When using ceramic output capacitors, additional circuitry is required to stabilize the closed loop system. For this circuit, the external components are R3, C5, C6, and C7. To determine the value of these components, first calculate the LC resonant frequency of the output filter:

Equation 14.

For this example the effective resonant frequency is calculated as 4109 Hz

The network composed of R1, R2, R3, C5, C6, and C7 has two poles and two zeros that are used to tailor the overall response of the feedback network to accommodate the use of the ceramic output capacitors. The pole and zero locations are given by the following equations:

Equation 15.

Equation 16.

Equation 17.

The final pole is located at a frequency too high to be of concern. The second zero, Fz2 as defined by Equation 17 uses 2.5 for the frequency multiplier. In some cases this may need to be slightly higher or lower. Values in the range of 2.3 to 2.7 work well. The values for R1 and R2 are fixed by the 3.3-V output voltage as calculated usingEquation 12. For this design R1 = 10 k? and R2 = 5.90 k?. With Fp1 = 426 Hz, Fz1 = 2708 Hz and Fz2 = 8898 Hz, the values of R3, C6 and C7 are determined using Equation 18, Equation 19, and Equation 20:

Equation 18.

Equation 19.

Equation 20.

For this design, using the closest standard values, C7 is 0.1 μF, R3 is 590 ?, and C6 is 1800 pF. C5 is added to improve load regulation performance. It is effectively in parallel with C6 in the location of the second pole frequency, so it should be small in relationship to C6. C5 should be less the 1/10 the value of C6. For this example, 150 pF works well.

For additional information on external compensation of the TPS5420 or other wide voltage range devices, see SLVA237  Using TPS5410/20/30/31 With Aluminum/Ceramic Output Capacitors