The converter continues to regulate the output voltage even if the input-to-output voltage ratio requires an on time less than the minimum on time of the converter with a given clock setting. This is accomplished using valley current control as shown in Figure 7-24.

In valley control mode, the inductor valley current is regulated, not inductor peak current.

Figure 7-24 Valley Current Operation

At all times, the compensation circuit dictates maximum peak and valley inductor currents. If for any reason, the valley current setpoint is exceeded, the clock cycle is extended until the valley current falls below that determined by the compensation circuit. If the converter is not operating in current limit, the maximum valley current is set above the peak inductor current, preventing valley control from being used unless there is a failure to regulate solely using peak current. If the input-to-output voltage ratio is too high, even though current exceeds the peak value dictated by compensation, the high-side switch cannot be turned off quickly enough to regulate the output voltage. As a result, the compensation circuit reduces both peak and valley currents. After a low enough current is established, the valley inductor current matches that being commanded by the compensation circuit. Under these conditions, the low-side switch is kept on and the next clock cycle is delayed until the inductor current drops below the desired valley current threshold. Because the on time is fixed at the minimum value, this type of operation resembles that of a device using a constant on-time (COT) control scheme.