8.3.1 Controller Circuit

The controlling circuit of the device is based on a current mode topology. The inductor current is regulated by a fast current regulator loop which is controlled by either a voltage control loop or a reference current. The controller also uses input and output voltage feedforward. Changes of the input and output voltages are monitored and immediately change the duty cycle in the modulator to achieve a fast response to those errors. The voltage error amplifier gets its feedback input from the FB pin. For the adjustable output voltage version, a resistive voltage divider must be connected to that pin. For the fixed output voltage version, the FB pin must be connected to the output voltage to directly sense the voltage. Fixed output voltage versions use a trimmed internal resistive divider. The feedback voltage is compared with the internal reference voltage to generate a stable and accurate output voltage. The reference current for average output current control is programmed with a resistor connected between the RI pin and GND.

The programming of the average output current also affects the maximum switch current in the main switch which basically is the input current. The lower the average output current is programmed, the lower the maximum input current. Now, maximum input power is controlled as well as the maximum peak current to achieve safe and stable operation under all possible conditions. Smaller inductors with lower saturation current ratings can be used, when lower average output currents are programmed.