LM4855IBLX National Semiconductor, LM4855IBLX Datasheet - Page 17

• Manufacturer Part Number: LM4855IBLX
• Manufacturer: National Semiconductor

Specifications of LM4855IBLX

• Operational Class: Class-AB
• Audio Amplifier Function: Headphone/Speaker
• Total Harmonic Distortion: 0.5@8Ohm@400mW%
• Single Supply Voltage (typ): 3V
• Dual Supply Voltage (typ): Not RequiredV
• Supply Current (max): 12@5VmA
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 62dB
• Single Supply Voltage (min): 2.6V
• Single Supply Voltage (max): 5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 18
• Package Type: uSMD
• Lead Free Status / RoHS Status: Not Compliant

APPLICATION INFORMATION
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4855 consists of three pairs of
output amplifier blocks (A4-A6). A4, A5, and A6 consist of
bridged-tied amplifier pairs that drive LOUT, ROUT, and
SPKROUT respectively. The LM4855 drives a load, such as
a speaker, connected between outputs, SPKROUT+ and
SPKROUT-. In the amplifier block A6, the output of the
amplifier that drives SPKROUT- serves as the input to the
unity gain inverting amplifier that drives SPKROUT+.
This results in both amplifiers producing signals identical in
magnitude, but 180˚ out of phase. Taking advantage of this
phase difference, a load is placed between SPKROUT- and
SPKROUT+ and driven differentially (commonly referred to
as ’bridge mode’). This results in a differential or BTL 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. 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 signal is not clipped.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
SPKROUT- and SPKROUT+ outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a typical 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.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is higher internal power dissi-
pation. The LM4855 has a pair of bridged-tied amplifiers
driving a handsfree speaker, SPKROUT. The maximum in-
ternal power dissipation operating in the bridge mode is
twice that of a single-ended amplifier. From Equation (2),
assuming a 5V power supply and an 8Ω load, the maximum
SPKROUT power dissipation is 634mW.
(Continued)
A
VD
= 2(R
f
/R
i
) = 2
(1)
17
The LM4855 also has 2 pairs of bridged-tied amplifiers driv-
ing stereo headphones, ROUT and LOUT. The maximum
internal power dissipation for ROUT and LOUT is given by
equation (3) and (4). From Equations (3) and (4), assuming
a 5V power supply and a 32Ω load, the maximum power
dissipation for LOUT and ROUT is 158mW, or 316mW total.
The maximum internal power dissipation of the LM4855
occurs when all 3 amplifiers pairs are simultaneously on; and
is given by Equation (5).
The maximum power dissipation point given by Equation (5)
must not exceed the power dissipation given by Equation
(6):
The LM4855’s TJMAX = 150˚C. In the IBL package, the
LM4855’s θ
DAP pad that expands to a copper area of 2.5in
the LM4855’s θ
ture T
dissipation supported by the IC packaging. Rearranging
Equation (6) and substituting P
in Equation (7). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4855’s maximum junction tem-
perature.
For a typical application with a 5V power supply and an 8Ω
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 104˚C for the
IBL package.
Equation (8) gives the maximum junction temperature T
MAX
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 (5) is
greater than that of Equation (6), then decrease the supply
. If the result violates the LM4855’s 150˚C, reduce the
P
A
DMAX-SPKROUT
P
P
, use Equation (6) to find the maximum internal power
P
DMAX-ROUT
DMAX-LOUT
DMAX-SPKROUT
JA
T
is 48˚C/W. In the LD package soldered to a
T
JA
JMAX
P
A
DMAX
is 42˚C/W. At any given ambient tempera-
= T
= 4(V
= 4(V
= P
= 4(V
JMAX
P
’ = (T
+ P
DMAX-TOTAL
DMAX-TOTAL
DD
DD
DMAX-LOUT
DD
- P
)
)
2
JMAX
2
/(2π
)
/(2π
DMAX-TOTAL
2
DMAX-TOTAL
/(2π
2
2
- T
R
2
R
=
L
A
θ
L
R
): Bridge Mode
): Bridge Mode
JA
)/ θ
+ P
L
): Bridge Mode (2)
+ T
JA
θ
DMAX-ROUT
for P
JA
A
DMAX
2
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on a PCB,
’ results
(3)
(4)
(5)
(6)
(7)
(8)
J -