Nonlinear features improve the controller response to severe repetitive load transient conditions.

When the controller is subjected to load transients at very high frequency, the output voltage may not be able to completely settle before the next transient event occurs. As a result, particularly during overshoot events, when the controller is firing pulses infrequently, the controller integration path can see error which does not completely settle. Accumulation of large overshoot error can cause the controller response to following undershoot events to be slower. To prevent excess accumulation of error during repetitive load transient events, the controller implements dynamic integration. When the output voltage overshoots its target by a certain voltage, V_DINT, the controller integration time constant can be changed to an alternate value, the dynamic integration time constant. Use the COMPENSATION_CONFIG command to configure the dynamic integration time constant and threshold voltage. Typically, set the dynamic integration constant to a longer time than the static integration time constant.

Figure 7-25 Dynamic integration, OSR, USR detection

Systems which use the dynamic phase shedding feature, may still have sudden and severe load transient events occur. The undershoot reduction (USR) feature allows the controller to add phases even before the output current reaches the dynamic phase adding thresholds. This ensures the transient undershoot event is stopped as quickly as possible. TPS53676 has two levels of USR. The USR1 threshold is used to quickly enable a configurable number of phases, USR1_PH. The USR2 threshold adds all enabled phases, assigned to that channel. Use the NONLINEAR_CONFIG command to configure the USR1 and USR2 features.

The overshoot reduction (OSR) feature reduces output voltage overshoot during severe load transient events, by turning off the low-side FETs of the powerstage devices (e.g. tri-stating the controller PWM pins), when an overshoot event occurs. The inductor current of each phase must remain continuous, forcing the output current through the body diode of each low-side FET. This dissipates excess energy more quickly than keeping the powerstage low-side FET fully conducting, due to the forward voltage drop characteristics of the body diodes. As a result, the transient overshoot is smaller when this technique is used, compared to simply turning on the low-side FET of each powerstage. However, this results in excess heat which must be properly managed in systems with highly repetitive transient conditions. Additionally, TPS53676 can be configured to truncate PWM pulses, to reduce the worst-case response time to overshoot events. The NONLINEAR_CONFIG command provides four controls for overshoot reduction: an enable bit for diode braking, an enable bit for pulse truncation, the OSR threshold, V_OSR, and the diode braking timeout, which limits the maximum amount of time during which diode braking takes place, to manage excess heating. Refer to the Technical Reference Manual for a register map of this command.