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Yang Wu The TPS53819A uses a cycle by cycle, Off-time, valley current limit mechanism that limits the inductor current and places the converter into a constant output current mode until the load current decreases or the output voltage drops sufficiently to trigger the output under voltage protection fault. After an On-time completes, the highside FET turns off and the low-side FET turns on, there are 3 signals gating the start of a new On-time. When all 3 conditions are met, a new on-time starts. 1) Minimum Off-time of 320ns (typ). This ensures enough time for the On-time energy to be reflected on the output voltage and for the current sense on the low-side FET to stabilize. 2) Current Limit programmed by the TRIP pin. The voltage on the SW pin during the on-time is compared to a scaled version of the voltage on the TRIP pin to set the current limit. If the negative voltage on switch pin exceeds the current limit threshold, the Off-time will be extended until the inductor current drops below the current limit, limiting the inductor valley current during the off-time. 3) The Reference Voltage. This generates a new on-time when the output voltage is below the target output voltage as long as the minimum off-time has been met and the inductor current is less than the current limit. OCP is "triggered" when Conditions 1 and 3 are met, but Condition 2 is not because the inductor current is still above the Over Current Limit threshold when the reference attempts to trigger a new on-time. So, the current is actually sensed continuously during the low-side FET on-time, but it only affects operation when the inductor current exceeds the current-limit when a new on-time would be created. By extending the off-time to limit the inductor current, the duty cycle is increased, reducing the output voltage until it triggers under-voltage protection. This ensures a smooth transition from D-CAP regulation to pulse by pulse valley current limit while limiting the inductor current to protect against saturation and allowing the output capacitors to help ride through a momentary over-current condition, such as exceeding the current limit during a load-step recovery response. |