Application Information

SUPPLY VOLTAGE SEQUENCING

Before applying any signal to the inputs or shutdown pins of

the LM48820, it is important to apply a supply voltage to the

V

applied to the shutdown pins (see MICRO POWER SHUT-

DOWN) and input pins.

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM48820 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows the

outputs of the LM48820 to be biased about GND instead of a

nominal DC voltage, like traditional headphone amplifiers.

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 out-

put, but also attenuates low frequencies, impacting the bass

response. Because the LM48820 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 LM48820 when compared to a traditional

headphone amplifier operating from the same supply voltage.

OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED

The LM48820 contains advanced circuitry that virtually elim-

inates output transients ('clicks and pops'). This circuitry pre-

vents 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 LM48820 has two internal opera-

tional amplifiers. The two amplifiers have internally configured

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

R

A

(1)

where R

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a suc-

cessful design. Equation 1 states the maximum power dissi-

pation point for a single-ended amplifier operating at a given

supply voltage and driving a specified output load.

P

2

(2)

Since the LM48820 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from Equation 2. Even

with large internal power dissipation, the LM48820 does not

require heat sinking over a large range of ambient tempera-

tures. From Equation 2, assuming a 3V power supply and a

16

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 3:

P

(3)

For this micro SMD package,

θ

C. Depending on the ambient temperature, T

surroundings, Equation 3 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 1, then

either the supply voltage must be decreased, the load

impedance increased or T

tion of a 3V power supply, with a 16

Ω load, the maximum

ambient temperature possible without violating the maximum

junction temperature is approximately 127°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 crit-

ical for low noise performance and high power supply rejec-

tion. Applications that employ a 3V power supply typically use

a 4.7µF capacitor in parallel with a 0.1µF ceramic filter ca-

pacitor to stabilize the power supply 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 LM48820's power supply pin and ground as short

as possible.

MICRO POWER SHUTDOWN

The voltage applied to the SD_LC (shutdown left channel) pin

and the SD_RC (shutdown right channel) pin controls the

LM48820’s shutdown function. When active, the LM48820’s

micropower shutdown feature turns off the amplifiers’ bias

circuitry, reducing the supply current. The trigger point is

0.45V (max) for a logic-low level, and 1.2V (min) for logic-high

level. The low 0.05µA (typ) shutdown current is achieved by

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

SD_LC/SD_RC pins. 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 mi-

croprocessor, or a microcontroller. When using a switch,

connect an external 100k

Ω pull-up resistor between the

SD_LC/SD_RC pins and V

the SD_LC/SD_RC pins and ground. Select normal amplifier

operation by opening the switch. Closing the switch connects

the SD_LC/SD_RC pins to ground, activating micro-power

shutdown. The switch and resistor guarantee that the

SD_LC/SD_RC pins will not float. This prevents unwanted

state changes. In a system with a microprocessor or micro-

controller, use a digital output to apply the control voltage to

the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins

with active circuitry eliminates the pull-up resistor.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM48820's performance requires properly se-

lecting external components. Though the LM48820 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

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