8.2.2.5 Step Five: Minimum Load DC COMP Voltage

The TPS54218 implements a minimum COMP voltage clamp for improved load-transient response. The COMP voltage tracks the peak inductor current, increasing as the peak inductor current increases, and decreasing as the peak inductor current decreases. During a severe load-dump event, for instance, the COMP voltage decreases suddenly, falls below the minimum clamp value, then settles to a lower DC value as the control loop compensates for the transient event. During the time when COMP reaches the minimum clamp voltage, turnon of the high-side power switch is inhibited, keeping the low-side power switch on to discharge the output voltage overshoot more quickly.

Proper application circuit design must ensure that the minimum load steady-state COMP voltage is above the +3 sigma minimum clamp to avoid unwanted inhibition of the high-side power switch. For a given design, the steady-state DC level of COMP must be measured at the minimum designed load and at the maximum designed input voltage, then compared to the minimum COMP clamp voltage shown in Figure 22. These conditions give the minimum COMP voltage for a given design. Generally, the COMP voltage and minimum clamp voltage move by about the same amount with temperature. Increasing the minimum load COMP voltage is accomplished by decreasing the output inductor value or the switching frequency used in a given design.