LM391 Audio Power Driver

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Application Hints (Continued) A 40W8X 60W4X AMPLIFIER Given Power Output 40W8X 60W4X 1V Max 100k 20 Hz–20 kHz g 025 dB Input Sensitivity Input Impedance Bandwidth Equations (1) and (2) give 40W8X V OPeak e 253V I OPeak e 316A 60W4X V OPeak e 219V I OPeak e 548A Therefore the supply required is g303V 316A reducing to g269V 548A With 15% regulation and high line we get g383V using equation (3) The minimum gain from equation (4) is A V 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 R f1 e 51k R IN e 100k C C e 5pF R f2 e100k C f 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 225 at 55A (I Opeak ) The MJE712 MJE722 driver pair are 80V transistors with a minimum beta of 50 at 250 mA This output combination guarantees I Opeak with 5 mA from the LM391 Output transistor heat sink requirements are found using equations (7) (9) and (10) P D e 04 (60) e 24W (7) 200 b 55 i JA s e 60CW for T AMAX e 55C (9) 24 i SA s 60 b 11 b 10 e 39CW (10) For both output transistors on one heat sink the thermal resistance should be 19CW Now using equation (8) we find the power dissipation in the driver P DRIVER e 24 e 12W (8) 20 150 b 55 i JA s e 79CW (9) 12 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 R E 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 TLH7146–10 The desired data points are V M e 80V V B e 47V I L e 3A I L e 11A Since the break voltage is not equal to the supply we will use two resistors to replace R 3 and move V B Circuit Used Thevenin Equivalent TLH7146–11 Where R TH e R A 3 ll R B 3 V TH e V b R A 3 R A 3 3( a RB TLH7146–12 10
Application Hints (Continued) The formulas for R E R 1 and R 2 do not change R E e 065 3A e 022X 80 b 065 R 2 e 1k R 1 e 1k e 120k 065 The formula for R 3 now gives R TH when the V a in the formula becomes V B V R TH e B R 2 I L R E b w b 1 ( 47 e 1k 11 (022) b 065 b 1 ( e 2555k V TH is the additional voltage added to the supply voltage to get V B V TH eb(V B b V a ) eb(47 b 30) eb17V Now we must find R A 3 and R B 3 using the Thevenin formulas Putting V TH V b and R TH into the appropriate formulas reduces to R B 3 e 076 RA 3 and 2555k e R A 3 ll R B 3 The easiest way to solve these equations is to iterate with standard values If we guess R A 3 e 62k then RB 3 e 4712k use 47k The Thevenin impedance comes out 267k which is close enough to 2555k Now we will use equation (5) to determine the heat sinking requirements of the drivers to insure thermal stability 022 (20 a 1) i JA s 57CW (5) 40 (0002) This value is lower than we got with equation (9) so we will use it in equation (10) i SA s 57 b 6 b 1 e 50CW (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 TLH7146–13 11
Page 1 and 2: LM391 Audio Power Driver General De
Page 3 and 4: Typical Performance Characteristics
Page 5 and 6: Application Hints GENERALIZED AUDIO
Page 7 and 8: Application Hints (Continued) TRANS
Page 9: Application Hints (Continued) Since

LM391 Audio Power Driver

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