Application Information

PIN OUT COMPATIBILITY WITH THE LM4863

The LM4873 pin out was designed to simplify replacing the

LM4863: except for the four bottom pins, which the imple-

ment the LM4873’s extra functionality, the LM4873MT/MTE

and LM4863MT/MTE pin outs match.(Note 22)

Note 22: If the LM4873 replaces an LM4863 and the input mux circuitry is

INPUT MUX

The has two inputs per channel. The Mux Control pin con-

trols which input is active. As shown in the Truth Table for

Logic Inputs, if the Mux Control is held low, input 1 is active.

If the Mux Control is held high, input 2 is active.

Figure 2 shows an example usage of the Mux Control circuit.

Mux input 1 is connected to a feedback network that in-

creases gain at low frequencies (bass boost). Mux input 2 is

connected to a simple gain circuit. The example circuit has

mux input 1 used to equalize the internal speaker and mux

input 2 used for line-out or headphone driving. In this case,

the Mux Control and HP In pins would be tied together, so

that when the headphone was plugged in, the feedback net-

work would automatically be changed. If the HP In and Mux

Control pins are not connected, the example circuit be used

for user-selectable bass-boost, so that independent of the

HP In state, the user could select bass-boost.

Since the Mux Control switches between the two inverting in-

puts of the amplifier, thereby changing the input signal

source or the feedback network, an audible click may be

generated during the transition from one mux input to the

other. For example, in the above example circuit, if the two

gains are markedly different, then, when a transition is made

between mux states, a click may be heard as the feedback

network, and therefore the gain, is suddenly changed.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The exposed-DAP package of the LM4873MTE requires

special attention to thermal design. If thermal design issues

are not properly addressed, an LM4873MTE driving 4

Ω will

go into thermal shutdown.

The exposed-DAP on the bottom of the LM4873MTE should

be soldered down to a copper pad on the circuit board. Heat

is conducted away from the exposed-DAP by a copper

plane. If the copper plane is not on the top surface of the cir-

cuit board, 8 to 10 vias of 0.013 inches or smaller in diameter

should be used to thermally couple the exposed-DAP to the

plane. For good thermal conduction, the vias must be

plated-through and solder-filled.

The copper plane used to conduct heat away from the

exposed-DAP should be as large as pratical. If the plane is

on the same side of the circuit board as the exposed-DAP,

Ω. If the heat

sink plane is buried or not on the same side as the exposed-

DAP, 5in

Ω. If the am-

bient temperature is higher than 25˚C, a larger copper plane

or

forced-air

cooling

will

be

required

to

keep

the

LM4873MTE junction temperature below the thermal shut-

down temperature (150˚C). See the power derating curve for

the LM4873MTE for derating information.

The LM4873MTE requires forced-air cooling when operating

into 3

with a 3

Ω load, and with an ambient temperature of 25˚C,

450 linear-feet per minute kept the part out of thermal shut-

down. In higher ambient temperatures, higher airflow rates

and/or larger copper areas will be required to keep the part

out of thermal shutdown.

See DEMOBOARD CIRCUIT LAYOUT for an example of an

exposed-DAP TSSOP circuit board layout.

3

Ω &4Ω LAYOUT CONSIDERATIONS

With low impedance loads, the output power at the loads is

heavily dependent on trace resistance from the output pins

of the LM4873. Traces from the output of the LM4873MTE to

the load or load connectors should be as wide as practical.

Any resistance in the output traces will reduce the power de-

Power Supply

Rejection Ratio

DS100993-21

Open Loop

Frequency Response

DS100993-22

Supply Current vs

Supply Voltage

DS100993-23

DS100993-70

FIGURE 2. Input Mux Example

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