Charging circuit, power management circuit, and electronic device using the same

A charging circuit comprising:

• a DC input terminal configured to receive a DC voltage from a DC power source;
• a battery terminal connected to a secondary battery;
• a system terminal;
• a first transistor disposed between the DC input terminal and the system terminal;
• a second transistor comprising:
• a first electrode connected to the system terminal;
• a second electrode connected to the battery terminal;
• a back gate;
• a first back gate switch disposed between the back gate and the first electrode;
• and
• a second back gate switch disposed between the back gate and the second electrode;
• a gate controller configured to control turning on/off of the first transistor;
• a charging controller configured to control a gate voltage of the second transistor to control charging of the secondary battery;
• a DC power detecting unit configured to detect whether the DC power source is connected to the charging circuit;
• a back gate controller comprising a comparator configured to compare a voltage of the system terminal with a voltage of the battery terminal, the back gate controller is configured to turn the first back gate switch on and the second back gate switch off when the voltage of the system terminal is higher than the voltage of the battery terminal, and turn the first back gate switch off and the second back gate switch on when the voltage of the system terminal is lower than the voltage of the battery terminal;
• and
• a comparator controller configured to turn off the comparator when the DC power detecting unit determines that the DC power source is not connected to the charging circuit and the voltage of the battery terminal is higher than the voltage of the system terminal.