SLVAF90 October   2021 TPS63805

 

  1.   Trademarks
  2. 1Introduction
  3. 2TPS63805 Features
  4. 3LC Filter Design for Extreme Low Output Voltage Ripple
  5. 4Summary
  6. 5References

3 LC Filter Design for Extreme Low Output Voltage Ripple

After adding a LC filter on the output side of buck-boost, the schematic is shown as Figure 3-1. To get lower than 10mV ripple, the LC filter will be designed to make an attenuation of 20dB at switching frequency. To effectively attenuate the output ripple, the resonant frequency must be lower than switching frequency.

Figure 3-1 TPS63805 Circuit with LC Filter

To avoid affecting loop stability, LC filter need to be configured cautiously. LC filter will bring double poles to the loop and might cause lower phase margin, so the resonant frequency must be far away from the gain crossover frequency fc at which the open loop gain of the buck-boost is zero. LC filter with high Q might lead to a low gain margin at resonant frequency due to peaking, so the resonant frequency should also be kept away from the phase crossover frequency fx where open loop phase is zero. Besides, a damping resistor could be used to damp the Q. Details are given in Design a second-stage filter for sensitive applications.

The frequency response tested on the TPS63805EVM-026 with default configuration is shown in Figure 3-2, the gain crossover frequency is 49.75kHz and the phase crossover frequency is 205.22kHz.

Figure 3-2 Bode Plot of TPS63805 with Default Configuration

According to the TPS63805 data sheet, the switching frequency of buck-boost mode is 1.4MHz. Since the gain of LC filter will drop with a slope of -40dB/dec, the desired resonant frequency is calculated by Equation 1.

Equation 1. f=fsw10A40=442.7kHz

where

  • fsw is switching frequency.
  • A is desired attenuation at switching frequency.

The inductor and capacitor can be calculated by Equation 2.

Equation 2. f=12πLC

Table 3-1 and Table 3-2 shows the LC filter configuration. This LC filter is added on the output side of TPS63805EVM-026 to filter the output ripple.

Table 3-1 Inductor of LC Filter Configuration
Inductor Value[μH] Saturation Current[A] DCR [mΩ] Part Number Manufacture Size(LxWxH mm)
0.33 6.3 21 DFE201612E-R33M MURATA 2.0×1.6×1.2
Table 3-2 Capacitor of LC Filter Configuration
Capacitor Value[μF] Voltage Rating[V] ESR [mΩ] Part Number Manufacture Size(LxWxH mm)
0.33 25 15 885012206074 Würth 1.6×0.8×0.8

Figure 3-3 shows the TPS63805 reference circuit used as voltage stabilizer.

Figure 3-3 Reference Circuit of Voltage Stabilizer

As shown in Figure 3-4, the frequency response is not much affected after adding the LC filter. Figure 3-5 shows the output voltage ripple filtered by LC, which is kept below 5mV when loading 1 A.

Figure 3-4 Bode Plot of TPS63805 with LC Filter
Figure 3-5 Output Voltage Ripple with LC Filter