LM4894 National Semiconductor, LM4894 Datasheet - Page 13

• Manufacturer Part Number: LM4894
• Description: 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
• Manufacturer: National Semiconductor

Application Information
mum power dissipation point obtained from Equation 3 must
not be greater than the power dissipation results from Equa-
tion 4:
The LM4894’s θ
Depending on the ambient temperature, T
surroundings, Equation 4 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 3 is greater than that of Equation 4,
then either the supply voltage must be decreased, the load
impedance increased, the ambient temperature reduced, or
the θ
traces near the output, V
lower the θ
heatsinking allowing higher power dissipation. For the typical
application of a 5V power supply, with an 8Ω load, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 30˚C pro-
vided that device operation is around the maximum power
dissipation point. Recall that internal power dissipation is a
function of output power. If typical operation is not around the
maximum power dissipation point, the LM4894 can operate
at higher ambient temperatures. Refer to the Typical Per-
formance Characteristics curves for power dissipation in-
formation.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the
bypass and power supply pins should be as close to the
device as possible. A larger half-supply bypass capacitor
improves PSRR because it increases half-supply stability.
Typical applications employ a 5V regulator with 10µF and
0.1µF bypass capacitors that increase supply stability. This,
however, does not eliminate the need for bypassing the
supply nodes of the LM4894. Although the LM4894 will
operate without the bypass capacitor C
may decrease. A 1µF capacitor is recommended for C
value maximizes PSRR performance. Lesser values may be
used, but PSRR decreases at frequencies below 1kHz. The
issue of C
and click and pop performance as explained in the section
Proper Selection of External Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4894 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4894 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
GND to set the LM4894 as either a "shutdown-high" device
or a "shutdown-low" device, respectively. The device may
then be placed into shutdown mode by toggling the Shut-
down Select pin to the same state as the Shutdown Mode
pin. For simplicity’s sake, this is called "shutdown same", as
the LM4894 enters shutdown mode whenever the two pins
JA
reduced with heatsinking. In many cases, larger
B
JA
selection is thus dependant upon desired PSRR
. The larger areas of copper provide a form of
P
JA
DMAX
in an MUA10A package is 190˚C/W.
= (T
DD
, and GND pins can be used to
JMAX
- T
A
)/θ
B
, although the PSRR
JA
(Continued)
A
, of the system
B
. This
DD
(4)
or
13
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 perfor-
mance. 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 when optimizing
device and system performance. Although the LM4894 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4894 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4894 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1Vrms are available
from sources such as audio codecs. Please refer to the
Audio Power Amplifier Design section for a more complete
explanation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LM4894
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V
allowable input common-mode voltage levels are actually a
function of V
Equation 5:
Special care must be taken to match the values of the
feedback resistors (R
matching the input resistors (R
Figure 1) more infront. Because of the balanced nature of
differential amplifiers, resistor matching differences can re-
sult in net DC currents across the load. This DC current can
increase power consumption, internal IC power dissipation,
reduce PSRR, and possibly damaging the loudspeaker. The
chart below demonstrates this problem by showing the ef-
fects of differing values between the feedback resistors while
assuming that the input resistors are perfectly matched. The
results below apply to the application circuit shown in Figure
1, and assumes that V
DC coupled inputs tied to ground.
V
V
CMi
CMi
<
<
DD
(V
(V
, R
DD
DD
i
-1.2)*((R
-1.2)*((R
, and R
F1
DD
and R
= 5V, R
f
f
f
+(R
+(R
and may be determined by
i1
F2
i
i
and R
L
)/(R
)/(R
) to each other as well as
= 8Ω, and the system has
f
f
)-V
)-V
i2
) to each other (see
DD
DD
*(R
*(R
i
i
/2R
/2R
www.national.com
DD
f
f
)
)
. Exact
(5)
(6)