LM4818M National Semiconductor, LM4818M Datasheet - Page 9

• Manufacturer Part Number: LM4818M
• Description: 350mW Audio Power Amplifier with Shutdown Mode
• Manufacturer: National Semiconductor

Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1 , the LM4818 consist of two operational
amplifiers. External resistors, R
gain of the first amplifier (and the amplifier overall), whereas
two internal 20k resistors set the second amplifier’s gain at
-1. The LM4818 is typically used to drive a speaker con-
nected between the two amplifier outputs.
Figure 1 shows that the output of Amp1 servers as the input
to Amp2, which results in both amplifiers producing signals
identical in magnitude but 180˚ out of phase. Taking advan-
tage of this phase difference, a load is placed between V
and V
’bridge mode’). This results in a differential gain of
Bridge mode is 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 a
distinct advantage over the single-ended configuration: its
differential output doubles the voltage swing across the load.
This results in four times the output power when compared
to a single-ended amplifier under the same conditions. This
increase in attainable output assumes that the amplifier is
not current limited or the output signal is not clipped. To
ensure minimum output signal clipping when choosing an
amplifier’s closed-loop gain, refer to the Audio Power Am-
plifier Design Example section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This results from biasing V
and V
that single supply, single-ended amplifiers require. Eliminat-
ing an output coupling capacitor in a single-ended configu-
ration forces a single supply amplifier’s half-supply bias volt-
age across the load. The current flow created by the half-
supply bias voltage increases internal IC power dissipation
and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful bridged or single-ended amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified load.
However, a direct consequence of the increased power de-
livered to the load by a bridged amplifier is an increase in the
internal power dissipation point for a bridge amplifier oper-
ating at the same given conditions. Equation (3) states the
maximum power dissipation point for a bridged amplifier
operating at a given supply voltage and driving a specified
load.
The LM4818 has two operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. However, even with this
substantial increase in power dissipation, the LM4818 does
not require heatsinking. From Equation (3), assuming a 5V
power supply and an 8
tion point is 633mW. The maximum power dissipation point
obtained from Equation (3) must not exceed the power dis-
sipation predicted by Equation (4):
02
02
P
P
at half-supply. This eliminates the coupling capacitor
DMAX
and driven differentially (commonly referred to as
DMAX
= 4(V
P
= (V
DMAX
DD
DD
)
= (T
A
2
)
VD
2
/(2
/(2
load, the maximum power dissipa-
= 2 *(R
JMAX
2
2
R
R
L
i
L
- T
and R
) (W) Single-ended
f
) (W) Bridge Mode
/R
A
)/
i
)
JA
F
set the closed-loop
(W)
(1)
(2)
(3)
(4)
01
01
9
For the M08A package,
for the LM4818. For a given ambient temperature, T
tion (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 the result of Equation (4), then
decrease the supply voltage, increase the load impedance,
or reduce the ambient temperature. For a typical application
using the M08A packaged LM4818 with a 5V power supply
and an 8
does not violate the maximum junction temperature is ap-
proximately 42˚C. It is assumed that a device is a surface
mount part operating around the maximum power dissipation
point. The assumption that the device is operating around
the maximum power dissipation point is incorrect for an 8
load. The maximum power dissipation point occurs when the
output power is equal to the maximum power dissipation or
50% efficiency. The LM4818 is not capable of the output
power level (633mW) required to operate at the maximum
power dissipation point for an 8
power dissipation, the graph Power Dissipation vs. Output
Power must be used. From the graph, the maximum power
dissipation for an 8
575mW. Substituting this value back into equation (4) for
P
maximum ambient temperature is 52˚C. Refer to the Typical
Performance Characteristics curves for power dissipation
information for lower output powers and maximum power
dissipation for each package at a given ambient tempera-
ture.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4818 as
possible. The capacitor connected between the bypass pin
and ground improves the internal bias voltage’s stability,
producing improved PSRR. The improvements to PSRR
increase as the bypass pin capacitor value increases. Typi-
cal applications employ a 5V regulator with 10µF and 0.1µF
filter capacitors that aid in supply stability. Their presence,
however, does not eliminate the need for bypassing the
supply nodes of the LM4818. The selection of bypass ca-
pacitor values, especially C
requirements, click and pop performance as explained in the
section, Proper Selection of External Components, as
well as system cost and size constraints.
SHUTDOWN FUNCTION
The voltage applied to the LM4818’s SHUTDOWN pin con-
trols the shutdown function. Activate micro-power shutdown
by applying V
LM4818’s micro-power shutdown feature turns off the ampli-
fier’s bias circuitry, reducing the supply current. The logic
threshold is typically 1/2V
down current is achieved by applying a voltage that is as
near as V
that is less than V
Avoid intermittent or unexpected micro-power shutdown by
ensuring that the SHUTDOWN pin is not left floating but
connected to either V
There are a few ways to activate micro-power shutdown.
These included using a single-pole, single-throw switch, a
microcontroller, or a microprocessor. When using a switch,
connect an external 10k to 100k pull-up resistor between
the SHUTDOWN pin and V
the SHUTDOWN pin and ground. Select normal amplifier
DMAX
and using
DD
load, the maximum ambient temperature that
as possible to the SHUTDOWN pin. A voltage
DD
to the SHUTDOWN pin. When active, the
JA
DD
load and a 5V supply is approximately
DD
= 170˚C/W for the M08A package, the
may increase the shutdown current.
or GND.
JA
DD
DD
B
= 170˚C/W and T
. The low 0.7µA typical shut-
. Connect the switch between
, depends on desired PSRR
load. To find the maximum
JMAX
www.national.com
A
= 150˚C
, Equa-