Texas Instruments LM4951A Evaluation Board LM4951ASDBD/NOPB LM4951ASDBD/NOPB Datenbogen
Produktcode
LM4951ASDBD/NOPB
SNAS453C – AUGUST 2008 – REVISED APRIL 2013
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in
, the LM4951A consists of two operational amplifiers that drive a speaker connected
between their outputs. The value of input and feedback resistors determine the gain of each amplifier. External
resistors R
resistors R
i
and R
f
set the closed-loop gain of AMP
A
, whereas two 20k
Ω
internal resistors set AMP
B
's gain to -1.
shows that AMP
A
's output serves as AMP
B
's input. This results in both amplifiers producing signals
identical in magnitude, but 180° out of phase. Taking advantage of this phase difference, a load is placed
between AMP
between AMP
A
and AMP
B
and driven differentially (commonly referred to as "bridge-tied load"). This results in a
differential, or BTL, gain of:
A
VD
= 2(R
f
/ R
i
)
(V/V)
(1)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single
amplifier's output and ground. For a given supply voltage, bridge mode has an advantage over the single-ended
configuration: its differential output doubles the voltage swing across the load. Theoretically, this produces four
times the output power when compared to a single-ended amplifier under the same conditions. This increase in
attainable output power assumes that the amplifier is not current limited and that the output signal is not clipped.
Under rare conditions, with unique combinations of high power supply voltage and high closed loop gain settings,
the LM4951A may exhibit low frequency oscillations.
amplifier's output and ground. For a given supply voltage, bridge mode has an advantage over the single-ended
configuration: its differential output doubles the voltage swing across the load. Theoretically, this produces four
times the output power when compared to a single-ended amplifier under the same conditions. This increase in
attainable output power assumes that the amplifier is not current limited and that the output signal is not clipped.
Under rare conditions, with unique combinations of high power supply voltage and high closed loop gain settings,
the LM4951A may exhibit low frequency oscillations.
Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by
biasing AMP1's and AMP2's outputs at half-supply. This eliminates the coupling capacitor that single supply,
single-ended amplifiers require. Eliminating an output coupling capacitor in a typical single-ended configuration
forces a single-supply amplifier's half-supply bias voltage across the load. This increases internal IC power
dissipation and may permanently damage loads such as speakers.
biasing AMP1's and AMP2's outputs at half-supply. This eliminates the coupling capacitor that single supply,
single-ended amplifiers require. Eliminating an output coupling capacitor in a typical single-ended configuration
forces a single-supply amplifier's half-supply bias voltage across the load. This increases internal IC power
dissipation and may permanently damage loads such as speakers.
POWER DISSIPATION
The LM4951A's dissipation when driving a BTL load is given by
. For a 7.5V supply and a single 8
Ω
BTL load, the dissipation is 1.42W.
P
DMAX-MONOBTL
= 4(V
DD
)
2
/ 2
π
2
R
L
(W)
(2)
The maximum power dissipation point given by
must not exceed the power dissipation given by
:
P
DMAX
= (T
JMAX
- T
A
) /
θ
JA
(3)
The LM4951A's T
JMAX
= 150°C. In the SD package, the LM4951A's
θ
JA
is 73°C/W when the metal tab is soldered
to a copper plane of at least 1in
2
. This plane can be split between the top and bottom layers of a two-sided PCB.
Connect the two layers together under the tab with an array of vias. At any given ambient temperature T
A
, use
to find the maximum internal power dissipation supported by the IC packaging. Rearranging
and substituting P
DMAX
for P
DMAX
' results in
. This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipation without violating the LM4951A's maximum
junction temperature.
junction temperature.
T
A
= T
JMAX
- P
DMAX-MONOBTL
θ
JA
(°C)
(4)
For a typical application with a 7.5V power supply and a BTL 8
Ω
load, the maximum ambient temperature that
allows maximum stereo power dissipation without exceeding the maximum junction temperature is 46°C for the
SD package.
SD package.
T
JMAX
= P
DMAX-MONOBTL
θ
JA
+ T
A
(°C)
(5)
gives the maximum junction temperature T
JMAX
. If the result violates the LM4951A's maximum
junction temperature of 150°C, reduce the maximum junction temperature by reducing the power supply voltage
or increasing the load resistance. Further allowance should be made for increased ambient temperatures.
or increasing the load resistance. Further allowance should be made for increased ambient temperatures.
The above examples assume that a device is operating around the maximum power dissipation point. Since
internal power dissipation is a function of output power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
internal power dissipation is a function of output power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
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