lm4874mh National Semiconductor Corporation, lm4874mh Datasheet - Page 15

• Manufacturer Part Number: lm4874mh
• Description: 2.1w Differential Input, Btl Output Stereo Audio Amplifier With Selectable Gain And Shutdown
• Manufacturer: National Semiconductor Corporation

Application Information
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3W AND 4W LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
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 also adversely affects maxi-
mum output power. A poorly regulated supply’s output volt-
age 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, each of the LM4874’s stereo channels
consists of two operational amplifiers. The LM4874 can be
used to drive a speaker connected between the two outputs
of each channel’s amplifiers.
Figure 1 shows that the output of Amp1 serves as the input
to Amp2, which results in both amplifiers producing signals
identical in magnitude, but 180˚ out of phase. Taking advan-
tage of this phase difference, a load is placed between
OUT+ and OUT- and driven differentially (commonly referred
to as ’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 assumes that the amplifier is
not current limited or the output signal is not clipped. To
ensure minimum output signal clipping when selecting one
of the amplifier’s four closed-loop gains, 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 OUT+
and OUT- at half-supply. This eliminates the coupling capaci-
tor that single supply, single-ended amplifiers require. Elimi-
nating an output coupling capacitor in a single-ended con-
A
VD
= 2(R
F
/R
I
)
(1)
15
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 an increase in the
internal power dissipation point for a bridge amplifier oper-
ating at the same given conditions.
The LM4874 has four operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. From Equation (3), assum-
ing a 5V power supply and an 8Ω load, the maximum power
dissipation point is 2W. The maximum power dissipation
point obtained from Equation (3) must not exceed the power
dissipation predicted by Equation (4):
For the exposed DAP TSSOP package, θ
T
perature T
internal power dissipation supported by the IC packaging. If
the result of Equation (3) is greater than that of Equation (4),
decrease the supply voltage, increase the load impedance,
or reduce the ambient temperature. For a typical application
with a 5V power supply and an 8Ω load, the maximum
ambient temperature that does not violate the maximum
junction temperature is approximately 68˚C. This further as-
sumes 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 am-
bient temperatures are allowed as output power decreases.
Refer to the Typical Performance Characteristics curves for
power dissipation information at lower output power levels.
BTL GAIN SELECTION
The LM4874 features four fixed, internally set, BTL voltage
gains: 6dB, 10dB, 15.6dB, and 21.6dB. Select one of the
four gains by applying a logic level signal to the GAIN0
(MSB) and GAIN1 (LSB) digital inputs.
The closed-loop gain of the first amplifier is adjustable, hav-
ing four different gains, whereas two internal 20kΩ resistors
set the second amplifier’s gain at -1. Table 1 below, shows
the state of the two logic inputs required to select one of the
four gain values.
JAMAX
= 150˚C for the LM4874. For a given ambient tem-
P
P
DMAX
A
DMAX
, Equation (4) can be used to find the maximum
= 4
P
= (V
DMAX
*
DD
(V
DD
= (T
)
2
/(2π
)
2
/(2π
JMAX
2
R
2
L
R
) Single-Ended
− T
L
) Bridge Mode
A
)/θ
JA
JA
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= 41˚C/W.
(2)
(3)
(4)