LM391

LM391 Audio Power Driver December 1994 LM391 Audio Power Driver General Description The LM391 audio power driver is designed to drive external power transistors in 10 to 100 watt power amplifier designs High power supply voltage operation and true high fidelity performance distinguish this IC The LM391 is internally protected for output faults and thermal overloads circuitry providing output transistor protection is user programmable Features Y Y Y Y Y Y Y High Supply Voltage Low Distortion Low Input Noise High Supply Rejection Gain and Bandwidth Selectable Dual Slope SOA Protection Shutdown Pin g 50V max 0 01% 3 mV 90 dB Equivalent Schematic and Connection Diagram TL H 7146 – 1 Dual-In-Line Package TL H 7146 – 2 Top View Order Number LM391N-100 See NS Package Number N16A C1995 National Semiconductor Corporation TL H 7146 RRD-B30M115 Printed in U S A Absolute Maximum Ratings If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage LM391N-100 Input Voltage Shutdown Current (Pin 14) g 50V or a 100V Package Dissipation (Note 1) Storage Temperature Operating Temperature Lead Temp (Soldering 10 sec ) Thermal Resistance iJC iJA 1 39W b 65 C to a 150 C 0 C to a 70 C 260 C 20 C W 63 C W Supply Voltage less 5V 1 mA Electrical Characteristics TA e 25 C (The following are for V a e 90% V a MAX and Vb e 90% VbMAX ) Parameter Quiescent Current LM391N-100 Output Swing Drive Current Noise (20 Hz – 20 kHz) Supply Rejection Total Harmonic Distortion Intermodulation Distortion Open Loop Gain Input Bias Current Input Offset Voltage Positive Current Limit VBE Negative Current Limit VBE Positive Current Limit Bias Current Negative Current Limit Bias Current Pin 10–9 Pin 9–13 Pin 10 Pin 13 Conditions Current in Pin 15 VIN e 0 Positive Negative Source (Pin 8) Sink (Pin 5) Input Referred Input Referred f e 1 kHz f e 20 kHz 60 Hz 7 kHz 4 1 f e 1 kHz 1000 70 Va b 7 Vb a 7 5 5 3 90 0 01 0 10 0 01 5500 01 5 650 650 10 10 100 100 10 20 0 25 Min Typ 5 Va b 5 Vb a 5 Max 6 Units mA V V mA mA mV dB % % % VV mA mV mV mV mA mA Pin 14 Current Comments Minimum pin 14 current required for shutdown is 0 5 mA and must not exceed 1 mA Maximum pin 14 current for amplifier not shut down is 0 05 mA The typical shutdown switch point current is 0 2 mA Note 1 For operation in ambient temperatures above 25 C the device must be derated based on a 150 C maximum junction temperature and a thermal resistance of 90 C W junction to ambient Typical Applications TL H 7146 – 3 FIGURE 1 LM391 with External Components Protection Circuitry Not Shown 2 Typical Performance Characteristics Output Power vs Supply Voltage Total Harmonic Distortion vs Frequency (RL e 8X) Total Harmonic Distortion vs Frequency (RL e 4X) Open Loop Gain vs Frequency Input Referred Power Supply Rejection vs Frequency Total Harmonic Distortion vs AB Bias Current TL H 7146 – 4 Pin Descriptions Pin No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Pin Name a Input b Input Comments Audio input Feedback input Sets the dominant pole Improves negative supply rejection Drives output devices and is emitter of AB bias VBE multiplier Base of VBE multiplier Collector of VBE multiplier Drives output devices Biases the IC and is used in protection circuits Base of positive side protection circuit transistor Diode used for dual slope SOA protection Diode used for dual slope SOA protection Base of negative side protection circuit transistor Shuts off amplifier when current is pulled out of pin Positive supply Negative supply Compensation Ripple Filter Sink Output BIAS BIAS Source Output Output Sense a Current Limit a SOA Diode b SOA Diode b Current Limit Shutdown Va Vb 3 External Components (Figure 1) Component CIN Typical Value 1 mF Comments Input coupling capacitor sets a low frequency pole with RIN 1 fL e 2qRINCIN Sets input impedance and DC bias to input Feedback resistor for minimum offset voltage at the output this should be equal to RIN Feedback resistor that works with Rf2 to set the voltage gain Rf AV e 1 a 2 Rf1 Feedback capacitor This reduces the gain to unity at DC for minimum offset voltage at the output Also sets a low frequency pole with Rf1 1 fL e 2qRf1Cf Compensation capacitor Sets gain bandwidth product and a high frequency pole 1 GBW fh e GBW e 2q5000CC AV Max fh for stable design 500 kHz AB bias resistor AB bias potentiometer Adjust to set bias current in the output stage Bypass capacitor for bias This improves high frequency distortion and transient response Ripple capacitor This improves negative supply rejection at midband and high frequencies CR if used must equal CC Bleed resistor This removes stored charge in output transistors Output compensation resistor This resistor and CO compensate the output stage This value will vary slightly for different output devices Output compensation capacitor This works with RO to form a zero that cancels fb of the output power transistors Emitter degeneration resistor This resistor gives thermal stability to the output stage quiescent current IRC PW5 type Shutdown resistor Sets the amount of current pulled out of pin 14 during shutdown Compensation capacitors for protection circuitry Used to isolate capacitive loads usually 20 turns of wire wrapped around a 10X 2W resistor RIN Rf2 Rf1 100k 100k 5 1k Cf 10 mF CC 5 pF RA RB CAB CR Reb RO CO RE RTH C2 C 2 XL 3 9k 10k 0 1 mF 5 pF 100X 2 7X 0 1 mF 0 3X 39k 1000 pF 10X ll 5 mH 4 Application Hints GENERALIZED AUDIO POWER AMP DESIGN Givens Power Output Load Impedance Input Sensitivity Input Impedance Bandwidth The power output and load impedance determine the power supply requirements Output signal swing and current are found from VOpeak e 02 RL PO (1) (2) To prevent thermal runaway of the AB bias current the following equation must be valid iJA s RE (bMIN a 1) VCEQMAX (K) (5) where iJA is the thermal resistance of the driver transistor junction to ambient in C W RE is the emitter degeneration resistance in ohms bmin is that of the output transistor VCEQMAX is the highest possible value of one supply from equation (3) K is the temperature coefficient of the driver base-emitter voltage typically 2 mV C Often the value of RE is to be determined and equation (5) is rearranged to be iJA (VCEQMAX) K (6) bMIN a 1 The maximum average power dissipation in each output transistor is PDMAX e 0 4 POMAX (7) RE t The power dissipation in the driver transistor is PDMAX PDRIVER(MAX) e (8) bMIN Heat sink requirements are found using the following formulas TJMAX b TAMAX PD iSA s iJA b iJC b iCS iJA s (9) (10) IOpeak e Add 5 volts to the peak output swing (VOP) for transistor voltage to get the supplies i e g (VOP a 5V) at a current of Ipeak The regulation of the supply determines the unloaded voltage usually about 15% higher Supply voltage will also rise 10% during high line conditions g (VOpeak a 5) (1 a regulation) (1 1) max supplies (3) The input sensitivity and output power specs determine the required gain VORMS (4) VIN VINRMS Normally the gain is set between 20 and 200 for a 25 watt 8 ohm amplifier this results in a sensitivity of 710 mV and 71 mV respectively The higher the gain the higher the THD as can be seen from the characteristics curves Higher gain also results in more hum and noise at the output The desired input impedance is set by RIN Very high values can cause board layout problems and DC offsets at the output The bandwidth requirements determine the size of Cf and CC as indicated in the external component listing The output transistors and drivers must have a breakdown voltage greater than the voltage determined by equation (3) The current gain of the drive and output device must be high enough to supply IOpeak with 5 mA of drive from the LM391 The power transistors must be able to dissipate approximately 40% of the maximum output power the drivers must dissipate this amount divided by the current gain of the outputs See the output transistor selection guide Table A AV t e 0R 2 PO L 0PO RL where TjMAX is the maximum transistor junction temperature TAMAX is the maximum ambient temperature iJA is thermal resistance junction to ambient iSA is thermal resistance sink to ambient iJC is thermal resistance junction to case iCS is thermal resistance case to sink typically 1 C W for most mountings 5 Application Hints (Continued) PROTECTION CIRCUITRY The protection circuits of the LM391 are very flexible and should be tailored to the output transistor’s safe operating area The protection V-I characteristics circuitry and resistor formulas are described below The diodes from the output to each supply prevent the output voltage from exceeding the supplies and harming the output transistors The output will do this if the protection circuitry is activated while driving an inductive load TURN-ON DELAY It is often desirable to delay the turn-ON of the power amplifier This is easily implemented by putting a resistor in series with a capacitor from pin 14 to ground The value of the Protection Circuitry with External Components resistor is set to limit the current to less than 1 mA (the absolute maximum) This resistor with the capacitor gives a time constant of RC The turn-ON delay is approximately 2 time constants Example Amplifier with maximum supply of 30V like the 20W 8X example in the data sheet requiring a delay of 1 second Time delay e 2 RC Re Max V a 1 mA So R e 30k Solving for C gives 16 7 mF Use C e 20 mF with a 30V rating Protection Characteristics TL H 7146 – 6 TL H 7146–5 Protection Circuit Resistor Formulas (VB e V a ) Type of Protection Current Limit Single Slope SOA Protection Dual Slope SOA Protection (VB e V a ) RE R w RE e IL RE e w IL w IL R1 R 1 Not Required R1 e R2 R2 R 2 Short R3 R 3 Not Required Not Required Va b1 I L RE b w   VM b w w VM b w w J J 1 kX RE e R1 e R2 1 kX R3 e R2 ( Note w is the current limit VBE voltage 650 mV Assumptions V a ll w VM ll w V a is the load supply voltage VM is the maximum rated VCE of the output transistors 6 Application Hints (Continued) TRANSIENT INTERMODULATION DISTORTION There has been a lot of interest in recent years about transient intermodulation distortion Matti Otala of University of Oulu Oulu Finland has published several papers on the subject The results of these investigations show that the open loop pole of the power amplifier should be above 20 kHz To do this with the LM391 is easy Put a 1 MX resistor from pin 3 to the output and the open loop gain is reduced to about 46 dB Now the open loop pole is at 30 kHz The current in this resistor causes an offset in the input stage that can be cancelled with a resistor from pin 4 to ground The resistor from pin 4 to ground should be 910 kX rather than 1 MX to insure that the shutdown circuitry will operate correctly The slight difference in resistors results in about 15 mV of offset The 40W 8X amplifier schematic shows the hookup of these two resistors BRIDGE AMPLIFIER A switch can be added to convert a stereo amplifer to a single bridge amplifer The diagram below shows where the switch and one resistor are added When operating in the bridge mode the output load is connected between the two outputs the input is VIN 1 and VIN 2 is disconnected OSCILLATIONS GROUNDING Most power amplifiers work the first time they are turned on They also tend to oscillate and have excess THD Most oscillation problems are due to inadequate supply bypassing and or ground loops A 10 mF 50V electrolytic on each power supply will stop supply-related oscillations However if the signal ground is used for these bypass caps the THD is usually excessive The signal ground must return to the power supply alone as must the output load ground All other grounds bypass output R-C protection etc can tie together and then return to supply This ground is called high frequency ground On the 40W amplifier schematic all the grounds are labeled Capacitive loads can cause instabilities so they are isolated from the amplifier with an inductor and resistor in the output lead AB BIAS CURRENT To reduce distortion in the output stage all the transistors are biased ON slightly This results in class AB operation and reduces the crossover (notch) distortion of the class B stage to a low level (see performance curve THD vs AB bias) The potentiometer RB from pins 6 – 7 is adjusted to give about 25 mA of current in the output stage This current is usually monitored at the supply or by measuring the voltage across RE Typical Applications (Continued) Bridge Circuit Diagram TL H 7146 – 7 Output Transistors Selection Guide Table A Power Output 20W 30W 40W 60W 8X 4X 8X 4X Driver Transistor PNP MJE711 MJE171 D43C8 MJE712 MJE172 D43C11 NPN MJE721 MJE181 D42C8 MJE722 MJE182 D42C11 PNP TIP42A 2N6490 2N5882 Output Transistor NPN TIP41A 2N6487 2N5880 7 Application Hints (Continued) A 20W 8X 30W 4X AMPLIFIER Givens Power Output Input Sensitivity Input Impedance Bandwidth 20W into 8X 30W into 4X 1V Max 100k 20 Hz–20 kHz g 0 25 dB Solving for Cf 1 e 7 8 mF use 10 mF 2qRf1fL The recommended value for CC is 5 pF for gains of 20 or larger This gives a gain-bandwidth product of 6 4 MHz and a resulting bandwidth of 320 kHz better than required The breakdown voltage requirement is set by the maximum supply we need a minimum of 58V and will use 60V We must now select a 60V power transistor with reasonable beta at IOpeak 3 87A The TIP42 TIP41 complementary pair are 60V 60W transistors with a minimum beta of 30 at 4A The driver transistor must supply the base drive given 5 mA drive from the LM391 The MJE711 MJE721 complementary driver transistors are 60V devices with a minimum beta of 40 at 200 mA The driver transistors should be much faster (higher fT) than the output transistors to insure that the R-C on the output will prevent instability To find the heat sink required for each output transistor we use equations (7) (9) and (10) PD e 0 4 (30) e 12W (7) Cf t 150 C b 55 C e 7 9 C W for TAMAX e 55 C (9) 12 iSA s 7 9 b 2 1 b 1 0 e 4 8 C W (10) If both transistors are mounted on one heat sink the thermal resistance should be halved to 2 4 C W The maximum average power dissipation in each driver is found using equation (8) iJA s PDRIVER(MAX) e Using equation (9) iJA s 155 b 55 e 237 C W 04 12 e 400 mW 30 Equations (1) and (2) give IOP e 2 24A 20W 8X VOP e 17 9V 30W 4X VOP e 15 5V IOP e 3 87A Therefore the supply required is g 23V 2 24A reducing to g 21V 3 87A With 15% regulation and high line we get g 29V from equation (3) Sensitivity and equation (4) set minimum gain AV t 020 c 8 1 e 12 65 We will use a gain of 20 with resulting sensitivity of 632 mV Letting RIN equal 100k gives the required input impedance For low DC offsets at the output we let Rf2 e 100k Solving for Rf1 gives Rf2 e 100k Rf1 e 100k e 5 26k use 5 1k 20 b 1 The bandwidth requirement must be stated as a pole i e the 3 dB frequency Five times away from a pole gives 0 17 dB down which is better than the required 0 25 dB Therefore 20 e 4 Hz fL e 5 fh e 20k c 5 e 100 kHz 8 Application Hints (Continued) Since the free air thermal resistance of the MJE711 MJE721 is 100 C W no heat sink is required Using this information and equation (6) we can find the minimum value of RE required to prevent thermal runaway 100 (30) (0 002) e 0 19X RE t (6) 30 a 1 We must now use the SOA data on the TIP42 TIP41 transistors to set up the protection circuit Below is the SOA curve with the 4X and 8X load lines Also shown are the desired protection lines Note the value of VB is equal to the supply voltage so we use the formulas in the table D C SOA of TIP42 TIP41 Transistors The data points from the curve are VM e 60V VB e 23V IL e 3A I L e 7A Using the dual slope protection formulas 0 65 e 0 22X RE e 3 R2 e 1k 60 b 0 65 R1 e 1k 91k 0 65 23 b1 24k R3 e 1k 7(0 22) b 0 65 Note that an RE of 0 22X satisfies equation (6) The final schematic of this amplifier is below If the output is shorted the current will be 1 8A and VCE is 23V Since the input is AC the average power is short PD e (1 8) (23) 21W   J J This power is greater than was used in the heat sink calculations so the transistors will overheat for long-duration shorts unless a larger heat sink is used TL H 7146 – 8 Typical Applications (Continued) 20W-8X 30W-4X Amplifier with 1 Second Turn-ON Delay TL H 7146 – 9 Additional protection for LM391N Schottky diodes and R j 100X 9 Application Hints (Continued) A 40W 8X 60W 4X AMPLIFIER Given Power Output Input Sensitivity Input Impedance Bandwidth Equations (1) and (2) give 40W 8X VOPeak e 25 3V 60W 4X VOPeak e 21 9V Therefore the supply required is g 30 3V 3 16A reducing to 40W 8X 60W 4X 1V Max 100k 20 Hz–20 kHz g 0 25 dB IOPeak e 3 16A IOPeak e 5 48A Since a heat sink is required on the driver we should investigate the output stage thermal stability at the same time to optimize the design If we find a value of RE that is good for the protection circuitry we can then use equation (5) to find the heat sink required for the drivers The SOA characteristics of the 2N5882 2N5880 transistors are shown in the following curve along with a desired protection line SOA 2N5882 2N5880 g 26 9V 5 48A With 15% regulation and high line we get g 38 3V using equation (3) The minimum gain from equation (4) is AV t 18 We select a gain of 20 resulting sensitivity is 900 mV The input impedance and bandwidth are the same as the 20 watt amplifier so the components are the same Rf1 e 5 1k RIN e 100k CC e 5 pF Rf2 e 100k Cf e 10 mF The maximum supplies dictate using 80V devices The 2N5882 2N5880 pair are 80V 160W transistors with a minimum beta of 40 at 2A and 20 at 6A This corresponds to a minimum beta of 22 5 at 5 5A (IOpeak) The MJE712 MJE722 driver pair are 80V transistors with a minimum beta of 50 at 250 mA This output combination guarantees IOpeak with 5 mA from the LM391 Output transistor heat sink requirements are found using equations (7) (9) and (10) PD e 0 4 (60) e 24W (7) 200 b 55 e 6 0 C W for TAMAX e 55 C (9) 24 iSA s 6 0 b 1 1 b 1 0 e 3 9 C W (10) For both output transistors on one heat sink the thermal resistance should be 1 9 C W Now using equation (8) we find the power dissipation in the driver iJA s 24 e 1 2W 20 150 b 55 e 79 C W iJA s 12 PDRIVER e (8) (9) Thevenin Equivalent TL H 7146 – 10 The desired data points are VB e 47V IL e 3A I L e 11A VM e 80V Since the break voltage is not equal to the supply we will use two resistors to replace R3 and move VB Circuit Used TL H 7146 – 11 A Where RTH e R 3 A R3 ll R B 3 VTH e Vb TL H 7146 – 12 A B R3 a R3 ( 10 Application Hints (Continued) The formulas for RE R1 and R2 do not change 0 65 e 0 22X RE e 3A 80 b 0 65 e 120k R1 e 1k R2 e 1k 0 65 The formula for R3 now gives RTH when the V a in the formula becomes VB RTH e R2 e 1k The easiest way to solve these equations is to iterate with A B standard values If we guess R 3 e 62k then R 3 e 47 12k use 47k The Thevenin impedance comes out 26 7k which is close enough to 25 55k Now we will use equation (5) to determine the heat sinking requirements of the drivers to insure thermal stability iJA s 0 22 (20 a 1) 40 (0 002) 57 C W (5) VB b1 I L RE b w ( ( 47 b 1 e 25 55k 11 (0 22) b 0 65 VTH is the additional voltage added to the supply voltage to get VB VTH e b(VB b V a ) e b(47 b 30) e b17V A B Now we must find R 3 and R 3 using the Thevenin formulas Putting VTH Vb and RTH into the appropriate formulas reduces to B A A B R 3 e 0 76 R 3 and 25 55k e R 3 ll R 3 This value is lower than we got with equation (9) so we will use it in equation (10) iSA s 57 b 6 b 1 e 50 C W (10) This is the required heat sink for each driver For low TIM we add the 1 MX resistor from pin 3 to the output and a 910k resistor from pin 4 to ground The complete schematic is shown below If the output is shorted the transistor voltage is about 28V and the current is 5A Therefore the average power is short PD e (28) 5 e 70W This is much larger than the power used to calculate the heat sinks and the output transistors will overheat if the output is shorted too long Typical Applications (Continued) 40W-8X 60W-4X Amplifier High Frequency Ground Input Ground Speaker Ground Note All Grounds Should be Tied Together Only at Power Supply Ground Additional protection for LM391N Schottky diodes and R j 100X TL H 7146 – 13 11 LM391 Audio Power Driver Physical Dimensions inches (millimeters) Molded Dual-In-Line Package (N) Order Number LM391N-100 NS Package Number N16A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which (a) are intended for surgical implant into the body or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected 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