LM4922 datasheet(17/21 Pages) NSC

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Application Information

SUPPLY VOLTAGE SEQUENCING

It is a good general practice to first apply the supply voltage

to a CMOS device before any other signal or supply on other

pins. This is also true for the LM4922 audio amplifier which is

a CMOS device.

Before applying any signal to the inputs or shutdown pins of

the LM4922, it is important to apply a supply voltage to the

DD pins. After the device has been powered, signals may

be applied to the shutdown pins (see MICRO POWER

SHUTDOWN) and input pins.

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM4922 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows

the outputs of the LM4922 to be biased about GND instead

of a nominal DC voltage, like traditional headphone amplifi-

ers. Because there is no DC component, the large DC

blocking capacitors (typically 220µF) are not necessary. The

coupling capacitors are replaced by two, small ceramic

charge pump capacitors, saving board space and cost.

Eliminating the output coupling capacitors also improves low

frequency response. In traditional headphone amplifiers, the

headphone impedance and the output capacitor form a high

pass filter that not only blocks the DC component of the

output, but also attenuates low frequencies, impacting the

bass response. Because the LM4922 does not require the

output coupling capacitors, the low frequency response of

the device is not degraded by external components.

In addition to eliminating the output coupling capacitors, the

ground referenced output nearly doubles the available dy-

namic range of the LM4922 when compared to a traditional

headphone amplifier operating from the same supply volt-

age.

OUTPUT TRANSIENT (’CLICK AND POPS’)

ELIMINATED

The LM4922 contains advanced circuitry that virtually elimi-

nates output transients (’clicks and pops’). This circuitry

prevents all traces of transients when the supply voltage is

first applied or when the part resumes operation after coming

out of shutdown mode.

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4922 has two internal opera-

tional amplifiers. The two amplifiers have internally config-

ured gain, the closed loop gain is set by selecting the ratio of

f to Ri. Consequently, the gain for each channel of the IC is

V = -(Rf /Ri) = 1.5 V/V

where R

F = 30k

Ω and R

i = 20k

Ω.

Since this is an output ground-referenced amplifier, by driv-

ing the headphone through R

OUT (Pin C2) and LOUT (Pin

D2), the LM4922 does not require output coupling capaci-

tors. The typical single-ended amplifier configuration re-

quires large, expensive output capacitors.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

DMAX =(VDD)

2 /(2

π2R

(1)

Since the LM4922 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from Equation 1. Even

with large internal power dissipation, the LM4922 does not

require heat sinking over a large range of ambient tempera-

tures. From Equation 1, assuming a 3V power supply and a

Ω load, the maximum power dissipation point is 28mW per

amplifier. Thus the maximum package dissipation point is

56mW. The maximum power dissipation point obtained must

not be greater than the power dissipation that results from

Equation 2:

DMAX =(TJMAX -TA)/(

JA)

(2)

For the micro SMD package,

JA = 105˚C/W. TJMAX = 150˚C

for the LM4922. Depending on the ambient temperature, T

of the system surroundings, Equation 2 can be used to find

the maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased, the load impedance increased or T

A reduced. For

the typical application of a 3V power supply, with a 16

Ω load,

the maximum ambient temperature possible without violating

the maximum junction temperature is approximately 144˚C

provided that device operation is around the maximum

power dissipation point. Power dissipation is a function of

output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature

may be increased accordingly.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 3V power supply typi-

cally use a 4.7µF capacitor in parallel with a 0.1µF ceramic

filter capacitor to stabilize the power supply’s output, reduce

noise on the supply line, and improve the supply’s transient

response. Keep the length of leads and traces that connect

capacitors between the LM4922’s power supply pin and

ground as short as possible.

AUTOMATIC STANDBY MODE

The LM4922 features Automatic Standby Mode circuitry

(patent pending). In the absence of an input signal, after

approximately 12 seconds, the LM4922 goes into low cur-

rent standby mode. The LM4922 recovers into full power

operating mode immediately after a signal, which is greater

than the input threshold voltage, is applied to either the left or

right input pins. The input threshold voltage is not a static

value, as the supply voltage increases, the input threshold

voltage decreases. This feature reduces power supply cur-

rent consumption in battery operated applications. Please

see also the graph entitled Representation of Automatic

Standby Mode Behavior in the Typical Performance Charac-

teristics section.

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