Fullscreen HTML5Med Res (??MB)HTML4

Manufacturer's data on the LM831 low voltage audio power amplifier National Semiconductor's LM831 is a dual low power audio amplifier optimised for operation at low voltages. It has two independent amplifiers, giving stereo or bridged mono operation. By DARREN YATES In the October and November issues of SILICON CHIP, we published an FM Radio Intercom which was mainly intended for communication between two or more motorbikes or between rider and passenger. One of the chips featured in the circuit is a new device from National Semiconductor, the LM831 stereo power amplifier. Although there are already a number of audio power ICs on the market, this one is different in a number of useful ways. The LM831 is a self-contained stereo power amplifier capable of driving loads as low as 4 ohms in either normal stereo (dual) opera- tion or in a mono power bridge (BTL) configuration. But what makes this 16 pin dual in-line (DIL) chip different is that it is optimised for very low voltage operation down to as low as 1.8 volts DC. The LM831 includes a patented compensation technique to reduce high frequency radiation, for optimum performance in AM radios. This compensation also reduces wideband noise and results in lower distortion. Main features The main features of the LM831 include: • Low voltage operation: 1.8 to 6.0 volts DC; • Typically 220 milliwatts output per channel into 40 loads or 440 milliwatts into 80 in bridge mode from a 3 volt supply; • Low AM radiation; • Low harmonic distortion typically 0.25% at 50mW; • Maximum gain: 46dB in dual (stereo) mode, 52dB in bridge mode; • System gain adjustable by external components; • Low quiescent current: typically 5 to 6mA. Both amplifiers can be connected in the inverting or non-inverting configurations and no input coupling capacitors are needed. Stereo operation A typical application for the LM831 is shown in Fig.2. It shows a stereo power amplifier which can drive 40 loudspeakers and run from just 3 volts. This could be supplied by two AA (penlite) cells. The gain of each channel is set internally by the 16k0 and 800 feedback resistors to 200. If need be, the gain can be easily reduced by connecting resistors in Typical Performance Characteristics Distortion vs Power Output Separation vs Frequency 11111 11 70 Ml u 50 z 0 i ~ •o IRAv • 2•on. Ctw • 270 pF) GAIN•C6 dB (R.1,ya.OO. C1w•O pF) ---- - r-,;;: ~ - - 10 kHz i !; i I.,._ ;;; t kHz ll 30 20 II Vout•200 mv I 1 11111 20 I 0.05 DUAL M OE 10 ~Yee• 3V, CH-A TO CH-8 0 SO 100 I 200 I I SOO 1K 2K FREOUENCY !HZ! 5K "" "" ,Hi:i .. 3,-. 111 l),EIJLe I 1111' .,, / Power Dissipation vs Power Output 10 10 10K 2DK 0.5 0.2 0.1 0.05 Ycc•3V, Rt•40 0.02 ~ C1w •RAv• 0 0.002 0.5 1 I ..,,. -~ Vcc•4V - :.,., ;..,- ~,; "' t1c•3V-- Vee• 2v 0.01 0.005 0.2 TH0-10 -;. Ill 0.02 ~ DUAL MODE I 1111 111 1 0.01 0.001 0.002 0.005 0.01 0.02 0.05 0.1 POWER OUTPUT IWATT) 1 Ycc•&V Vcc•5V ,-DUAL MODE 110TH DRIVE) Rt~•P·1 '1~111~Hz 0.001 0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 POWER OUTPUT (WAIT) 2 5 Fig.1: these graphs show the performance of the LM831 in stereo mode. The chip can be operated on single supplies up to 6V DC and it generally performs with lower distortion at the higher voltage. 112 SILICON CHIP 500pF VIN 13 47 - +22~ O.JJJ LM831 4(l VIN 1D0 lOk>-,.....:.14;:_t.:...:l:,..N....;j..--1 ~SPEAKER .,. vs 1 I BY~q 9 47 47 VIN + 22+ 47 .,. SPEAKER GNDt +3V - .,. BSr VIN > + 10D - 1000 80(l 16k AV GND R 2--6--1 + +~ D.JJl ... 4(l ~SPEAKER 10! Fig.2 (above): this circuit shows how the LM831 is connected for stereo operation. Because the input transistors are PNP, no input coupling capacitors are required. Fig.3 (right): this is an improved version of the circuit ► shown in Fig.2 with bigger feedback, bootstrapping and output coupling capacitors. series with the external 101,lF feedback capacitors (ie between pin 2 and the ' capacitor for one channel, and between pin 15 and the capacitor for the other channel). For example, if a 2200 resistor is installed in series with the capacitor, the gain drops to about 54. Both channels are independent of each other and channel separatiqn is better than 52dB (with reference to lkHz). However, this figure drops quickly if the supply voltage goes below 2 volts. Maximum usable output power in the stereo mode is about 450mW into 40 per channel with a 6V DC supply. Total harmonic distortion at this power level is about 3% but at lower power levels it is typically around 0.5%. With a 3V supply, the maximum power per channel into 40 loads is about 220 milliwatts. The circuit of Fig.2 has a quoted frequency response of 250Hz to 35kHz. Fig.3 is an improved circuit with larger output and bootstrapping capacitors VIN DM8~0(l◄J :_-_-_-:~GN. . . ·.;......_ _ ]l!!!!.._IJG"-'N:::.D_ _ 2 .,.6 1 6 + ...JZ 11 ~ ~ 47r.- 15 + 10+ 0.1 Fig 4: here's how to connect the LM831 to drive a single loudspeaker in bridge mode. Note that, as far as we can determine, the 10µF feedback capacitor connected to pin 15 should be omitted although we have drawn the circuit as shown in National Semiconductor literature. and improved DC supply bypassing. It has a frequency response from 20Hz to 20kHz. Bridge mode In bridge mode, both amplifiers in the LM831 drive a single 80 load to get close to the same total power as the device can deliver into two 40 loads in stereo mode. You might ask, "Why bother'?" The answer is that the bridge mode delivers more DECEMBER1989 113 VIN Fig.5: this improved version of the bridge circuit uses bigger feedback and bootstrap capacitors. Again, we think that the 2400 resistor and 22,uF capacitor connected at pin 15 should be omitted. power into the most readily available speaker load (ie, 80) than can otherwise be achieved from a low supply voltage. The bridge mode also eliminates the need for output coupling capacitors which is an important point. In bridge mode, two power amplifiers are driven so that their outputs are 180° out of phase. When connected to a common load, the output voltages are added and so the effective power is quadrupled. In the LM831 however, the minimum load impedance that each channel can drive is 40, so the minimum load for bridge mode is 80. This is because each channel amplifier " sees" half the actual load impedance. Fig.4 shows the method of connecting the LM831 for the bridge mode. It requires a O.lµF capacitor to be connected between pins 1 and 13. This has the effect of making channel B work in the inverting mode with a gain of unity and with the signal coupled internally via a 16k0 resistor from the output of channel A. Because of the effect of signal addition, the maximum gain of the bridge mode circuit is 5 ZdB. Note that the 80 speaker is connected directly across the outputs and no output coupling capacitor is used. The maximum output power in this mode is close to 800mW into 80 at 3% harmonic distortion from a DC supply of 6 volts. At 3 volts, the maximum power is about 440 milliwatts into an 80 load. Overall bandwidth of the circuit of Fig.4 is quoted as 250Hz to 25kHz. Readers who saw the FM Radio Intercom project featured in the October and November issues may recall that in that circuit the LM831 was used to drive two 80 speakers in parallel, giving a load of 40. The LM831 will happily drive such a load although it will give no more power than it would into an 80 load. The reason we used two 80 speakers is that they are readily available and we wanted to have a speaker for each ear. While the LM831 works satisfactorily into a 40 load in bridge mode, ...---------------01 BOOTSTRAP tAI BOOTSTRAP (BI 120-,....-....----1---------BYPASS Vsum, JSO!! 16,o--------------Al OUTPUT (BI 10 ~.,_-+---+------+--+----l__,,.Y,,._I\N~-0 11 POWER GROUNO (II OUTPUT (A l ....,..,~H-----+-+-----J---"""""-+-<J 8 ~ ~ 1 \ / \ ,...... SIGNAL GROUNO POWER GROUNO !Al Fig.6: internal schematic of the LM831, as shown in the National Semiconductor Linear Databook, Volume 3. It has a bias circuit which automatically sets the amplifier outputs at close to ½Vee. 114 SILICON CHIP Distortion vs Power Output 10 - ·I i SILICON CHIP BINDERS I ' I , I i r-,...~ I lOkHl 0.5 I 0.2 I I 0.1 -.... 1kHZ ! * High quality up to 14 issues * Hold 80mm internal width * Gold with * SILICONprinted CHIP logo 0.05 ~ BTL MOOE 0.02 1-Vcc•JV. RL•BO C1w•R.1v=0 D.01 0.001 0.0D2 0.005 0.01 0.02 0.05 0.1 POWER OUTPUT (WATT) ' 0.2 0.5 1 Power Dissipation vs Power Output on spine IJ - Vcc•5V ,.... 0.5 ..,.,i., 0.2 ~~ 0.1 0.05 Ycc•6V ~ ~~ - ,f, Ycc~4V Ycca3Y I I ,..~ 1~Holit. THo.:.10% Vcc•2V ,, .,. ,,. 0.02 BTL MODE RL•8U . \~ 1 ~Hl 1 11 D.D1 0.D01 0.D02 0.005 0.01 0.01 0.05 0.1 0.2 0.5 POWER OUTPUT (WA TT) 1 2 S These two graphs show the performance of the LM831 in bridge mode. Good power output is available with only a 3V supply rail. the supply voltage should not exceed 4.5 volts otherwise internal dissipation may become too high. Fig.5 shows an improved bridge mode circuit with larger capacitors to improve the low frequency bandwidth. It also has a 330pF capacitor between the output and noninverting inputs of each amplifier, to curtail the high frequency response. Overall bandwidth is quoted as 20Hz to 20kHz. Fig.6 shows the internal schematic of the LM831 although note that this does not show all the semiconductor details. The input stage of each amplifier is a single transistor rather than the usual differential input stage of most op amps. The DC biasing is arranged so that the output is always at half the supply voltage for supplies above 2 volts. This bias voltage is filtered by the external capacitor between pin 16 and ground. Since the input transistor is a PNP type, no input coupling capacitor is required provided the signal source is referenced to OV. ~ These beautifully made binders protect your copies of SILICON CHIP. In distinctive two-tone green high-quality vinyl specially selected for SILICON CHIP, and with heavy board covers, each binder holds a year's issues (the 14 issues of Vol. I or the 12 issues of Vol.2). It will look great on your bookshelf. -------------------~-Yes! Please send me _ _ _ _ SILICON CHIP binder(s) at $A 11 .95 plus $A3 p&p each (NZ p&p $A6). Name _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ (PLEASE PRINT) Street_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ __ Suburb/town, _ _ _ _ _ _ _ _ _ _ _ _ Postcode_ _ _ _ __ Enclosed is my cheque/money order for $ _ _ _ _ or please debit my □ Bankcard □ Visa Card □ MasterCard I L ~I~~~ l~---'-~~I ~I-'--'--_._____,I l~---'-----'---'I Card No. Signature_ _ _ _ _ _ _ _ _ _ _ _ Card expiry date_~-w w w - -. . . - - - - - - - - - - - .... - -- - - - - - - - - . _ __ ._-----..1 DECEMBER1989 115