Audio Power (APA) Operation and Measurement

Stephen Crump http://e2e.ti.com Amplifier Applications Audio and Imaging Products 18 August 2010 Contents • Operation • Class-D APA Operation • Measuring Class-D and Class-AB Outputs Audio Power Amplifier Operation

APA Classes Input Configurations Output Configurations Fully Differential APAs Audio Power Amplifier Classes • There are two classes of audio power

in common use. – Class-AB – continuous output. The traditional configuration. Continuous output, amplitude proportional to input

– Class-D – switching output. We will examine Class-D in detail later. – Class-D output is the Switched output, short-term average of duty cycle and short-term the switching waveform. average proportional to input Advantages and Disadvantages • Advantages and disadvantages of Class-AB. – Simple application. – Inexpensive (but not necessarily in SYSTEM cost). – Low efficiency, high power drain and heat generation. • Advantages and disadvantages of Class-D. – High efficiency, low power drain and heat generation. – Somewhat more expensive (but not in SYSTEM cost). – More complicated application. • Class-D advantages usually are compelling. APA Input Configurations • There are two common input configurations.

– Single-ended – single input line referred to Input referred

ground. Traditional S to APA ground configuration.

– Differential – a pair of

input lines. A superior S configuration. Input referred – May be connected to a to the source, differential source OR not APA ground a single-ended source. Single-Ended Inputs: Disadvantages • Input DC blocking cap are practically always required in single-supply systems. • No rejection of input noise or interference. APA input = sum Noisy power currents in the of these signals ground between source & APA produce voltages that S add to the APA input signal.

APA input = sum of these signals Radiation from other circuits

near APA induces voltages S that add to the input signal. Differential Inputs: Advantages • Input blocking caps may not be required. • High rejection of input noise and interference.

With ground reference at net input = only source, voltages induced by intended signal

noisy ground currents are S the same at both inputs & are rejected by APA CMRR.

net input = only If input leads are closely intended signal spaced, voltages induced

by radiation are essentially S the same at both inputs and are rejected by APA CMRR. Differential Inputs Cont’d. • Differential inputs may be connected to either differential or single-ended sources.

caps may be optional - If the audio source DC is within the APA common-mode range, DC blocking caps are optional. Differential - However, be sure source DC Source offset is not a problem! - Input caps may still be used if

high-pass filtering is needed. - Input DC blocking caps are required with a single-supply Single- ended single-ended source. Source Differential Inputs Cont’d. • Psuedo-differential sources use a single output with a midrail bias.

caps may be optional

Psuedo- Differential Source • Treat these like differential sources for wiring to the differential inputs of APAs. Differential Input Connections • Keep the 2 input leads close together. • With single-ended sources connect the APA input ground lead at source ground, NOT at APA ground.

• This lets the CMRR of the APA reject any common-mode radiation or any ground noise between the APA and the source. APA Output Configurations • There are two common output configurations.

– Single-ended – single output line. Traditional configuration. Output referred to APA ground Zload

– Differential – a pair of output lines. Also called BTL (for Bridge-Tied Zload Load). Output independent – Must be connected to a of APA ground floating load. Single-Ended Outputs: Disadvantages • A large output DC blocking cap is required in

single-supply systems. The blocking capacitor is required to prevent high DC current through the load. Zload

The cap must be very big for good low-frequency response: - 8Ω, 50Hz: C ~= 390μF. Zload - 4Ω, 20Hz: C ~= 2200μF! Differential Outputs: Advantages • Output power is nearly 4 times S/E output power.

+ The 2 outputs are opposite in 2 x phase, so their voltages sum - Zload across the load to provide Outputs sum to twice the voltage and 4 times 2 x S/E outputs the power of single-ended. • DC blocking capacitor is not required.

NO output cap is required – when input is zero, output is Zload DC load current a small DC offset, so DC is negligible load current is negligible. Fully Differential APAs • Fully differential APAs use differential circuits at

inputs, outputs and all intermediate stages. S Zload

• They have all the advantages of differential inputs and outputs, with increased CMRR, PSRR and RF immunity from balanced differential operation throughout the IC. • All recent differential APAs from TI use fully differential architecture. Fully Differential vs. Traditional • APAs with differential inputs and outputs, like master-slave IC’s, may not be fully differential. • These cannot match the performance of fully differential APAs.

Noise on input Gain 1 Noise coupled 1 amp Noise on into inputs is amplified to 2 output the outputs

2 RF coupled into inputs 2 or outputs can cause Noise on RF Rectification – Inverting amp output BAD! Class-D Audio Power Amplifier Operation

Benefits Block Diagram and Circuit Description of Operation Output Waveforms AD and BD Modulation Class-D APA Benefits • Class-D audio power amplifiers offer greater efficiency than amplifiers like Class-AB. • They therefore reduce power consumption of products in which they’re used. – Product power budgets are reduced. – Battery life is extended in portable products. – Heat generation is reduced. • These benefits reduce product cost and improve product performance. Class-D APA Block Diagram • Below is a block diagram for a fully differential

Class-D audio power amplifier.

feedback OUT+ Vcc

PGA

- - - + PWM Vin LOGIC H-Bridge + + + -

Vcc feedback OUT- • Most TI Class-D amplifiers are fully differential. • Single-ended implementations are possible. Class-D Differential APA Circuits • A programmable-gain feeds a differential integrator and comparator. • The integrator takes feedback from the output pulse train, subtracts it from the input signal and low-pass filters the result. • The comparator compares integrator output to a triangle wave to set output pulse width. • PWM (pulse width modulation) interface logic drives output FET gates. • A MOSFET bridge supplies switching pulses to a loudspeaker, which low-pass filters them to produce an audio output. Class-D Analog/PWM Conversion • The integrator produces an error voltage at its output that reflects the input after feedback.

OUT+ Error Voltage

Triangle - + Wave

+ -

Comparator OUT- Outputs

• The comparator switches when the error voltage crosses the output of the triangle wave oscillator. • PWM logic converts the comparator outputs to gate drive signals for the H-bridge. Class-D Output Waveforms • The PWM output switches at a frequency well above the audio frequency range. • Its short-term average is the audio-band output.

ON Vcc off Duty cycle determines the Q1 Q2 short-term average, the amplitude of the audio output + + - - Positive Output Q4 Q3 Polarity off ON

off Vcc ON Q1 Q2 Negative Output - + - + Polarity Q4 Q3 ON off AD Modulation • AD modulation, the simplest technique, puts the full differential output voltage across the load at all times, varying the duty cycle to control output. – (Differential or BTL AD modulation is shown on the preceding page. In differential AD modulation the outputs are always switched in opposite phase.) • AD modulation is a powerful technique, but it can generate high ripple current in the load at the switching frequency. • So AD modulation generally requires an LC filter before the load to eliminate the ripple current. AD Modulation Ripple Current • Without the LC filter, AD modulation ripple current wastes power and may increase the power handling requirement of the speaker.

With no LC filter, ripple current With no input signal, is limited only by loudspeaker switching at 250kHz, inductance, usually 20 to 60 uH. approximate peak ripple current would be Vcc * 1uS / L.

For Vcc = 12V and L = 30uH, peak ripple current would be ~ 0.4A.

With an 8Ω load, including extra power burned in the APA, this would waste nearly 1/2 watt. BD (Filter-Free) Modulation • A newer technique, BD modulation, permits operation without an output LC filter.

ON Vcc ON Q1 Q2 OUTP + - OUTN OUTP

Q4 Q3 OUTN off off

Differential off Vcc off Load Voltage Q1 Q2 OUTP + - OUTN Ripple Current Q4 Q3 ON ON BD Modulation Characteristics • BD modulation requires a differential output. • When there is no input, BD modulation switches the opposing outputs nearly in parallel. • So the differential voltage across the load is limited to very low duty cycle and ripple current is reduced dramatically. BD Modulation Waveforms • As input increases, output duty cycles are modulated in opposite phase to produce a net load voltage at twice the switching frequency.

OUTP

OUTN

Differential +5V Load 0V Voltage -5V

Load Current

Current Current Increasing Decaying

Filter free modulation output voltage and current waveforms, example signal A Note About Output Filtering • BD modulation eliminates the problem of ripple current without an output LC filter. • However, a output filter may be required for EMC even with BD modulation. • This will depend on the system or product configuration! Measuring Class-D and Class-AB Outputs Viewing Class-D Outputs • Look again at an earlier graph of Class-D output.

Duty cycle determines the The audio output short-term average, the may be an ordinary Positive audio output amplitude 1kHz signal. Output Polarity However, it’s very difficult to see any Negative audio output in the Output switching Polarity waveforms!

• The switching waveform doesn’t look much like the audio output. RC Filter for Viewing Class-D Output • To view the audio content of a Class-D output use an RC low-pass filter at each output.

Audio unclear in switching output Rflt Cflt

Class-D

Rflt Cflt RC filter shows audio with small ripple

• Filter frequency should be 30 to 40 kHz. • Recent work shows that 330Ω+15nF works best. Measuring Differential Outputs • Single-ended outputs are measured between output and ground. • HOWEVER ! – measure differential outputs BETWEEN the 2 output lines to be accurate.

Rflt Cflt • Measure single-ended outputs to ground.

• Measuring a differential output vs. Class-D ground is NOT accurate, and it overlooks half the output voltage. Rflt • Connect a scope probe to each side Cflt and use a math difference function. • Do not connect probe ground to a differential output – that will short it to ground. Class-D Output Rise and Fall • A Class-D switching waveform has very fast rise and fall, or equivalent slew rate. • Very few other devices can match this.

The audio output may be an ordinary 1kHz signal.

But the switching output may rise and fall in 10nS.

With a typical 12V supply, this means an equivalent slew rate of 1200V/uS! Filters for Measuring Class-D APAs • Many audio analyzers require filtering because extreme slew rates of Class-D waveforms cause slew-induced in their input stages. • A first-order RC filter with time constant around 4.7μS eliminates this problem in most cases. • At high gains such analyzers may require second-order filters. These may be cascaded RCs, with time constants around 2.7μS.

• Be aware that there is some frequency response rolloff in the audio band! It is generally not large enough to cause significant loss in results. 1st and 2nd Order Filter Responses • Schematics and frequency responses for suggested 1st and 2nd order -00 filters appear

at right. -10-10

-20-20

-30-30

-40-40 1.0KHz 3.0KHz 10KHz 30KHz 100KHz 300KHz 1.0MHz 1kHzDB(V(_V1.1)) DB(V(_V2.2)) 3kHz 10kHz 30kHz 100kHz 300kHz 1MHz F requency Other Filter Possibilities • It’s possible active filters could be used for measuring outputs of Class-D amplifiers. – HOWEVER, active filters can have the same slewing problems as analyzers. • It’s possible transformers could be used for measuring outputs of Class-D amplifiers. – HOWEVER, transformers often have problems like saturation and overshoot. • MAKE SURE YOUR FILTER DOES NOT ADD TROUBLE ! QUESTIONS ?