LM2876

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SNAS088C – AUGUST 1995 – REVISED MARCH 2013

REACTIVE LOADING

It is hard for most power amplifiers to drive highly capacitive loads very effectively and normally results in

oscillations or ringing on the square wave response. If the output of the LM2876 is connected directly to a

capacitor with no series resistance, the square wave response will exhibit ringing if the capacitance is greater

than about 0.2

μF. If highly capacitive loads are expected due to long speaker cables, a method commonly

employed to protect amplifiers from low impedances at high frequencies is to couple to the load through a 10

Ω

resistor in parallel with a 0.7

μH inductor. The inductor-resistor combination as shown in the Typical Application

Circuit isolates the feedback amplifier from the load by providing high output impedance at high frequencies thus

allowing the 10

Ω resistor to decouple the capacitive load and reduce the Q of the series resonant circuit. The LR

combination also provides low output impedance at low frequencies thus shorting out the 10

Ω resistor and

allowing the amplifier to drive the series RC load (large capacitive load due to long speaker cables) directly.

GENERALIZED AUDIO POWER AMPLIFIER DESIGN

The system designer usually knows some of the following parameters when starting an audio amplifier design:

Desired Power Output

Input Level

Input Impedance

Load Impedance

Maximum Supply Voltage

Bandwidth

The power output and load impedance determine the power supply requirements, however, depending upon the

application some system designers may be limited to certain maximum supply voltages. If the designer does

have a power supply limitation, he should choose a practical load impedance which would allow the amplifier to

provide the desired output power, keeping in mind the current limiting capabilities of the device. In any case, the

output signal swing and current are found from (where PO is the average output power):

(5)

(6)

To determine the maximum supply voltage the following parameters must be considered. Add the dropout

voltage (4V for LM2876) to the peak output swing, Vopeak, to get the supply rail value (i.e. ± (Vopeak + Vod) at a

current of Iopeak). The regulation of the supply determines the unloaded voltage, usually about 15% higher.

Supply voltage will also rise 10% during high line conditions. Therefore, the maximum supply voltage is obtained

from the following equation:

Max. supplies

≈ ± (Vopeak + Vod)(1 + regulation)(1.1)

(7)

The input sensitivity and the output power specs determine the minimum required gain as depicted below:

(8)

Normally the gain is set between 20 and 200; for a 40W, 8

Ω audio amplifier this results in a sensitivity of 894 mV

and 89 mV, respectively. Although higher gain amplifiers provide greater output power and dynamic headroom

capabilities, there are certain shortcomings that go along with the so called “gain.” The input referred noise floor

is increased and hence the SNR is worse. With the increase in gain, there is also a reduction of the power

bandwidth which results in a decrease in feedback thus not allowing the amplifier to respond quickly enough to

nonlinearities. This decreased ability to respond to nonlinearities increases the THD + N specification.

The desired input impedance is set by RIN. Very high values can cause board layout problems and DC offsets at

the output. The value for the feedback resistance, Rf1, should be chosen to be a relatively large value (10

k

Ω–100 kΩ), and the other feedback resistance, Ri, is calculated using standard op amp configuration gain

equations. Most audio amplifiers are designed from the non-inverting amplifier configuration.

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