAX232 driver can output positive and negative RS232 levels in line with the standard, but it is not a very accurate and stable DC voltage, which fluctuates up and down under the influence of the load current and Vcc voltage. And its ability to carry load is very limited, as a 5V to 12V booster chip, the effect is difficult to be satisfactory.