Application Information

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4912 has three operational

amplifiers internally. Two of the amplifier’s have externally

configurable gain while the other amplifier is internally fixed

at the bias point acting as a unity-gain buffer. The closed-

loop gain of the two configurable amplifiers is set by select-

ing the ratio of R

channel of the IC is

A

By driving the loads through outputs V

acting as a buffered bias voltage the LM4912 does not

require output coupling capacitors. The classical single-

ended amplifier configuration where one side of the load is

connected to ground requires large, expensive output cou-

pling capacitors.

A configuration, such as the one used in the LM4912, has a

major advantage over single supply, single-ended amplifiers.

Since the outputs V

V

nates the need for output coupling capacitors which are

required in a single-supply, single-ended amplifier configura-

tion. Without output coupling capacitors in a typical single-

supply, single-ended amplifier, the bias voltage is placed

across the load resulting in both increased internal IC power

dissipation and possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. When operating in capacitor-coupled

mode, 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.

P

(1)

Since the LM4912 has two operational amplifiers in one

package, the maximum internal power dissipation point is

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

Equation 1, assuming a 3V power supply and a 32

Ω load,

the maximum power dissipation point is 14mW per amplifier.

Thus the maximum package dissipation point is 28mW.

The maximum power dissipation point obtained from Equa-

tion 1 must not be greater than the power dissipation that

results from Equation 2:

P

θ

JA

(2)

For package MUB10A,

θ

the LM4912. Depending on the ambient temperature, T

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

the typical application of a 3V power supply, with an 32

Ω

load, the maximum ambient temperature possible without

violating the maximum junction temperature is approximately

144˚C provided that device operation is around the maxi-

mum power dissipation point. Thus, for typical applications,

power dissipation is not an issue. 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. Refer to the

Typical Performance Characteristics curves for power dissi-

pation information for lower output powers.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is important

for low noise performance and high power supply rejection.

The capacitor location on the power supply pins should be

as close to the device as possible.

Typical applications employ a 3V regulator with 10mF tanta-

lum or electrolytic capacitor and a ceramic bypass capacitor

which aid in supply stability. This does not eliminate the need

for bypassing the supply nodes of the LM4912. A bypass

capacitor value in the range of 0.1µF to 1µF is recommended

for C

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4912’s shutdown function. Activate micro-power shut-

down by applying a logic-low voltage to the SHUTDOWN

pin. When active, the LM4912’s micro-power shutdown fea-

ture turns off the amplifier’s bias circuitry, reducing the sup-

ply current. The trigger point varies depending on supply

voltage and is shown in the Shutdown Hysteresis Voltage

graphs in the Typical Performance Characteristics section.

The low 0.1µA(typ) shutdown current is achieved by apply-

ing a voltage that is as near as ground as possible to the

SHUTDOWN pin. A voltage that is higher than ground may

increase the shutdown current. There are a few ways to

control the micro-power shutdown. These include using a

single-pole, single-throw switch, a microprocessor, or a mi-

crocontroller. When using a switch, connect an external

100k

Ω pull-up resistor between the SHUTDOWN pin and

V

ground. Select normal amplifier operation by opening the

switch. Closing the switch connects the SHUTDOWN pin to

ground, activating micro-power shutdown.

The switch and resistor guarantee that the SHUTDOWN pin

will not float. This prevents unwanted state changes. In a

system with a microprocessor or microcontroller, use a digi-

tal output to apply the control voltage to the SHUTDOWN

pin. Driving the SHUTDOWN pin with active circuitry elimi-

nates the pull-up resistor.

Shutdown enable/disable times are controlled by a combina-

tion of C

on/off times from Shutdown. Smaller Vdd values also in-

crease turn on/off time for a given value of C

shutdown times also improve the LM4912’s resistance to

click and pop upon entering or returning from shutdown. For

a 2.4V supply and C

seconds to enter or return from shutdown. This longer shut-

down time enables the LM4912 to have virtually zero pop

and click transients upon entering or release from shutdown.

Smaller values of C

cost of increased pop and click and reduced PSRR. Since

shutdown enable/disable times increase dramatically as

supply voltage gets below 2.2V, this reduced turn-on time

may be desirable if extreme low supply voltage levels are

used as this would offset increases in turn-on time caused by

the lower supply voltage.

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