LM4844TLEVAL National Semiconductor, LM4844TLEVAL Datasheet - Page 19

• Manufacturer Part Number: LM4844TLEVAL
• Description: BOARD EVALUATION LM4844TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4844TLEVAL

• Amplifier Type: Class AB
• Output Type: 2-Channel (Stereo) with Stereo Headphones
• Max Output Power X Channels @ Load: 1.2W x 2 @ 8 Ohm; 80mW x 2 @ 32 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4844
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Both the feedback resistor, R
internally set.
Bridge mode amplifiers are different from single-ended am-
plifiers 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. Theoretically, 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 and that the output sig-
nal is not clipped.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
LLS- and LLS+ outputs at half-supply. This eliminates the
coupling capacitor that single supply, single-ended amplifiers
require. Eliminating an output coupling capacitor in a typical
single-ended configuration forces a single-supply amplifier's
half-supply bias voltage across the load. This increases in-
ternal IC power dissipation and may permanently damage
loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful single-ended or bridged amplifier.
A direct consequence of the increased power delivered to the
load by a bridge amplifier is higher internal power dissipation.
The LM4844 has 2 sets of bridged-tied amplifier pairs driving
LLS and RLS. The maximum internal power dissipation op-
erating in the bridge mode is twice that of a single-ended
amplifier. From Equation (3) and (4), assuming a 5V power
supply and an 8Ω load, the maximum power dissipation for
LLS and RLS is 634mW per channel.
The LM4844 also has a pair of single-ended amplifiers driving
LHP and RHP. The maximum internal power dissipation for
ROUT and LOUT is given by equation (5) and (6). From
Equations (5) and (6), assuming a 5V power supply and a
32Ω load, the maximum power dissipation for LOUT and
ROUT is 40mW per channel.
The maximum internal power dissipation of the LM4844 oc-
curs during output modes 3, 8, and 13 when both loudspeaker
and headphone amplifiers are simultaneously on; and is given
by Equation (7).
P
P
DMAX-RHP
DMAX-LHP
P
P
DMAX-RLS
DMAX-LLS
= (V
= (V
= 4(V
= 4(V
DD
DD
)
)
2
DD
2
DD
/ (2π
/ (2π
)
)
2
2
/ (2π
/ (2π
f
2
2
, and the input resistor, R
R
R
L
2
L
2
): Single-ended
): Single-ended
R
R
L
L
): Bridged
): Bridged
i
, are
(3)
(4)
(5)
(6)
19
The maximum power dissipation point given by Equation (7)
must not exceed the power dissipation given by Equation (8):
The LM4844's T
LM4844's θ
T
sipation supported by the IC packaging. Rearranging Equa-
tion (8) and substituting P
Equation (9). This equation gives the maximum ambient tem-
perature that still allows maximum stereo power dissipation
without violating the LM4844's maximum junction tempera-
ture.
For a typical application with a 5V power supply, stereo 8Ω
loudspeaker load, and the stereo 32Ω headphone load, the
maximum ambient temperature that allows maximum stereo
power dissipation without exceeding the maximum junction
temperature is approximately 100°C for the TL package.
Equation (10) gives the maximum junction temperature
T
maximum junction temperature by reducing the power supply
voltage or increasing the load resistance. Further allowance
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 (7) is greater
than that of Equation (8), then decrease the supply voltage,
increase the load impedance, or reduce the ambient temper-
ature. If these measures are insufficient, a heat sink can be
added to reduce θ
ditional copper area around the package, with connections to
the ground pin(s), supply pin and amplifier output pins. Ex-
ternal, 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 Characteris-
tics curves for power dissipation information at lower output
power levels.
A
JMAX
, use Equation (8) to find the maximum internal power dis-
P
DMAX-LLS
. If the result violates the LM4844's 150°C, reduce the
JA
T
T
+ P
JMAX
P
A
is 62°C/W. At any given ambient temperature
JA
DMAX
= T
DMAX-RLS
JMAX
is the sum of θ
JA
= P
JMAX
. The heat sink can be created using ad-
P
' = (T
DMAX-TOTAL
DMAX-TOTAL
= 150°C. In the TL package, the
- P
JMAX
+ P
SA
DMAX-TOTAL
DMAX-TOTAL
DMAX-LHP
is the sink-to-ambient thermal
- T
=
JC
A
θ
) / θ
JA
, θ
+ T
CS
JA
θ
+ P
CS
for P
JA
, and θ
A
is the case-to-sink
DMAX-RHP
DMAX
SA
www.national.com
. (θ
' results in
JC
is the
(10)
(7)
(8)
(9)