Datasheet

LM4853
SNAS155E –JANUARY 2002–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
BRIDGED AND SINGLE-ENDED OPERATION
As shown in Typical Application, the LM4853 contains three operational amplifiers (A1-A3). These amplifiers can
be configured for SE or BTL modes.
In the SE mode, the LM4853 operates as a high current output dual op amp. A1 and A3 are independent
amplifiers with an externally configured gain of A
V
= - R
F
/R
I
. The outputs of A1 and A3 are used to drive an
external set of headphones plugged into the headphone jack. Amplifier A2 is shut down to a high output
impedance state in SE mode. This prevents any current flow into the mono bridge-tied load, thereby muting it.
In BTL mode, A3 is shut down to a high impedance state. The audio signal from the RIGHT IN pin is directed to
the inverting input of A1. As a result, the LEFT IN and RIGHT IN audio signals, V
INL
and V
INR
, are summed
together at the input of A1. A2 is then activated with a closed-loop gain of A
V
= -1 fixed by two internal 20kΩ
resistors. The outputs of A1 and A2 are then used to drive the mono bridged-tied load.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION
The LM4853's exposed-DAP (die attach paddle) package (LD) 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 copper traces, ground plane, and surrounding air. The result is a low voltage audio power
amplifier that produces 1.7W at ≤ 1% THD+N with a 4Ω load. This high power is achieved through careful
consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4853's
high power performance and activate unwanted, though necessary, thermal shutdown protection.
The LD 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 continuous unbroken copper. This plane forms a thermal mass, 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 4(2x2) vias. The via diameter should be 0.012in-0.013in with a 1.27mm pitch. Ensure efficient
thermal conductivity by plating through the vias.
Best thermal performance is achieved with the largest practical heat sink area. If the heatsink and amplifier share
the same PCB layer, a nominal 2.5in
2
area is necessary for 5V operation with a 4Ω load. Heatsink areas not
placed on the same PCB layer as the LM4853 should be 5in
2
(min) for the same supply voltage and load
resistance. The last two area recommendations apply for 25°C ambient temperature. Increase the area to
compensate for ambient temperatures above 25°C. The LM4853's power de-rating curve in the Typical
Performance Characteristics
LD Specific Characteristics shows the maximum power dissipation versus temperature. An example PCB layout
for the LD package is shown in the Demonstration Board Layout section. For further detailed and specific
information concerning PCB layout, fabrication, and mounting an NHE (WSON) package, see TI's AN-1187
Application Report.
BRIDGE CONFIGURATION EXPLANATION
When the LM4853 is in BTL mode, the output of amplifier A1 serves as the input to amplifier A2, which results in
both amplifiers producing signals identical in magnitude, but out of phase by 180°. Consequently, the differential
gain for the mono channel is:
A
VD
= V
OUT
/ (V
INL
+ V
INR
) = 2 × (R
F
/ R
I
) (1)
Driving a load differentially through the BTL OUT- and BTL OUT+ outputs is an amplifier configuration commonly
referred to as "bridged mode". Bridged mode operation is different from the classical single-ended amplifier
configuration where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration. It drives a load
differentially, which doubles output swing for a specified supply voltage. This produces four times the output
power as that produced by a single-ended amplifier under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier's closed-loop
gain without causing excessive output signal clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN
section.
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