LP3921

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SNVS580A – AUGUST 2008 – REVISED MAY 2013

Table 23. Feedback Resistor Mis-match

Tolerance

RF1

RF2

V02 - V01

ILOAD

20%

0.8R

1.2R

-0.500V

62.5 mA

10%

0.9R

1.1R

-0.250V

31.25 mA

5%

0.95R

1.05R

-0.125V

15.63 mA

1%

0.99R

1.01R

-0.025V

3.125 mA

0%

R

R

0

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/8

Ω Audio Amplifier

Given:

•

Power Output: 1 Wrms

•

Load Impedance: 8

Ω

•

Input Level: 1 Vrms

•

Input Impedance: 20 k

Ω

•

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 VOPEAK using Equation 8 and add the dropout voltages.

(8)

Using the Output Power vs. Supply Voltage graph for an 8

Ω 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)

Rf / Ri = AVD

(10)

From Equation 10, the minimum AVD is 2.83. Since the desired input impedance was 20 kΩ, a ratio of 2.83:1 of

Rf to Ri results in an allocation of Ri = 20 kΩ for both input resistors and Rf = 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.

fH = 20 kHz * 5 = 100 kHz

(11)

The high frequency pole is determined by the product of the desired frequency pole, fH , and the differential gain,

AVD. With AVD = 2.83 and fH = 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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