a power - supply device has a first power supply circuit adapted to generate from an input voltage a first output voltage and a second power supply circuit adapted to generate from the first output voltage a second output voltage . the second power supply circuit monitors the first output voltage to check if it is higher than a first threshold voltage , and also monitors the first power - supply circuit to check if it has started operation for generating the first output voltage , so that the second power supply circuit stays on stand - by , even when the first output voltage is higher than the first threshold voltage , until the first power supply circuit starts operation for generating the first output voltage .