Connect a feedforward capacitor (C_FF) between the OUT pin and the FB pin. C_FF improves transient, noise, and PSRR performance. If a higher capacitance C_FF is used, the start-up time increases. For a detailed description of the C_FF tradeoffs, see the Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator application note.

As shown in Figure 7-3, poor layout practices and using long traces at the FB pin results in the formation of a parasitic capacitor (C_FB).

Figure 7-3 Formation of a Parasitic Capacitor at the FB Pin

C_FB, along with the feedback resistors R₁ and R₂ potentially results in the formation of an uncompensated pole in the transfer function of the loop gain. A C_FB value as small as 6pF potentially causes the parasitic pole frequency, given by Equation 8, to fall within the LDO bandwidth and result in instability.

Adding a feedforward capacitor (C_FF) creates a zero in the loop gain transfer function that compensates for the parasitic pole created by C_FB. Figure 7-4 shows this compensation. Equation 9 and Equation 10 calculate the pole and zero frequencies.

Figure 7-4 A Feedforward Capacitor Compensates the Effects of the Parasitic Capacitor

The C_FF value that makes f_P equal to f_Z depends on the values of C_FB and the feedback resistors used in the application. This C_FF value also results in a pole-zero cancellation. Alternatively, if the feedforward capacitor is selected so that C_FF ≫ C_FB, then the pole and zero frequencies from Equation 9 and Equation 10 are related as:

In most applications, particularly where a 3.3V or 5V V_OUT is generated, this ratio is not very large. Thus implying that the frequencies are located close to each other and therefore the parasitic pole is compensated. A C_FF value of approximately 100pF ≤ C_FF ≤ 10nF typically helps prevent instability caused by the parasitic capacitance on the feedback node. This C_FF range helps even for large V_OUT values, where this ratio is potentially as large as 20.