PLL-based converter controls light source

You can digitally control light intensity of a lamp with the circuit shown in Figure 1. The control loop is based on a PLL where the VCO is made of a light to frequency converter (TSL220) coupled to a light source which is driven by a switching regulator (L4970). The output of the phase/frequency comparator (4046) is low pass filtered and used as control voltage for the switching regulator. The control voltage is proportional to the frequency error between the reference frequency (fREF) and the signal frequency (fIN) coming from the light to frequency converter. By changing the reference frequency, the voltage applied to the lamp is properly regulated to force the output of the TSL220 to be locked to fREF. The two resistors at the output of the 4046 provide an attenuation of 1000 which guarantees the loop stability. As an example, the L4970A switching regulator was used to drive a 50W 12V halogen lamp. The control loop of Figure 1 works over a frequency range from dc up to 500KHz. The maximum allowable value of fREF should not be larger than the saturation frequency of the TSL220 to avoid that the system enters a positive feedback condition. This value depends on the integrating capacitor used for the light to frequency converter and must not be larger than 750 KHz. To avoid lamp damage, the 10K( trimmer is used which limits the voltage Vo applied to the light source