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LM391N-100 Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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LM391N-100 Datasheet(HTML) 5 Page - National Semiconductor (TI) |
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5 / 12 page  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 02RL PO (1) IOpeak e 02PO RL (2) Add 5 volts to the peak output swing (VOP) for transistor voltage to get the supplies ie g (VOP a 5V) at a current of Ipeak The regulation of the supply determines the unload- ed voltage usually about 15% higher Supply voltage will also rise 10% during high line conditions max supplies g (VOpeak a 5) (1 a regulation) (11) (3) The input sensitivity and output power specs determine the required gain AV t 0PO RL VIN e VORMS VINRMS (4) 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 out- put 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 approxi- mately 40% of the maximum output power the drivers must dissipate this amount divided by the current gain of the out- puts See the output transistor selection guide Table A To prevent thermal runaway of the AB bias current the fol- lowing equation must be valid iJA s RE (bMIN a 1) VCEQMAX (K) (5) where iJA is the thermal resistance of the driver transistor junc- tion to ambient in CW 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 RE t iJA (VCEQMAX)K bMIN a 1 (6) The maximum average power dissipation in each output transistor is (7) PDMAX e 04 POMAX The power dissipation in the driver transistor is PDRIVER(MAX) e PDMAX bMIN (8) Heat sink requirements are found using the following formu- las iJA s TJMAX b TAMAX PD (9) (10) iSA s iJA b iJC b iCS 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 CW for most mountings 5 |
Similar Part No. - LM391N-100 |
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Similar Description - LM391N-100 |
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