Here is a cut out of the TPS92070 with some of the internal circuitry. The top left most arrow coming from the resistor divider provides a signal to the SENSE pin; this signal is representative of the rectified AC line, including any phase cutting. It is represented by the topmost waveform, a blow up of which can be seen in the blue circle. The DTC pin contains a couple of internal switches that provide a current path from the dimmer through the bridge diodes to ground. As the AC line approaches zero crossing, the weaker transistor (which can only sink up to 250 µA) can be turned on. This is done to start discharging the EMI capacitor in the TRIAC circuit. As the line voltage falls further, ~10 V for instance, the stronger transistor (capable of sinking up to 20 mA) can be turned on. This provides a path for the TRIAC trigger current to flow. When the TRIAC conducts after the zero cross, this path to ground is no longer needed and the transistors are turned off to minimize power consumption during the rest of the line cycle. One last thing to note from this slide is that the two threshold comparators in the chip feed a flip-flop that creates a phase signal proportional to the TRIAC conduction angle. The next slide will examine how this signal is used to control the dimming.