TPS53355: current limit

Hamamoto-san,

Valley current limit keeps the on time constant while extending the off time to reduce current below the limit. So it would be natural to think of using this type of current limit in converters using constant on time control modes.   Constant on time control sets the on time to a fixed value and uses a hysteretic comparator to end the off time when the fed back portion off the output voltage falls below a preset level. The valley current limit will override this control signal if the current is above the limit, increasing the off time. So here we come to an advantage of valley current limit over peak current. Valley current limit is applied at the end of the off time, before any switching transition. There is no need for any blanking time.

Another consideration is the physical location of the sensing circuit. For current limit, the actual current is not used, but instead it is a voltage proportional to the current. This may be derived directly from the respective switch or more typically from a smaller mirrored element. For current mode control, this sensing element is at the high side switch and is sensed relative to the input voltage rail. The input voltage may be variable of a large range and may also have significant ripple. Valley current limit on the other hand senses the low side switch current and the sensing element is referenced to the much quieter and constant circuit ground. While this is an advantage, it does have a significant limitation as well. Since valley current limit senses current at the low side switch, it is generally limited to synchronous converters.