Regulating circuit

摘要

A comparator (17) to check input voltage (VIN) and resistors (24, 25) are provided in the synchronous regulator. The comparator (17) checks whether low voltage batteries or high voltage AC adaptors are utilized, by comparing the voltage in the comparator (17) with a reference voltage, and outputs the compared result to a PWM comparator (20). In the PWM comparator (20), the ON duty ratio of the FETs (11, 12) is regulated as much as possible to keep the output voltage (VOUT) constant. Further, in the FET (11), in which the largest electric power loss is generated, the output voltage is varied within the range in which the change of the output voltage is allowable for the required output voltage, by setting the ON duty ratio lower. Such a regulating circuit can be used in notebook or lap-top personal computers to prolong battery operating time and reduce AC adaptor heat dissipation.