Figure 4-4 shows how capacitors are placed for both input and output side of IBB converter. The input capacitor, C_IN is required to provide a low-impedance input voltage source to the converter. A low equivalent series resistance (ESR) X5R or X7R ceramic capacitor is best for input voltage filtering and minimizing interference with other circuits. As shown in Figure 3-3, PMP23333 uses 3 x 10µF ceramic capacitors (C2, C3, C4) from V­_IN to ground (system ground, not –V_OUT). Note that C1 is only used for validation. The C_IN capacitor value can be increased without any limit for better input voltage filtering.

Figure 4-4 Inverting Buck-Boost with Additional Capacitors C_IO, C_{IO_HF}

There is importance to place new bypass capacitors C_IO, C_{IO_HF} for IBB topology to provide a low impedance source for the internal gate drivers. C_IO, C_{IO_HF} can be connected from V_IN to -V­_OUT. Therefore, the capacitors must be properly sized for the voltage difference between V_IN and -V­_OUT. The values for the bypass capacitance, C_IO, C_{IO_HF} can be chosen using input capacitance recommendations from the buck converter data sheet. These bypass capacitors provide an AC path from V_IN to -V­_OUT. When V_IN is applied to the circuit, this dV/dt across the capacitor from V_IN to -V­_OUT creates a current that must return to ground (the return of the input supply) to complete the loop.

PMP23333 uses 2 x 2.2uF, 2 x 220nF (C12 approximately C15) as input bypass capacitors. As shown in Figure 4-5 to Figure 4-10, installing bypass capacitors can reduce the input, output voltage ripple and improve the transient response. Without the bypass capacitors, the converter cannot filter the switching noise which is around 400kHz in PMP2333 design. It also causes worse transient performance which can impact to the system. Therefore, installing proper C_IO, C_{IO_HF} can reduce the size of C_IN, C_OUT with optimization.

Figure 4-5 V_IN Ripple Waveform With C12 - C15

Figure 4-7 V_OUT Ripple Waveform With C12 - C15

Figure 4-9 V_OUT Transient Waveform (0-2.7A) With C12 - C15

Figure 4-6 V_IN Ripple Waveform Without C12 - C15

Figure 4-8 V_OUT Ripple Waveform Without C12 - C15

Figure 4-10 V_OUT Transient Waveform (0-2.7A) Without C12 - C15

In IBB converter, the output current is discontinuous. The output capacitors supply energy to the load during the on time when energy stored in the inductor is increasing. During the off time, the inductor is delivering energy to both load and the output capacitors. The output capacitance can also follow buck converter data sheet recommendation. PMP23333 uses 5 x 22uF. The output capacitance can be increased or reduced by the output ripple and transient requirements.