4 A Simple Solution

The actual output ripple in real system circuit is not always equal to the value measured in the EVM. And this high frequency noise doesn’t always impact the system loading:

• If there is an LDO between the boost output and the loading, The LDO and its input and output capacitor would help to filter such high frequency noise.
• If there is long cable between the boost output and the loading, the ESR, ESL of the cable and the bypass capacitor closed to the loading forms RLC filter to reduce the high frequency noise.
• Such high frequency noise may not impact the performance the system loading.

But if the loading is sensitive to the high frequency noise, or EMI performance is critical to system, a ferrite bead with high impedance at high frequency can greatly reduce such noise. The location of the ferrite bead is shown in Figure 4-1. The impedance of the FB should be much higher than C_OUT2 at frequency closed to 300 MHz.

Taking BLM18KG30TH1 as an example, its impedance is higher than 35Ω at frequency from 200 MHz to 1000 MHz. the impedance is much higher than a capacitor as in Figure 4-2.

After adding ferrite bead and moving one of three capacitors to location C_OUT2 in the TPS61022EVM, The output voltage ripple is shown in Figure 4-3. The high frequency noise is greatly reduced and the ripple is mainly the switching frequency ripple.