LP3921 Datasheet, PDF (26/34 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

nals that are 180° out of phase with each other. The LP3921
can be used, however, as a single ended input amplifier while
still retaining its fully differential benefits. In fact, completely
unrelated signals may be placed on the input pins. The
LP3921 simply amplifies the difference between them.
A bridged configuration, such as the one used in the LP3921,
also creates a second advantage over single ended ampli-
fiers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
assumes that the input resistor pair and the feedback resistor
pair are properly matched. BTL configuration eliminates the
output coupling capacitor required in single supply, single-
ended amplifier configurations. If an output coupling capacitor
is not used in a single-ended output configuration, the half-
supply bias across the load would result in both increased
internal IC power dissipation as well as permanent loud-
speaker damage. Further advantages of bridged mode oper-
ation specific to fully differential amplifiers like the LP3921
include increased power supply rejection ratio, common-
mode noise reduction, and click and pop reduction.
FIGURE 9. Audio Block
30069841
EXPOSED-DAP PACKAGE MOUNTING
CONSIDERATIONS
The LP3921's exposed-DAP (die attach paddle) package
(LLP) provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. this allows
rapid heat transfer from the die to the surrounding PCB cop-
per traces, ground plane and, finally, surrounding air. Failing
to optimize thermal design may compromise the LP3921's
high-power performance and activate unwanted, though nec-
essary, thermal shutdown protection. The LLP package must
have its DAP soldered to a copper pad on the PCB> The
DAP's PCB copper pad is connected to a large plane of con-
tinuous unbroken copper. This plane forms a thermal mass
and heat sink and radiation area. Place the heat sink area on
either outside plane in the case of a two-sided PCB, or on an
inner layer of a board with more than two layers. Connect the
DAP copper pad to the inner layer or backside copper heat
sink area with a thermal via. The via diameter should be 0.012
in. to 0.013 in. Ensure efficient thermal conductivity by plating-
through and solder-filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. In all circumstances and condi-
tions, the junction temperature must be held below 150°C to
prevent activating the LP3921's thermal shutdown protection.
Further detailed and specific information concerning PCB lay-
out, fabrication, and mounting an LLP package is available
from National Semiconductor's package Engineering Group
under application note AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load's impedance. As load impedance
decreases, load dissipation becomes increasingly dependent
on the interconnect (PCB trace and wire) resistance between
the amplifier output pins and the load's connections. Residual
trace resistance causes a voltage drop, which results in power
www.national.com
26