The XC9128/XC9129 series are synchronous step-up DC/DC converters with a 0.2?(TYP.) N-channel driver transistor and a synchronous 0.2?(TYP.) P-channel switching transistor built-in. A highly efficient and stable current can be supplied up to 1.0A by reducing ON resistance of the built-in transistors. With a high switching frequency of 1.2MHz, a small inductor is selectable so that the series are ideally suited for the applications requiring low profile limitation or space-saving. With the MODE pin, the series provide mode selection of PWM control or PFM/PWM automatic switching control. In the PWM/PFM automatic switching mode, the series enters from PWM to PFM to reduce switching loss when load current is small. When load current is large, the series enters automatically to the PWM mode so that high efficiency is achievable over a wide range of load conditions. The series provide small output ripple from light to large loads by using the built-in circuit which enables the transition between PWM and PFM smoothly. When voltage higher than the input voltage is applied to the output during shut-down, the input and the output are isolated and the IC allows parallel drive with such as AC adaptor.