In the previous article, we introduced the fundamental role of the inductor in a Class-D amplifier: it forms, together with a capacitor, LC low-pass filter , attenuate the PWM switching carrier while preserving the audio signal. However, in practical circuits, two additional issues arise— ① 　Why do BTL audio channels that use LC output filtering typically require two inductors? ② 　Can two separate inductors be used directly? Why bother combining them into a single unit? 1 Why does a BTL channel require two inductors? BTL stands for… Bridge-Tied Load , commonly translated into Chinese as “bridged load” or “bridge‑type load,” refers to speakers that are bridged across the same channel. OUT+ and OUT− Between, two output terminals Opposite phase , driven by the voltage difference between the two. OUT+ and OUT− They are all High-frequency switching output terminal , all of them generate PWM waveforms and require filtering before being fed to the speaker. Therefore, in order to suppress the PWM switching carrier and high-frequency components, OUT+, OUT− Two switch branches typically each require Series connection One grain Inductance . and Output capacitor Together constitute Differential LC Low-Pass Filter Network 。

When using a differential LC output filter, the OUT+ and OUT− branches typically each include an inductor. For example, TI… TAS6424-Q1 , the 4 BTL channels provide a total of 8 power output paths, thus requiring 8 inductor units 。


 

Figure 1 | Typical four-channel Class-D amplifier circuit based on the TI TAS6424‑Q1. If discrete inductors are used, they must be mounted. 8 pieces ; After adopting the two-in-one inductor, with each package integrating L1 and L2 of the same audio channel, the number can be reduced to: 8 inductor units → 4 two-in-one devices What is reduced is the number of packages, not the number of inductor units. For BTL output, what is even more critical is— Are the two inductors in the same channel identical? 。2 Can an independent inductor be used? It can be used directly, but it will require separate mounting space. Mounting position, and the two components’ There may be some variation in the inductance value, DCR, and temperature characteristics. For differential BTL outputs, in addition to ensuring that each inductor individually meets its specifications, it is also necessary to consider… Consistency between L1 and L2 。

For example, an individual inductor rated at 3.3 μH with a tolerance of ±20% has a specification range of: 

2.64 μH to 3.96 μH 

Within the specification limits The worst-case theoretical combination Under these conditions, the inductance difference between the two devices can be as high as: 

1.32 μH 

However, it should be noted that this is merely a specification‑limit calculation and does not reflect the typical variation observed among devices from the same production batch. 

3 How can the two-in-one be resolved? 

With SPFDA1009S-3R3MT For example, the same channel’s… L1, L2 Integrated within a single package, with consistency control implemented across both units. 

By means of paired screening, the difference in inductance values between the two units is kept within ±0.1 μH , improve the consistency of the two filtering branches.

Figure 2 | Comparison of ±20% tolerance for a single inductor and ±0.1 μH matching error for a two‑in‑one, two‑unit configuration4 What are the characteristics of a 2-in-1 inductor? 1. Reduce PCB footprint Integrate L1 and L2 within the same package. The four-channel BTL power amplifier can deliver… 8 independent inductors Reduced to 4 2-in-1 devices , simplifies the layout and helps shorten the output filtering loop.

Figure 3 | Comparison of Package Structures 2. Enhancing the consistency of the two filter branches is beneficial for EMI and common-mode noise suppression. Two units adopt Paired design , by controlling the inductance and DCR differences between the two units, the OUT+ and OUT− filter branches are made more symmetrical.

Figure 4 | Consistency Comparison of the Filtering Branch 5 Lika Electric Control: Advantages of the Two-in-One Inductor Unlike traditional Class-D amplifiers, which use inductors… Round wire + cold pressing Compared to the method, Lika Electric Control SPFDA Series Adopt Metal powder cores, T-CORE hot-pressing technology, and flat enameled copper wire ：

Advantage One: Electroplated Electrodes vs. Welded Electrodes A copper–nickel–tin electroplated layer is deposited on the magnet’s surface to form an electrode, resulting in an integral monolithic structure with exceptionally high vibration resistance. The lead-out terminal strength is as high as 500 N. , with vibration and shock resistance exceeding 10G, it is well-suited to the complex operating environments of automotive electronics.

Advantage Two: T-CORE Hot-Pressing Process vs. Cold-Pressing The T-CORE hot-pressing process ensures tighter molding of the magnetic core, Enhance inductor saturation characteristics , reducing core losses and minimizing damage to the wire. 
 

Advantage Three: Flat Wire vs. Round Wire By employing flat-wire winding for the coil, the overall DCR of the inductor is reduced, significantly improving high-frequency current performance. Skin effect , with overall output power capacity superior to that of its peers.

5 Two-in-one power amplifier How do you choose a feeling? 

Taking the existing data for SPFDA1009S-3R3MT as an example: parameter, typical specification, nominal inductance. 3.3 μH±20% Two-unit inductance matching error ±0.1 μH DCR 5.4 mΩ typical / 7.1 mΩ maxIrms 14 .0 A typ , ΔT = 40 K Isat 28.0 A typ , ΔL% = 30% of the rated voltage 75 VDC In practical selection, one should also make a comprehensive assessment based on the power amplifier’s switching frequency, speaker impedance, filter capacitor values, output power, ambient temperature, and the PCB’s thermal‑dissipation conditions. It also needs to be combined with Current, temperature rise, frequency, and impedance curves Assess their actual work capabilities.

Figure 5a | Impedance vs. Frequency

Figure 5b | Temperature Rise vs. Current 
For two-in-one Why do BTL audio channels employing LC output filtering typically require two inductors?

Class-D amplifier inductors—besides inductance value, DCR, and rated current—customers also pay attention to the inductor’s actual audio performance when integrated into the system, such as: THD+N across the full frequency range, distortion variation at different output power levels, and dual-channel consistency. 。
The figure below shows the SPFDA1009S-100MT after being connected to a Class-D amplifier system. THD+N Frequency Sweep Test Example , used to assess the system’s distortion levels and inter-channel consistency across different audio frequencies.

Figure 5c | Full‑band THD+N sweep and dual‑channel consistency after incorporating the two‑in‑one inductor 

As shown in the figure, Lika’s electronically controlled filter inductors exhibit excellent consistency. The inductance value, DCR (DC resistance), and self-resonant frequency all demonstrate low parameter variation, ensuring uniform frequency response across multiple channels and consistent EMI filtering performance for the entire system. This reduces overall debugging complexity, minimizes variations in sound quality and EMC characteristics between production batches, and facilitates large-scale mass production of power amplifier products. 

Conclusion 

For a BTL Class‑D amplifier employing LC output filtering, both OUT+ and OUT− typically require filter inductors. 

A two-in-one power amplifier inductor is not simply “two inductors crammed into a single housing”; its primary value lies in: 

Reduce the number of packages, save PCB footprint, and improve the matching between the two filter branches in the same channel. 

When selecting components, in addition to the nominal inductance value, you should also pay particular attention to: 

Two‑unit matching error, DCR, Irms, Isat, temperature rise, and magnetic shielding performance.