An analogue-digital circuit for thermal control of the resonance frequency of optical ring resonators is suggested. This approach is characterised by a low power dissipation of 209 mu W and needs only 2800 mu m(2) of chip area in 0.16 mu m CMOS technology. Therefore, it is well appropriate to be used to control each of the cells of large optical switch matrices. Compared with a purely analogue heater control approach the power dissipation is reduced to 7%.