LP3921 Datasheet, PDF (31/41 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

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Tolerance
20%
10%
5%
1%
0%
LP3921
R_F1
0.8R
0.9R
0.95R
0.99R
R
SNVS580A – AUGUST 2008 – REVISED MAY 2013
Table 23. Feedback Resistor Mis-match
R_F2
1.2R
1.1R
1.05R
1.01R
R
V₀₂ - V₀₁
-0.500V
-0.250V
-0.125V
-0.025V
0
I_LOAD
62.5 mA
31.25 mA
15.63 mA
3.125 mA
0
Similar results would occur if the input resistors were not carefully matched. Adding input coupling capacitors in
between the signal source and the input resistors will eliminate this problem, however, to achieve best
performance with minimum component count it is highly recommended that both the feedback and input resistors
matched to 1% tolerance or better.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier
Given:
• Power Output: 1 Wrms
• Load Impedance: 8Ω
• Input Level: 1 Vrms
• Input Impedance: 20 kΩ
• Bandwidth: 100 Hz–20 kHz ± 0.25 dB
A designer must first determine the minimum supply rail to obtain the specified output power. To determine the
minimum supply rail is to calculate the required V_OPEAK using Equation 8 and add the dropout voltages.
(8)
Using the Output Power vs. Supply Voltage graph for an 8Ω load, the minimum supply rail just about 5V. Extra
supply voltage creates headroom that allows the LP3921 to reproduce peaks in excess of 1W without producing
audible distortion. At this time, the designer must make sure that the power supply choice along with the output
impedance does not violate the conditions explained in POWER DISSIPATION. Once the power dissipation
equations have been addressed, the required differential gain can be determined from Equation 9.
(9)
R_f / R_i = A_VD
(10)
From Equation 10, the minimum A_VD is 2.83. Since the desired input impedance was 20 kΩ, a ratio of 2.83:1 of
R_f to R_i results in an allocation of R_i = 20 kΩ for both input resistors and R_f = 60 kΩ for both feedback resistors.
The final design step is to address the bandwidth requirement which must be stated as a single -3 dB frequency
point. Five times away from a -3 dB point is 0.17 dB down from pass band response which is better than the
required ±0.25 dB specified.
f_H = 20 kHz * 5 = 100 kHz
(11)
The high frequency pole is determined by the product of the desired frequency pole, f_H , and the differential gain,
A_VD. With A_VD = 2.83 and f_H = 100 kHz, the resulting GBWP = 150 kHz which is much smaller than the LP3921
GBWP of 10 MHz. This figure displays that if a designer has a need to design an amplifier with a higher
differential gain, the LP3921 can still be used without running into bandwidth limitations.
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