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| 型號： | LM4901 |
| 文件頁數(shù)： | 12/22頁 |
| 文件大?。?/td> | 1177K |
| 代理商： | LM4901 |
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1
, the LM4901 has two internal opera-
tional amplifiers. The first amplifier’s gain is externally con-
figurable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
f
to R
i
while
the second amplifier’s gain is fixed by the two internal 20k
resistors.
Figure 1
shows that the output of amplifier one
serves as the input to amplifier two which results in both
amplifiers producing signals identical in magnitude, but out
of phase by 180. Consequently, the differential gain for the
IC is
A
VD
= 2 *(R
f
/R
i
)
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the
Audio Power Amplifier
Design
section.
A bridge configuration, such as the one used in LM4901,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4901 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 1.
P
DMAX
= 4*(V
DD
)
2
/(2
π
2
R
L
)
(1)
It is critical that the maximum junction temperature T
JMAX
of
150C is not exceeded. T
JMAX
can be determined from the
power derating curves by using P
DMAX
and the PC board foil
area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of
θ
JA
,
resulting in higher P
DMAX
values without thermal shutdown
protection circuitry being activated.Additional copper foil can
be added to any of the leads connected to the LM4901. It is
especially effective when connected to V
, GND, and the
output pins. Refer to the application information on the
LM4901 reference design board for an example of good heat
sinking. If T
still exceeds 150C, then additional
changes must be made. These changes can include re-
duced supply voltage, higher load impedance, or reduced
ambient temperature. Internal power dissipation is a function
of output power. Refer to the
Typical Performance Charac-
teristics
curves for power dissipation information for differ-
ent output powers and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the device as possible. Typical appli-
cations employ a 5V regulator with 10 μF tantalum or elec-
trolytic capacitor and a ceramic bypass capacitor which aid
in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4901. The selection of
a bypass capacitor, especially C
, is dependent upon PSRR
requirements, click and pop performance (as explained in
the section,
Proper Selection of External Components
),
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4901 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4901 con-
tains a Shutdown Mode pin, allowing the designer to desig-
nate whether the part will be driven into shutdown with a high
level logic signal or a low level logic signal. This allows the
designer maximum flexibility in device use, as the Shutdown
Mode pin may simply be tied permanently to either V
DD
or
GND to set the LM4901 as either a ’shutdown-high’ device or
a ’shutdown-low’ device, respectively. The device may then
be placed into shutdown mode by toggling the Shutdown pin
to the same state as the Shutdown Mode pin. For simplicity’s
sake, this is called ’shutdown same’, as the LM4901 enters
shutdown mode whenever the two pins are in the same logic
state. The trigger point for either shutdown high or shutdown
low is shown as a typical value in the Supply Current vs
Shutdown Voltage graphs in the
Typical Performance
Characteristics
section. It is best to switch between ground
and supply for maximum performance. While the device may
be disabled with shutdown voltages in between ground and
supply, the idle current may be greater than the typical value
of 0.1μA. In either case, the shutdown pin should be tied to
a definite voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut-
down high or low application). This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4901 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
L
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