The LM644A2-Q1 continues to regulate output voltage. This statement is true even if the input-to-output voltage ratio requires an on-time less than the minimum on-time of the chip with a given clock setting. This requirement is accomplished using valley current control. At all times, the compensation circuit dictates both a maximum peak inductor current and a maximum valley inductor current. If, for any reason, valley current is exceeded, the clock cycle is extended until valley current falls below that determined by the compensation circuit. If not operating in current limit, the maximum valley current is set above the peak inductor current. This prevents valley control from being used unless there is a failure to regulate using peak current only. If the input-voltage to output-voltage ratio is too high, even though current exceeds the peak value dictated by compensation, the high-side device cannot be turned off quickly enough to regulate output voltage. See t_{ON_MIN} in the Electrical Characteristics. As a result, the compensation circuit reduces both peak and valley current. After a low enough current is selected by the compensation circuit, valley current matches that being commanded by the compensation circuit. Under these conditions, the low-side device is kept on and the next clock cycle is prevented from starting until inductor current drops below the desired valley current. Because on-time is fixed at the minimum value, this type of operation resembles that of a device using a COT control scheme. See Figure 7-21.

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

Figure 7-21 Valley Current Mode Operation