SLYY205A March 2021 – October 2022 TPS562211 , TPS562212 , TPS562231 , TPS563211 , TPS563212 , TPS563231 , TPS56339

Smart loop-bandwidth control

Unlike PCM control, where direct inductor current information is in the loop, AECM uses emulated inductor current information. The output filter of AECM control introduces one double pole like the D-CAP2 control topology. Thus, the Bode plot of AECM control is similar to the D-CAP2 control topology.

In D-CAP2 control, the A_cp is constant and the DC gain of the open-loop transfer function changes with V_OUT. While in AECM control, the A_cp adapts to the changing value of R₂ per the V_OUT setting to keep the A_cp × V_OUT a constant value for a fixed DC gain. As shown in Figure 13, assuming that the double poles for different outputs are the same, the loop bandwidths under different outputs should be much closer to each other when compared to the loop bandwidths of D-CAP2 control.

Figure 13 Bode plot under different V_OUT conditions of AECM.

Figure 14 shows the measured Bode plot of AECM control under different VOUT conditions. The DC gains are almost the same. The crossing frequency and phase margin have slight differences because of the output double-pole shift.

Figure 14 Measured Bode plot of 5-V and 1.05-V outputs.

Table 1 Comparing PCM and AECM buck converters.

	Traditional PCM Buck Converter (internal compensation)	AECM Buck Converter
Load- transient response	Slow The fixed internal zero is set relatively low. Error amplifier delay.	Fast Smartloop-bandwidth control provides a relatively high internal zero. No error amplifier delay.
Light load pulse	Single or nonsingle. Pulse controlled by the clamping control voltage.	Single Pulse controlled by the on-time under PFM operation mode.
Wide output stability	Difficult The fixed and relatively low internal zero makes it hard to support a wide output range.	Easy Smartloop-bandwidth control provides an adjustable bandwidth.

Table 2 Comparing D-CAP2 buck converters and AECM buck converters.

	D-CAP2 Buck Converter	AECM Buck Converter
Frequency	Pseudo-fixed and hard to support high frequencies. Frequency depends on the on-time generator, resulting in a large frequency variation.	Truly fixed and easy to support a high frequency. The frequency depends on the internal clock, resulting in a small frequency variation.
High V_OUT	Less than 7 V Limitation from the internal emulated ramp-generator circuit.	Higher than 7 V Improved internal emulated ramp-generator circuit and smart loop-bandwidth control.
Large duty cycle	Difficult Limitation from the internal emulated ramp-generator circuit. Requires a long minimum off-time.	Easy Improved internal emulated ramp-generator circuit and smart loop-bandwidth control. Requires a short minimum off-time. On-time extension function.