LP3921 Datasheet, PDF (28/41 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

LP3921
SNVS580A – AUGUST 2008 – REVISED MAY 2013
Differential Amplifier Explanation
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Bit
0
Name
amp_en
Table 22. Register Address 8h'19 Audio_Amp
Function
(if the powerup default is "amplifier disabled")
Bit set to '0' - amplifier disabled
Bit set to '1' - amplifier enabled
The LP3921 contains 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 VDD/2. When setting the differential gain, the amplifier can
be considered to have "halves". Each half uses an input and feedback resistor (Ri1 and RF1) to set its respective
closed-loop gain. (See Figure 10.) With Ri1 = Ri2 and RF1 = RF2, the gain is set at -RF / Ri for each half. This
results in a differential gain of:
AVD = −RF/Ri
(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. 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 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 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.
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 LP3921 include increased power supply rejection ratio, common-mode noise reduction, and click and pop
reduction.
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Figure 10. Audio Block
Product Folder Links: LP3921
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