Datasheet

LM4898, LM4898MMBD
www.ti.com SNAS216E –MAY 2003–REVISED APRIL 2013
APPLICATION INFORMATION
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4898 is a fully differential audio amplifier that features differential input and output stages. Internally this is
accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the
output voltages so that the average value remains V
DD
/2. When setting the differential gain, the amplifier can be
considered to have two "halves". Each half uses an input and feedback resistor (R
i
1 and R
f
1) to set its respective
closed-loop gain (see Figure 4). With R
i
1 = R
i
2 and R
f
1 = R
f
2, the gain is set at -R
f
/R
i
for each half. This results
in a differential gain of
A
VD
= -R
f
/R
i
(1)
It is extremely important to match the input resistors to each other, as well as the feedback resistors to each
other for best amplifier performance. See the PROPER SELECTION OF EXTERNAL COMPONENTS section for
more information. A differential amplifier works in a manner where the difference between the two input signals is
amplified. In most applications, this would require input signals that are 180° out of phase with each other. The
LM4898 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 LM4898 simply amplifies the
difference between them.
All of these applications, either single-ended or fully differential, provide what is known as a "bridged mode"
output (bridge-tied-load, BTL). This results in output signals at Vo1 and Vo2 that are 180° out of phase with
respect to each other. Bridged mode operation is different from the single-ended amplifier configuration that
connects the load between the amplifier output and ground. A bridged amplifier design has distinct advantages
over the single-ended configuration: it provides differential drive to the load, thus doubling maximum possible
output swing for a specific supply voltage. Four times the output power is possible compared with 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 excess clipping,
please refer to the AUDIO POWER AMPLIFIER DESIGN.
A bridged configuration, such as the one used in the LM4898, also creates a second advantage over single-
ended amplifiers. 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
(see PROPER SELECTION OF EXTERNAL COMPONENTS). 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 loudspeaker damage. Further advantages of bridged mode
operation specific to fully differential amplifiers like the LM4898 include increased power supply rejection ratio,
common-mode noise reduction, and click and pop reduction.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS
The LM4898’s exposed-DAP (die attach paddle) package (NGZ0010B) 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, finally, surrounding air. The result is a low voltage
audio power amplifier that produces 1.4W at ≤1% THD with a 4Ω load. This high power is achieved through
careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the
LM4898’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The
NGZ0010B 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 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 4 (2x2) vias. The via diameter should be 0.012in - 0.013in with a 0.050in pitch. Ensure
efficient thermal conductivity by plating through and solder-filling the vias.
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