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參數(shù)資料
| 型號(hào)： | LM391 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | LM391 Audio Power Driver |
| 中文描述： | LM391音頻功率驅(qū)動(dòng)器 |
| 文件頁(yè)數(shù)： | 5/12頁(yè) |
| 文件大?。?/td> | 236K |
| 代理商： | LM391 |
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:
V
Opeak
e
0
2 R
L
P
O
(1)
I
Opeak
e
0
2 P
O
R
L
(2)
Add 5 volts to the peak output swing (V
OP
) for transistor
voltage to get the supplies, i.e.,
g
(V
OP
a
5V) at a current
of I
peak
. 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
(V
Opeak
a
5) (1
a
regulation) (1.1)
The input sensitivity and output power specs determine the
required gain.
A
V
t
P
O
R
L
V
IN
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.
(3)
e
V
ORMS
V
INRMS
(4)
The desired input impedance is set by R
IN
. Very high values
can cause board layout problems and DC offsets at the out-
put. The bandwidth requirements determine the size of C
f
and C
C
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 I
Opeak
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:
i
JA
s
R
E
(
b
MIN
a
1)
V
CEQMAX
(K)
where:
(5)
i
JA
is the thermal resistance of the driver transistor, junc-
tion to ambient, in
§
C/W.
R
E
is the emitter degeneration resistance in ohms.
b
min
is that of the output transistor.
V
CEQMAX
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 R
E
is to be determined and equation (5)
is rearranged to be:
R
E
t
i
JA
(V
CEQMAX
) K
b
MIN
a
1
(6)
The maximum average power dissipation in each output
transistor is:
P
DMAX
e
0.4 P
OMAX
The power dissipation in the driver transistor is:
(7)
P
DRIVER(MAX)
e
P
DMAX
b
MIN
(8)
Heat sink requirements are found using the following formu-
las:
i
JA
s
T
JMAX
b
T
AMAX
P
D
(9)
(10)
i
SA
s
i
JA
b
i
JC
b
i
CS
where:
T
jMAX
is the maximum transistor junction temperature.
T
AMAX
is the maximum ambient temperature.
i
JA
is thermal resistance junction to ambient.
i
SA
is thermal resistance sink to ambient.
i
JC
is thermal resistance junction to case.
i
CS
is thermal resistance case to sink, typically 1
§
C/W for
most mountings.
5
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