lm4899 National Semiconductor Corporation, lm4899 Datasheet - Page 13

• Manufacturer Part Number: lm4899
• Description: 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select And Fixed 6db Gain
• Manufacturer: National Semiconductor Corporation

Application Information
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation versus a single-ended amplifier
operating at the same conditions.
Since the LM4899 has bridged outputs, the maximum inter-
nal power dissipation is 4 times that of a single-ended am-
plifier. Even with this substantial increase in power dissipa-
tion, the LM4899 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8Ω load,
the maximum power dissipation point is 625mW. The maxi-
mum power dissipation point obtained from Equation 3 must
not be greater than the power dissipation results from Equa-
tion 4:
The LM4899’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 LM4899 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
JA
reduced with heatsinking. In many cases, larger
P
DMAX
JA
. The larger areas of copper provide a form of
P
= 4*(V
JA
DMAX
in an MUA10A package is 190˚C/W.
DD
= (T
DD
)
2
, and GND pins can be used to
JMAX
/ (2π
2
- T
R
L
) Bridge Mode
A
) / θ
JA
(Continued)
A
, of the system
(2)
(3)
13
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 LM4899. Although the LM4899 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.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4899 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4899 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 LM4899 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 LM4899 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 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.
B
selection is thus dependant upon desired PSRR
B
, although the PSRR
www.national.com
B
. This
DD
or