5 Optimizing Load Transient Response

Virtually all electronic devices require a supply voltage that is held within a specific tolerance range, even when the load current varies. The voltage regulator must be able to hold it's output voltage constant as the load current demand varies anywhere from zero up to full load, even if the change occurs in a relatively short time. How much the output voltage changes will be dependent on the speed of the regulator, amount and type of output capacitor and the di/dt of the load current change. The di/dt of the load current is related with the application, thus the amount and type of the capacitor and the speed of the regulator are the two main things to improve how much the output voltage changes.

During the time interval while the regulator's control loop is adjusting for the change in load, the only thing supplying the difference in load current between the former steady-state value and the new load current value is the output capacitor. So the output capacitors will significantly affect transient response. The most obvious way to improve the output voltage deviation is to use a larger capacitance. Figure 5-6 shows the load transient of TPS563257 with 2.2 uH inductor and 10 uF capacitor. Figure 5-2 shows the load transient of TPS563257 with 2.2 uH inductor and 22 uF *3 capacitor.

The faster the regulator's control loop can respond, the less the output capacitor's voltage will change before the loop corrects to the new value of the load current. The response time of the controller is directly related to the bandwidth of the control loop. A higher bandwidth allows the controller to respond faster. Since for TPS56x25x family, it is internal compensated and the bandwidth is primarily affected by the corner frequency of the LC filter, which forms a double pole in the contrl loop. As shown in Table 3-1, higher LC corner frequencies allow for higher control loop bandwidths. To increase the LC corner frequency, decrease the product of the inductance and capacitance or choose appropriate feedforward capacitor.