LM4876M/NOPB National Semiconductor, LM4876M/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM4876M/NOPB
• Description: IC AMP AUDIO PWR 1.5W MONO 8SOIC
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4876M/NOPB

• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 1.5W x 1 @ 8 Ohm
• Voltage - Supply: 2 V ~ 5.5 V
• Features: Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Amplifier Class: AB
• No. Of Channels: 1
• Output Power: 1.1W
• Supply Voltage Range: 2V To 5.5V
• Load Impedance: 8ohm
• Operating Temperature Range: -40°C To +85°C
• Amplifier Case Style: SOIC
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM4876M; *LM4876M/NOPB; LM4876M

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Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4876 consists of two opera-
tional amplifiers. External resistors R
loop gain of Amp1, whereas two internal 40kΩ resistors set
Amp2’s gain at -1. The LM4876 drives a load, such as a
speaker, connected between the two amplifier outputs, V
and V
Figure 1 shows that the Amp1 output serves as the Amp2
input, which results in both amplifiers producing signals iden-
tical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between V
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 power assumes that the ampli-
fier is not current limited or that 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 Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This results from biasing V
and V
tor that single supply, single-ended amplifiers require. Elimi-
nating an output coupling capacitor in a single-ended con-
figuration forces a single-supply amplifier’s half-supply bias
voltage across the load. The current flow created by the
half-supply bias voltage increases internal IC power dissipa-
tion 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 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 LM4876 has two operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. Equation (3) states the
maximum power dissipation for a bridge amplifier. However,
even with this substantial increase in power dissipation, the
LM4876 does not require heatsinking. From Equation (3),
assuming a 5V power supply and an 8Ω load, the maximum
power dissipation point is 633mW.
The maximum power dissipation point given by Equation (3)
must not exceed the power dissipation given by Equation
(4):
The LM4876’s T
LM4876’s θ
T
A
o
2 and driven differentially (commonly referred to as
, use Equation (4) to find the maximum internal power
o
o
2 .
2 at half-supply. This eliminates the coupling capaci-
P
JA
P
DMAX
DMAX
is 140˚C/W. At any given ambient temperature
JMAX
= 4*(V
P
= (V
DMAX
= 150˚C. In the M08A package, the
A
DD
DD
VD
)
= (T
2
)
2
= 2 * (R
/(2π
/(2π
JMAX
2
2
R
R
L
-T
f
f
/R
) Single-Ended
L
and R
A
) Bridge Mode
i
) /θ
)
JA
i
set the closed-
o
1 and
(1)
(2)
(3)
(4)
o
o
1
1
8
dissipation supported by the IC packaging. Rearranging
Equation (4) results in Equation (5). This equation gives the
maximum ambient temperature that still allows maximum
power dissipation without violating the LM4876’s maximum
junction temperature.
For a typical application with a 5V power supply and an 8W
load, the maximum ambient temperature that allows maxi-
mum power dissipation without exceeding the maximum
junction temperature is approximately 61˚C.
For the MSOP10A package, θ
shows that T
ambient temperature of 25˚C and using the same 5V power
supply and an 8Ω load. This violates the LM4876’s 150˚C
maximum junction temperature when using the MSOP10A
package. Reduce the junction temperature by reducing the
power supply voltage or increasing the load resistance. Fur-
ther, allowance should be made for increased ambient tem-
peratures. To achieve the same 61˚C maximum ambient
temperature found for the MO8 package, the MSOP10 pack-
aged part should operate on a 4.1V supply voltage when
driving an 8Ω load. Alternatively, a 5V supply can be used
when driving a load with a minimum resistance of 12Ω for the
same 61˚C maximum ambient temperature.
Fully charged Li-ion batteries typically supply 4.3V to por-
table applications such as cell phones. This supply voltage
allows the LM4876 to drive loads with a minimum resistance
of 9Ω without violating the maximum junction temperature
when the maximum ambient temperature is 61˚C.
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 (3) is greater than that of Equation
(4), 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. When
adding a heat sink, the θ
( θ
case-to-sink thermal impedance, and θ
ambient thermal impedance.) Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation at lower output 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 10µF in parallel with a 0.1µF filter capacitors to stabi-
lize 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 local bypass ca-
pacitance at the LM4876’s supply pins. Keep the length of
leads and traces that connect capacitors between the
LM4876’s power supply pin and ground as short as possible.
Connecting a 1µF capacitor between the BYPASS pin and
ground improves the internal bias voltage’s stability and
improves the amplifier’s PSRR. The PSRR improvements
increase as the bypass pin capacitor value increases. Too
large, however, and the amplifier’s click and pop perfor-
JC
is the junction-to-case thermal impedance, θ
JA
. The heat sink can be created using additional
JMAX
, for the MSOP10 package, is 158˚C for an
T
T
JMAX
A
= T
JA
JMAX
= P
is the sum of θ
DMAX
JA
- P
= 210˚C/W. Equation (6)
DMAX
θ
JA
+ T
θ
SA
JA
JC
A
, θ
is the sink-to-
CS
, and θ
CS
is the
SA
(5)
(6)
.