lm4873iblx National Semiconductor Corporation, lm4873iblx Datasheet - Page 14

• Manufacturer Part Number: lm4873iblx
• Description: Dual 2.1w Audio Amplifier Plus Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
power. The LM4873LQ achieves the same output power
level without forced air cooling. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4873’s thermal shutdown
protection. The LM4873’s power de-rating curve in the Typi-
cal Performance Characteristics shows the maximum
power dissipation versus temperature. Example PCB layouts
for the exposed-DAP TSSOP and LQ packages are shown in
the Demonstration Board Layout section. Further detailed
and specific information concerning PCB layout, fabrication,
and mounting an LQ (LLP) package is available from Na-
tional Semiconductor’s AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω 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
*
Pin out shown for the 28-pin Expoased-DAP TSSOP package. Refer to the Connection Diagrams for the pin out of the
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4873 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. (Though the following discusses
channel A, it applies equally to channel B.) External resistors
R
internal 20kΩ resistors set Amp2A’s gain at −1. The LM4873
drives a load, such as a speaker, connected between the two
amplifier outputs, −OUTA and +OUTA.
Refer to the section Selecting Proper External Components, for a detailed discussion of C
f
and R
i
set the closed-loop gain of Amp1A, whereas two
20-pin Exposed-DAP TSSOP, Exposed-DAP LLP, and micro SMD package.
FIGURE 2. Typical Audio Amplifier Application Circuit
(Continued)
14
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 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.
Figure 2 shows that Amp1A’s output serves as Amp2A’s
input. This 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 −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
B
size.
A
VD
= 2
*
(R
f
/R
i
)
10099331
(1)