Apparatus (40) comprising a multistage charge pump (10) having an output (41) for connecting a load (Cout, KL). The charge pump (10) comprises m gain stages for charging and discharging m external stage capacitors (C) in order to provide an output voltage (Vout) at the output (41) that is about m times higher than a supply voltage (Vdd) of the charge pump (10). The charging and discharging is influenced by switches inside said charge pump (10) that are controlled by a switching signal having a switching frequency (fosc). A monitoring circuit (20) is provided that monitors temperature-induced changes of the value of an external reference capacitor (Cref). Furthermore, means (30) for adjusting the switching frequency (fosc) are employed in order to compensate variations of the gain of said charge pump (10) that are caused by the changes of the value of the m external stage capacitors (C).