The internal overload timer in the UCC28630 uses the same output load current measurement that is used by the CC loop. This measurement tracks the power stage thermal stress, and protects the power stage against output overload. If the output is overloaded for too long such that the power stage would be over-stressed, then the PWM shuts down, and enters low-power mode for a time period of tRESET(long); thereafter the device discharges VDD to the VDD(reset) level and initiates a hiccup mode restart.
The overload timer operates by taking an estimate of output current, squaring it (assuming the power stage losses are dominated by resistive I2 losses) to produce (K x I2OUT), where K is a scaling gain factor. The overload timer is constantly running at every load level, and accumulates at a rate dependent on the difference between (K x I2OUT) and the previous level of the timer. If (K x I2OUT) is greater than the previous timer level, the timer level continues to increase; if (K x I2OUT) is less than the previous timer level, then the timer level decreases. At any steady load, the overload timer level eventually settles at a level proportional to I2OUT. Because the overload timer level adjusts at a rate dependent on the difference between (K xI2OUT) and the previous level, the timer initially reacts faster to larger differences, but over time settles exponentially at a level proportional to (K x I2OUT).
As shown in Figure 34, in both the first and second examples, the initial steady load allows the timer to integrate and settle at a level proportional to the load. The margin to the over-load trip level depends on the historical loading, lower prior average loading results in greater future over-load capability, and vice versa. The rate at which the timer reacts to different load steps is set by the chosen time constant (or response rate) per Table 1.
The overload timer can cope with pulsed loads and loads with a complex waveform. Because the rate of increase and decrease also depends on the load change from the previous load, it also times out faster for bigger overloads, or allows a smaller overload to run for much longer. The overload timer operates in both normal CV mode and overload CC mode, or a dynamic mix of both modes.