LM48860 datasheet(12/18 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 LM48860 audio amplifier which

is a CMOS device.

Before applying any signal to the inputs or shutdown pins of

the LM48860, 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 SHUT-

DOWN) and input pins.

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM48860 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows the

outputs of the LM48860 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 LM48860 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 LM48860 when compared to a traditional

headphone amplifier operating from the same supply voltage.

OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED

The LM48860 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 LM48860 has two internal opera-

tional amplifiers. The two amplifiers have internally configured

gain.

Since this is an output ground-referenced amplifier, the

LM48860 does not require output coupling capacitors.

POWER DISSIPATION

From the graph (THD+N vs Output Power , V

DD = 3V, RL =

16Ω, f = 1kHz, 22kH BW, two channels in phase, page 6)

assuming a 3V power supply and a 16

Ω load, the maximum

power dissipation point and thus the maximum package dis-

sipation point is 281mW. The maximum power dissipation

point obtained must not be greater than the power dissipation

that results from Equation 1.

DMAX = (TJMAX - TA) / (θJA)

(1)

For the micro SMD package

JA = 59.3°C/W. TJMAX = 150°C

for the LM48860. Depending on the ambient temperature,

A, of the system surroundings, Equation 1 can be used to

find the maximum internal power dissipation supported by the

IC packaging. If the maximum power dissipation from the

graph is greater than that of Equation 1, then either the supply

voltage must be decreased, the load impedance increased or

A reduced (see power derating curves). For the application

of a 5V power supply, with a 16

Ω load, the maximum ambient

temperature possible without violating the maximum junction

temperature is approximately 110°C provided that device op-

eration is around the maximum power dissipation point. Pow-

er 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'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 LM48860'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

LM48860’s shutdown function. When active, the LM48860’s

micropower shutdown feature turns off the amplifiers’ bias

circuitry, reducing the supply current. The trigger point is

0.45V for a logic-low level, and 1.2V for logic-high level. The

low 0.01µA (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 in-

crease the shutdown current. Do not let SD_LC/SD_RC float,

connect either to high or low.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM48860's performance requires properly se-

lecting external components. Though the LM48860 operates

well when using external components with wide tolerances,

best performance is achieved by optimizing component val-

ues.

Charge Pump Capacitor Selection

Use low ESR (equivalent series resistance) (<100m

Ω) ce-

ramic capacitors with an X7R dielectric for best performance.

Low ESR capacitors keep the charge pump output

impedance to a minimum, extending the headroom on the

negative supply. Higher ESR capacitors result in reduced

output power from the audio amplifiers.

Charge pump load regulation and output impedance are af-

fected by the value of the flying capacitor (C4). A larger valued

C4 (up to 3.3uF) improves load regulation and minimizes

charge pump output resistance. Beyond 3.3uF, the switch-on

resistance dominates the output impedance.

The output ripple is affected by the value and ESR of the out-

put capacitor (C3). Larger capacitors reduce output ripple on

the negative power supply. Lower ESR capacitors minimize

the output ripple and reduce the output impedance of the

charge pump.

The LM48860 charge pump design is optimized for 2.2uF, low

ESR, ceramic, flying and output capacitors.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitors (C1 and C2 in Figure 1). A high val-

ue capacitor can be expensive and may compromise space

efficiency in portable designs. In many cases, however, the

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