7.3.2 Load Step Response

The LMR10530 has a fixed internal loop compensation, which results in a small-signal loop bandwidth highly related to the output voltage level. In general, the loop bandwidth at low voltage is larger than at high voltage due to the increased overall loop gain. The limited bandwidth at high output voltage may pose a challenge when loop step response is concerned. In this case, one effective approach to improving loop step response is to add a feed-forward capacitor C_FF) in the range of 27 nF to 100 nF in parallel with the upper feedback resistor (assuming the lower feedback resistor is 2 kΩ), as shown in Figure 21. The feedforward capacitor introduces a zero-pole pair which helps compensate the loop. The position of the zero-pole pair is a function of the feedback resistors and capacitor:

Equation 1.

Equation 2.

Note the factor in parenthesis is the ratio of the output voltage to the feedback voltage. As the output voltage gets close to 0.6V, the pole moves towards the zero, tending to cancel it out. Consequently, adding C_FF will have less effect on the step response at lower output voltages.

As an example, Figure 23 shows that at the output voltage of 3.3 V, 47 nF of C_FF can boost the loop bandwidth to 117kHz, from the original 23kHz as shown in Figure 22. Correspondingly, the responses to a load step between 0.3 A and 3 A without and with C_FF are shown in Figure 24 and Figure 25 respectively. The higher loop bandwidth as a result of C_FF reduces the total output excursion by more than half.

Aside from the above approach, increasing the output capacitance is generally also effective to reduce the excursion in output voltage caused by a load step. This approach remains valid for applications where the desired output voltages are close to the feedback voltage.

Figure 21. Adding a C_FF Capacitor

Figure 22. Loop Gain and Phase Without C_FF

Figure 24. Load Step Response Without C_FF

Figure 23. Loop Gain and Phase With C_FF

Figure 25. Load Step Response With C_FF