Reaching EMI targets with free for audio class D applications

YuQing Yang , Peter Cao, Xiaolin Qin HPA MPC Audio Power Amplifiers Introduction Attractive price gap between power inductor and Ferrite bead drive class-D audio amplifier filter design enter into inductor-free period. In the meanwhile, with Ferrite bead, filter’s cut off frequency will increased dramatically from several KHZ to several MHZ. It will weaken EMI suppression effect of the filter. So the requirement of the low EMI noise of class-D is raised up. In audio class D inductor free applications, good EMI results rely on board level tuning and proper PCB layout. Ferrite bead with a proper will slow down class-D output edge rate, however, it could also generate some ringing which may deteriorate conductive EMI, Zobel circuit is added to decrease this ringing.

This article will introduce some board level tuning skills, include ferrite bead selection principles (slow down edge rate), tuning methods (decrease rings), proper PCB layout etc. With TI’s latest EMI optimized audio Class D TPA3140D2, these solutions helped customer save lots of system design cost and achieved excellent audio performance.

Inductor free filter Inductor free design motivation is to use one low cost ferrite bead replace expensive inductor to achieve system level low EBOM target for customer. Ferrite bead is equal to a multilayer chip inductor. Limited by current Ferrite bead materials and manufacturing technology, it is very hard for this kind of inductor to tolerate both large current and high impedance. Take TOKO’s Multilayer chip inductor as example, most of inductor value will be < 1uH if engineer set the rated DC current target >2.5A. Another industry presenter SUNLORD ferrite bead serial (UPZ2012) also show similar conclusion: ferrite bead’s equal inductor value will be < 0.6uH if Max Rated Current requirement is >2.5A. Table 1 shows UPZ2012 series ferrite bead’s impedance at 100MHz, Max Rated Current and Max DC Resistance of different ferrite beads.

Table 1 Impedance vs Max current of 2012 package SMD ferrite bead Impedance Impedance Test Frequency Max Rated Current Max DC Resistance Thickness (Ω) (MHz) (mA) (mΩ) (mm) 120 100 4000 20 220 100 3000 40 330 100 2500 50 0.85±0.2 600 100 2000 90 1000 100 1500 120

As figure1 shows, the equal inductor of ‘120Ω@100MHz ferrite bead’ is 0.39uH, and for a 600Ω@100MHz ferrite bead, the equal inductor value is 1.59uH. 50Ω/(2*3.14159*20MHz) 200Ω/(2*3.14159*20MHz)

L=Z/(2*pi*f) 0.39uH=50Ω/(2*3.14159*20MHz) 1.59uH=200Ω/(2*3.14159*20MHz) Figure 1 Equal inductor value of ferrite beads Ferrite bead works equal as one parallel resonance circuit, as an inductor in low frequency domain (<100MHz), as a capacitor in high frequency domain (>100MHz), as a pure resistance at its own resonance frequency point. Using ferrite bead to setup an output filter, the basis is to use its inductance characteristic. Since every LC filter has its own resonance frequency, at this frequency point, the gain of filter is large and it will cause ringing after filter. R1 and C1 will absorb the ringing energy which caused by IC itself, usually use 10Ω and 330pF. R2 and C2 will absorb the ringing energy which caused by the filter itself.

Select proper ferrite bead to slower edge rate

a. Ferrite bead’s equal module b. Filter with ferrite bead Figure 2 Filter design with ferrite bead How to achieve low EMI target with inductor free filter?  Consideration1: ferrite bead selection to slow down edge rate serveral technologies had been implemented into TI devices to minimize conducted EMI noise within 5MHZ bandwdith (this frequency is usually ferrite bead filter’s cutoff frequency). Spread spectrum, phase-shift between L and R channel (stereo audio Class D) etc. are all show some help. For EMI band (>5MHz), especially when switching frequency is around 300kHz (for good efficency), experiment result show Slowing down edge rate is an effective method to decrease EMI.

Figure 3 edge rate @ different impedance ferrite bead In figure3, larger ferrite bead impedance can achieve slower edge rate of class-D output , with 600ohm@100MHz ferrite bead, class-D output can get the slowest edge rate output and finally achieve the best EMI result at high frequency band. However, large impedance means smaller Rated current. In table1, Max rated current is 2A with impedance=600ohm@100MHz. Take TV customer as the example: TV Application case: PVDD=12V, Speaker Load=8Ω, BD mode, ingore the Rdson and DC resistance of PCB and ferrite bead . Max current=12/8=1.5A. Engineer can use 600ohm@100MHz ferrite bead to design the filter in PVDD=12V/8Ω speaker case. Figure 4 shows ferrite bead’s effect to conductive EMI

Improve EMI@5MHz~30MHz

a. 120Ω@100MHz Ferrite bead +1nF b. 600Ω@100MHz Ferrite bead + 1nF Figure 4 Ferrite bead’s effect to conduct EMI Figure 5 shows ferrite bead’s effect to radiative EMI

a. 120Ω@100MHz Ferrite bead +1nF b. 600Ω@100MHz Ferrite bead + 1nF Figure 5 Ferrite bead’s effect to radiative EMI

 Consideration2: zobel network to minimize ringings. Figure6 show typical circuit we designed to decrease ringing effects on output filter circuit. R1 and C1 will absorb the ringing energy which caused by IC itself. R2 and C2 are designed to to absorb ringings which caused by the filter’s resonant frequency.

Tuning to decrease rings

Figure 6 Tuning to decrease rings and slower edge rate The 350ns period ringing (around 2.85MHz) in Figure7.a captured in conducted EMI test noise band, the energy had been attenuated a lot after zobel network, and more margin gain back. Table 2 filter and zobel network setting

Filter and zobel network setting C3 C2 R2 Idle current of PVCC Confgure1 1nF 1000pF 68Ω 65mA Confgure2(no zobel) 1nF DNP DNP 45mA Confgure3 2.2nF 1000pF 68Ω 80mA Confgure4(no zobel) 2.2nF DNP DNP 60mA

Horizontal: 200nS/div Vertical: 4V/div

a. amplitude-frequency curves of filter b. zobel network’s effect Figure 7 Tuning zobel network and capacitor (decrease rings and slower edge rate)

However, another challenge come out, Figure8 shows that 2MHz~4MHz band noise was deteriorated by rings (rings will get worse when class-D output current increase). Theoretically higher harmonic components should get lower amplitude, however the filter’s resonant frequency point let situation change. Look into Figure7.a, compare with Configure4, configure 3 has a better noise depress capability in 2MHz~5MHz band. Finally Configure3 show the best tuning result for ringing decrease, slower edge rate and good 2MHz-5MHz EMI margin achieved. Depress noise with configure 3

Figure 8 2MHz~4MHz band noise deteriorated by rings (configure4) PCB layout Figure 9 is the reference design board for TI’s inductor free audio Class D (TPA3140D2). Figure10 is the typical output application circuit schematic.

a. PCB area of filter (inductor free) b. PCB area of filter (with inductor) Figure 9 TPA3140 EVM board (left) save a lot of filter PCB area

Figure 10 TPA3140 Typical output application circuit schematic  Filter ‘s PCB layout In order to minimize the filter’s current loop (current flow back to GND), make sure the current loop is small. 1) Place ferrite bead as close as possible to the output Pin. 2) Try to minimize the current loop between filter’s GND (C8 to Class D’s Ground Pin). 3) Try to make sure the bottom layer of the filter and Class D device is a complete Ground plane. 4) If add Zobel network to minimize the ringing, place Zobel network as close as possible to filter. 5) Place snubber circuit as close as possible to device’s output Pin.

Ferrite bead

Device’s Ground PIN

Ferrite bead

Figure 10 Filter layout  PVCC’s layout  Place Bypass  Capacitor  close to device

 Output route  and PVCC  route square  crossing

 Place Bypass  Capacitor  close to device

Top layer Bot layer Figure 11 PVCC layout Conclusion TI’s latest inductor free stereo audio Class D (TPA3140) makes inductor free to come true in real mid-power class-D application. Based on demands of different length of speaker wire and output power (current), audio system engineers can use some board level tuning skills, include ferrite bead selection principles (slow down edge rate), ZOBEL network tuning methods (decrease rings), and proper PCB layout etc which introduced in this article, finally TPA3140 can achieve enough more margin in EMI test on customer system level test. Current customer design win feedback show TI TPA3140 is one real inductor free mid-power class-D which can help customer to do best balance of decreased system BOM cost, smaller PCB size, good EMC margin and stable good audio performance.