LM4819 datasheet(13/27 Pages) TI

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LM4819, LM4819MBD

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SNAS133D – FEBRUARY 2001 – REVISED MARCH 2013

In order to eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching VDD

may not allow the capacitors to fully discharge, which may cause "clicks and pops".

AUDIO POWER AMPLIFIER DESIGN EXAMPLE

The following are the desired operational parameters:

Given:

Power Output

100mW

Load Impedance

Ω

Input Level

1Vrms (max)

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.25dB

The design begins by specifying the minimum supply voltage necessary to obtain the specified output power. To

find this minimum supply voltage, use the Output Power vs. Supply Voltage graph in the Typical Performance

Characteristics section. From the graph for a 16

Ω load, (graphs are for 8Ω, 16Ω, and 32Ω loads) the supply

voltage for 100mW of output power with 1% THD+N is approximately 3.15 volts.

Additional supply voltage creates the benefit of increased headroom that allows the LM4819 to reproduce peaks

in excess of 100mW without output signal clipping or audible distortion. The choice of supply voltage must also

not create a situation that violates maximum dissipation as explained above in the Power Dissipation section. For

example, if a 3.3V supply is chosen for extra headroom then according to Equation 3 the maximum power

dissipation point with a 16

Ω load is 138mW. Using Equation 4 the maximum ambient temperature is 121°C for

the DGK0008A package and 126°C for the D0008A package.

After satisfying the LM4819's power dissipation requirements, the minimum differential gain is found using

Equation 6.

(6)

Thus a minimum gain of 1.27 V/V allows the LM4819 to reach full output swing and maintain low noise and

THD+N performance. For this example, let AVD = 1.27. The amplifier's overall gain is set using the input (Ri) and

feedback (RF) resistors. With the desired input impedance set to 20kΩ, the feedback resistor is found using

Equation 7.

RF/Ri = AVD/2 (V/V)

(7)

The value of RF is 13kΩ.

The last step in this design example is setting the amplifier's -3dB frequency bandwidth. To achieve the desired

±0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the

lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth

limit. The gain variation for both response limits is 0.17dB, well with in the ±0.25dB desired limit.

The results are:

fL = 100Hz/5 = 20Hz

fH = 20 kHz*5 = 100kHz

As mentioned in the External Components section, Ri and Ci create a high pass filter that sets the amplifier's

lower band pass frequency limit. Find the coupling capacitor's value using Equation 8.

Ci ≥ 1/(2πRifc) (F)

(8)

Ci ≥ 0.398µF, a standard value of 0.39µF will be used. The product of the desired high frequency cutoff (100kHz

in this example) and the differential gain, AVD, determines the upper pass band response limit. With AVD = 1.27

and fH = 100kHz, the closed-loop gain bandwidth product (GBWP) is 127kHz. This is less than the LM4819's

900kHz GBWP. With this margin the amplifier can be used in designs that require more differential gain while

avoiding performance restricting bandwidth limitations.

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