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 making the series ideally suited for applications requiring low profile or space saving solutions. With the MODE pin, the series provides mode selection of PWM control or PFM/PWM automatic switching control. In the PWM/PFM automatic switching mode, the series switches from PWM to PFM to reduce switching loss when load current is small. When load current is large, the series switches automatically to the PWM mode so that high efficiency is achievable over a wide range of load conditions. The series also provides small output ripple from light to large loads by using the built-in circuit which enables the smooth transition between PWM and PFM. With a adaptor enable function of the XC9128 series, when a voltage higher than the input voltage is applied to the output, the input and the output become isolated making it possible for the IC to work in parallel with the likes of an AC adapter.