LM4868MT National Semiconductor, LM4868MT Datasheet - Page 15

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

Application Information
tical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between -OUTA
and +OUTA and driven differentially (’commonly referred to
as bridge mode’). This results in a differential gain of
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 as-
sumes 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 cou-
pling capacitor 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
permanently 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 LM4868 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 an
4Ω load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
The LM4868’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 LM4868’s TJMAX = 150˚C. In the LQ package soldered
to a DAP pad that expands to a copper area of 5in
PCB, the LM4868’s θ
P
P
DMAX
DMAX
= 4
P
= (V
DMAX
*
JA
A
DD
(V
VD
’ = (T
)
DD
is 42˚C/W. In the MTE package
2
/(2π
= 2
)
2
/(2π
JMAX
2
*
R
(R
L
2
):
R
f
− T
L
/R
): Bridge Mode
Single-Ended
i
A
)
)/θ
(Continued)
JA
2
on a
(1)
(2)
(3)
(4)
15
soldered to a DAP pad that expands to a copper area of 2in
on a PCB, the LM4868’s θ
temperature T
nal power dissipation supported by the IC packaging. Rear-
ranging Equation (4) and substituting P
sults in Equation (5). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4868’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 43˚C for the LQ
package and 45˚C for the MTE package.
Equation (6) gives the maximum junction temperature
T
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
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
case−to−sink
sink−to−ambient thermal impedance.) Refer to the Typical
Performance Characteristics curves for power dissipation
information 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 a local 1.0µF
tantalum bypass capacitance connected between the
LM4868’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 LM4868’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
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, increases
turn−on time and can compromise the amplifier’s click and
JMAX
B
, between the BYPASS pin and ground improves the
. If the result violates the LM4868’s 150˚C, reduce the
JA
. The heat sink can be created using additional
A
JA
, use Equation (4) to find the maximum inter-
thermal
T
is the sum of θ
A
T
thermal
= T
JMAX
JMAX
JA
impedance,
= P
is 41˚C/W. At any given ambient
− 2 X P
DMAX
impedance,
JC
, θ
θ
CS
JA
DMAX
, and θ
DMAX
+ T
and
θ
A
JA
θ
for P
SA
θ
CS
SA
www.national.com
. (θ
DMAX
JC
is
is
is the
’ re-
the
the
(5)
(6)
2