5.10 Output Current Sensor and Short-Circuit Protection
The Output Rectifier stage is followed by output filter. This LC filter is common to buck derived topologies. To implement Average Current mode control on the output inductor, a shunt-based current sensor is implemented. It is important to place this current sensor before the Output Capacitor stage as it helps to see the real inductor current for the average current control loop implementation. Placing the current sensor at this location also helps enable/disable SRs above the worst-case CCM/DCM (30±4A) limit to protect against reverse currents coming from the Output Capacitor stage going back into SR MOSFETs. This can potentially destroy output SR MOSFETs, therefore, a reverse current protection becomes almost necessary for controlling this topology.
Two 0.2mR shunt resistors are used in parallel to increase current sensing capability, even though the average output current is limited to IOUT = 180A for safe operation. The output inductor current can go higher to 250 Apk during load transients. Therefore, the current sensor is designed with a maximum limit of 250 Apk giving an output of ISENSE = 3V@250 Apk.
An offset of 500 mV is added and maximum control output voltage of the current sensor is limited to 3V by design to add provision for implementing Peak Current mode control using the DAC+slope compensation peripheral. This adds to the advantage of very fast control loop execution as current control work purely in peripherals and only a digital voltage loop is implemented by the CPU.