The definition of propagation delay (T_pd) is the period between the input current reaching 10% of the final value and the output voltage reaching 10% of the final value, for an input current step sufficient to cause a 1V change in the output voltage, as shown in Figure 4-2.

Propagation delay is an important parameter during normal operation. For solar applications, average current sampling control is usually used. A large propagation delay can cause the sampling signal to have a big error compared with the real value. As shown in Figure 4-3, the yellow curve is the original input current, red curve is the hall sensor's signal chain output. Although the leading competitor has a higher bandwidth, due to high propagation delay, at the triggering point of ADC sampling, the output of the hall sensor experiences a 1.1us delay compared to the input current. This 1.1us delay causes a large additional offset error when sampling the average value of a triangular current in DC/DC converter. As shown in Figure 4-4, due to a much lower propagation delay, when sampling the average current, this helps minimize the additional offset error, makes the sampling signal chain more accurate.

Figure 4-3 Propagation Delay Comparison of Triangular Current Measurement: Leading Competitor: BW Approximately 400kHz, Tprop Approximately 1.1µs

Figure 4-4 Propagation Delay Comparison of Triangular Current Measurement: TMCS1123: BW Approximately 250kHz, Tprop Approximately 0.4µs

As shown in Figure 4-5, at AC current sampling side, due to the time-varying characteristic of grid voltage (in red curve), the voltage applied to the boost inductor is also changing (in green curve). According to L*di/dt=ΔV, the slew rate of inductor current, di/dt, is time-varying. This varying slew rate causes the additional offset error also be time-varying, as shown in Figure 4-5.

For example, to see the time varying offset error more obviously, in the peak of grid voltage, the slew rate of inductor current is 0.156A/µs, so a 3.2µs total signal chain delay can bring a 0.5A offset to the AC current sensing at the grid voltage peak. But in the middle of grid voltage, the slew rate of inductor current is 0.466A/µs, so a 3.2 µs total delay can bring a 1.5A offset to the AC current sampling result. Figure 4-5 shows a high signal chain delay that brings a big time-varying error to the current loop in AC current sampling.

A time-varying offset error is hard to compensate for, which makes AC output current have worse total harmonic distortion (THD). TI's in-package Hall sensor has a very low propagation delay, which minimizes this time-varying error.

Figure 4-5 Time Varying Offset Error with Respect to Slew Rate

To more intuitively reflect the impact of the time varying offset error, the simulation is done by adding two different T_pd to the feedback of inductor current while keeping other parts of the control loop the same. The current control is a normal PI controller. As shown in Figure 4-6, the inductor current is filtered with a 10kHz low pass filter to lower down the switching frequency ripple and see the current waveform more clearly.

In this example, red curve is with 1us T_pd, inductor current THD(iTHD) is 4.36%. The green curve is with 3us T_pd, iTHD is 4.73%, 0.37% higher than 1us T_pd delay example. It's obvious that a lower propagation delay can decrease the time varying offset error and improve the output current THD. Another point can be observed in the right part of Figure 4-6, is the third harmonic component is 80% lower with a 2us lower T_pd, this helps the converter have a 2% more accurate amplitude in current (0.7A).

Figure 4-6 Inductor current with Different Propagation Delays