When a ceramic output capacitor is used, the stability criteria in Equation 2 cannot be satisfied. The ripple injection approach as shown in Figure 7-4 is implemented to increase the ripple on the VFB pin and make the system stable. In addition to the selections made using Section 7.2.2.2.1 through Section 7.2.2.2.5, use the information in the Section 7.2.2.3 section to select the ripple injection components. The C2 value can be fixed at 1 nF. Select a value for C1 between 10 nF and 200 nF.

Equation 14.

where

• N is the coefficient to account for L and C_OUT variation.

N is also used to provide enough margin for stability. TI recommends N = 2 for V_OUT ≤ 1.8 V and N = 4 for V_OUT ≥ 3.3 V or when L ≤ 250 nH. The higher V_OUT needs a higher N value because the effective output capacitance is reduced significantly with higher DC bias. For example, a 6.3-V, 22-µF ceramic capacitor can have only 8 µF of effective capacitance when biased at 5 V.

Because the VFB pin voltage is regulated at the valley, the increased ripple on the VFB pin causes the increase of the VFB DC value. The AC ripple coupled to the VFB pin has two components, one coupled from SW node and the other coupled from the VOUT pin and they can be calculated using Equation 15 and Equation 16 when neglecting the output voltage ripple caused by equivalent series inductance (ESL).

Equation 15.

Equation 16.

TI recommends that V_{INJ_SW} to be less than 50 mV. If the calculated V_{INJ_SW} is higher than 50 mV, then other parameters must be adjusted to reduce it. For example, C_OUT can be increased to satisfy Equation 14 with a higher R7 value, thereby reducing V_{INJ_SW}.

The DC voltage at the VFB pin can be calculated by Equation 17:

Equation 17.

And the resistor divider value can be determined by Equation 18:

Equation 18.