LC Filter
4.5 LC Filter
The LC filter serves two purposes in this design.
1)Reduces EMI
2)Enables overcurrent (OC) protection.
The outputs of the TAS5111 are square waves with fast rise and fall times. The square waves produce harmonics up to 500 MHz. The speaker wire makes transmission lines for these frequencies. The LC filter attenuates the high frequencies that are transmitted over the speaker wires and increase EMI. The LC filter is shown in Figure 4−9.
Figure 4−9. APA100 Output Filter
TAS51xx
Output A | L |
| |
| C1A |
| C2 |
| C1B |
Output B | L |
|
R(Load)
To limit
The inductor must have 8 ∝H of inductance or more at 15 A, for overcurrent (OC) protection to be effective. The TAS5111 data sheet recommends 5 ∝H of inductance, but that is for a switching frequency of 380 kHz. The APA100 switches at 250 kHz and needs more inductance to protect the device.
The modulation scheme used by the APA100 (based on the TPA2001D1 modulation scheme) can be used without a filter if EMI and OC protection are not important. For more information on the filter−free modulation, see the application section of the TPA2001D1 data sheet.