lm4923mmx National Semiconductor Corporation, lm4923mmx Datasheet - Page 10

• Manufacturer Part Number: lm4923mmx
• Description: 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
• Manufacturer: National Semiconductor Corporation

www.national.com
the Demonstration Board Layout section. Further detailed
and specific information concerning PCB layout, fabrication,
and mounting an LLP package is available from National
Semiconductor's package Engineering Group under applica-
tion note AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load's impedance. As load impedance
decreases, load dissipation becomes increasingly dependent
on the interconnect (PCB trace and wire) resistance between
the amplifier output pins and the load's connections. Residual
trace resistance causes a voltage drop, which results in power
dissipated in the trace and not in the load as desired. This
problem of decreased load dissipation is exacerbated as load
impedance decreases. Therefore, to maintain the highest
load dissipation and widest output voltage swing, PCB traces
that connect the output pins to a load must be as wide as
possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply's output voltage de-
creases with increasing load current. Reduced supply voltage
causes decreased headroom, output signal clipping, and re-
duced output power. Even with tightly regulated supplies,
trace resistance creates the same effects as poor supply reg-
ulation. Therefore, making the power supply traces as wide
as possible helps maintain full output voltage swing.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful amplifer, whether the amplifier is bridged or single-
ended. Equation 2 states the maximum power dissipation
point for a single-ended amplifier operating at a given supply
voltage and driving a specified output load.
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 op-
erating at the same conditions.
Since the LM4923 has bridged outputs, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
Even with this substantial increase in power dissipation, the
LM4923 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 maximum power dis-
sipation point obtained from Equation 3 must not be greater
than the power dissipation results from Equation 4:
The LM4923's θ
pending on the ambient temperature, T
roundings, 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
P
P
DMAX
DMAX
= 4 * (V
P
= (V
DMAX
JA
DD
in an LQB08A package is 140°C/W. De-
DD
)
= (T
2
)
/ (2π
2
JMAX
/ (2π
2
R
2
- T
L
R
) Single-Ended
L
A
) Bridge Mode
) / θ
JA
A
, of the system sur-
(2)
(3)
(4)
10
impedance increased, the ambient temperature reduced, or
the θ
near the output, V
θ
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 62°C provided that device op-
eration is around the maximum power dissipation point. Re-
call that internal power dissipation is a function of output
power. If typical operation is not around the maximum power
dissipation point, the LM4923 can operate at higher ambient
temperatures. Refer to the Typical Performance Charac-
teristics curves for power dissipation information.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion 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 ap-
plications 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
LM4923. The LM4923 will operate without the bypass capac-
itor C
recommended for C
mance. Lesser values may be used, but PSRR decreases at
frequencies below 1kHz. The issue of C
dependant upon desired PSRR and click and pop perfor-
mance as explained in the section Proper Selection of Ex-
ternal Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4923 contains shutdown circuitry that is used to turn off the
amplifier's bias circuitry. The device may then be placed into
shutdown mode by toggling the Shutdown Select pin to logic
low. The trigger point for shutdown 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 un-
wanted 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 solution
is to use a single-throw switch in conjunction with an external
pull-up resistor. 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 using
integrated power amplifiers is critical when optimizing device
and system performance. Although the LM4923 is tolerant to
a variety of external component combinations, consideration
of component values must be made when maximizing overall
system quality.
The LM4923 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4923 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize signal to noise ratio. Low gain configurations
JA
. The larger areas of copper provide a form of heatsinking
JA
B
, although the PSRR may decrease. A 1µF capacitor is
reduced with heatsinking. In many cases, larger traces
DD
, and GND pins can be used to lower the
B
. This value maximizes PSRR perfor-
B
selection is thus