A feedforward capacitor (C_FF) is recommended to be connected between the OUT pin and the FB pin. C_FF improves transient, noise, and PSRR performance. A higher capacitance C_FF can be used; however, the start-up time increases. For a detailed description of 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 Parasitic Capacitor at the FB Pin

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

Adding a feedforward capacitor (C_FF), as shown in Figure 7-4, creates a zero in the loop gain transfer function that can compensate for the parasitic pole created by C_FB. Equation 9 and Equation 10 calculate the pole and zero frequencies.

Figure 7-4 Feedforward Capacitor Can Compensate the Effects of the Parasitic Capacitor

The C_FF value that makes f_P equal to f_Z, and result in a pole-zero cancellation, depends on the values of C_FB and the feedback resistors used in the application. Alternatively, if the feedforward capacitor is selected so that C_FF ≫ C_FB, then the pole and zero frequencies given by 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, implying that the frequencies are located close to each other and therefore the parasitic pole is compensated. Even for large V_OUT values, where this ratio can be as large as 20, a C_FF value in the range 100pF ≤ C_FF ≤ 10nF typically helps prevent instability caused by the parasitic capacitance on the feedback node.