A protection circuit which protects a load circuit operating with power supplied from an AC adapter, the protection circuit comprising:
- a power supply line extending from an input terminal connected to the AC adapter to an output terminal connected to the load circuit via a DC/DC converter;
- a switch unit provided between the input terminal and the DC/DC converter;
- and
- a protection circuit unit that stops the operation of the DC/DC converter to protect the load circuit, when an overvoltage or an overcurrent is detected, wherein
- the protection circuit unit includes:
- an overvoltage detection element that is turned on when a supplied voltage to the input terminal exceeds a predetermined overvoltage threshold value;
- an overcurrent detection element that is turned on when a current flowing through the power supply line exceeds a predetermined overcurrent threshold value;
- and
- a control signal output unit having an abnormal detection resistor element connected in series between a ground potential and a connection point where the overvoltage detection element and the overcurrent detection element are connected to each other in parallel, and an abnormal detection transistor that is turned on/off at a terminal voltage of the abnormal detection resistor element, the control signal output unit outputting a control signal for enabling the DC/DC converter to operate when the abnormal detection transistor is off and for stopping the operation of the DC/DC converter when the abnormal detection transistor is on, wherein
- the switch unit includes:
- a switching element connected in series between the input terminal and the DC/DC converter;
- a current limiting resistor element which is connected in parallel to the switching element and has a resistance value larger than on-resistance of the switching element;
- and
- a switch control element that turns on the switching element when a current flows through the current limiting resistor element, wherein
- a timing at which the switching element is turned on is delayed from a point at which a supply of power is started from the AC adapter, according to a delay time of currents flowing through the current limiting resistor element and the switch control element, and
- wherein
- the protection circuit unit includes a low-voltage protection unit that generates an enable signal in which a voltage level of the control signal is reduced with a predetermined voltage dividing ratio by a level conversion element provided between an output terminal of the abnormal detection transistor and the ground potential, the low-voltage protection unit controlling to set a voltage level of the enable signal to a voltage less than an operating threshold voltage of the DC/DC converter when the supplied voltage from the input terminal is less than a predetermined low voltage threshold value, in order to stop the operation of the DC/DC converter even if the abnormal detection transistor is off.