LM4841 National Semiconductor, LM4841 Datasheet - Page 16

• Manufacturer Part Number: LM4841
• Description: Stereo 2W Amplifiers with DC Volume Control/ Transient Free Outputs/ and Cap-less Headphone Drive
• Manufacturer: National Semiconductor

www.national.com
Application Information
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage
swing across the load. This produces four times the output
power when compared to a single-ended amplifier under the
same conditions. This increase in attainable output power
assumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. 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 higher internal
power dissipation for the same conditions.
The LM4841 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and a
4Ω load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
The LM4841’s power dissipation is twice that given by Equa-
tion (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not ex-
ceed the power dissipation given by Equation (4):
The LM4841’s T
to a DAP pad that expands to a copper area of 5in
PCB, the LM4841’s θ
ages soldered to a DAP pad that expands to a copper area
of 2in
For the LM4841MT package, θ
ambient temperature T
mum internal power dissipation supported by the IC packag-
ing. Rearranging Equation (4) and substituting P
P
DMAX
2
' results in Equation (5). This equation gives the maxi-
on a PCB, the LM4841MH’s and LQ’s θ
P
DMAX
P
DMAX
= 4
P
JMAX
= (V
DMAX
*
JA
DD
(V
= 150˚C. In the LQ package soldered
' = (T
A
DD
)
is 20˚C/W. In the MH and LQ pack-
, use Equation (4) to find the maxi-
2
/(2π
)
2
/(2π
JMAX
2
R
2
JA
L
R
) Single-Ended
− T
L
= 80˚C/W. At any given
) Bridge Mode
A
)/θ
JA
(Continued)
JA
is 41˚C/W.
DMAX
2
on a
(2)
(3)
(4)
for
16
mum ambient temperature that still allows maximum stereo
power dissipation without violating the LM4841’s maximum
junction temperature.
For a typical application with a 5V power supply and an 4Ω
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 45˚C for the MH
package.
Equation (6) gives the maximum junction temperature
T
reduce the maximum junction temperature by reducing the
power supply voltage or increasing the load resistance. Fur-
ther allowance should be made for increased ambient tem-
peratures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
If the result of Equation (2) is greater than that of Equation
(3), then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to
reduce θ
copper area around the package, with connections to the
ground pin(s), supply pin and amplifier output pins. External,
solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the θ
junction-to-case thermal impedance, θ
thermal impedance, and θ
impedance.) Refer to the Typical Performance Character-
istics curves for power dissipation information at lower out-
put power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10 µF in parallel with a 0.1 µF filter capacitor to
stabilize the regulator’s output, reduce noise on the supply
line, and improve the supply’s transient response. However,
their presence does not eliminate the need for a local 1.0 µF
tantalum bypass capacitance connected between the
LM4841’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
capacitors between the LM4841’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
internal bias voltage’s stability and the amplifier’s PSRR. The
PSRR improvements increase as the bypass pin capacitor
value increases. Too large a capacitor, however, increases
turn-on time and can compromise the amplifier’s click and
pop performance. The selection of bypass capacitor values,
especially C
click and pop performance (as explained in the following
section, Selecting Proper External Components), system
cost, and size constraints.
JMAX
B
, between the BYPASS pin and ground improves the
. If the result violates the LM4841’s 150˚C T
JA
. The heat sink can be created using additional
B
, depends on desired PSRR requirements,
JA
T
T
A
is the sum of θ
JMAX
= T
JMAX
= P
SA
DMAX
– 2*P
is the sink-to-ambient thermal
JC
θ
DMAX
, θ
JA
CS
+ T
CS
, and θ
θ
A
JA
is the case-to-sink
SA
. (θ
JC
JMAX
is the
(5)
(6)
,