Application Information

GENERAL AMPLIFIER FUNCTION

The LM48861 headphone amplifier features National’s

ground referenced architecture that eliminates the large DC-

blocking capacitors required at the outputs of traditional head-

phone amplifiers. A low-noise inverting charge pump creates

a negative supply (CPV

(V

supplies, with the amplifier outputs biased about GND, in-

stead of a nominal DC voltage (typically V

amplifiers. Because there is no DC component to the head-

phone output signals, the large DC-blocking capacitors (typ-

ically 220

μF) are not necessary, conserving board space and

system cost, while improving frequency response.

COMMON MODE SENSE

The LM48861 features a ground (common mode) sensing

feature. In noisy applications, or where the headphone jack is

used as a line out to other devices, noise pick up and ground

imbalance can degrade audio quality. The LM48861 COM in-

put senses and corrects any noise at the headphone return,

or any ground imbalance between the headphone return and

device ground, improving audio reproduction. Connect COM

directly to the headphone return terminal of the headphone

jack (Figure 2). No additional external components are re-

quired. Connect COM to GND if the common-mode sense

feature is not in use.

30054101

FIGURE 2.

MICRO POWER SHUTDOWN

The voltage applied to the shutdown (SHDN) pin controls the

LM48861’s shutdown function. Activate micro-power shut-

down by applying a logic-low voltage to the SHDN pin. When

active, the LM48861’s micro-power shutdown feature turns

off the amplifier’s bias circuitry, reducing the supply current.

The trigger point is 0.4V (max) for a logic-low level, and 1.4V

(min) for a logic-high level. The low 0.1

μA (typ) shutdown cur-

rent is achieved by applying a voltage that is as near as

ground as possible to the SHDN 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 mi-

croprocessor, or a microcontroller. When using a switch,

connect an external 100k

Ω pull-up resistor between the

SHDN pin and GND. Connect the switch between the

SHDN pin and V

ing the switch. Opening the switch connects the SHDN pin to

ground, activating micro-power shutdown. The switch and re-

sistor guarantee that the SHDN pin will not float. This prevents

unwanted state changes. In a system with a microprocessor

or microcontroller, use a digital output to apply the control

voltage to the SHDN pin. Driving the SHDN pin with active

circuitry eliminates the pull-up resistor.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier, especially one in mobile devices. In the LM48861,

the power dissipation comes from the charge pump and two

operational amplifiers. Refer to the Power Dissipation vs Out-

put Power curve in the Typical Performance Characteristics

section of the datasheet to find the power dissipation associ-

ated the output power level of the LM48861. The power

dissipation should not exceed the maximum power dissipa-

tion point of the micro SMD package given in equation 1.

P

(1)

For the LM48861TM micro SMD package,

θ

T

tem surroundings.

PROPER SELECTION OF EXTERNAL COMPONENTS

Power Supply Bypassing/Filtering

Proper power supply bypassing is critical for low noise per-

formance and high PSRR. Place the supply bypass capaci-

tors as close to the supply pins as possible. Place a 1

μF

ceramic capacitor from V

tance may be added as required.

Charge Pump Capacitor Selection

Use low ESR ceramic capacitors (less than 100m

Ω) for opti-

mum performance.

Charge Pump Flying Capacitor (C5)

The flying capacitor (C5) affects the load regulation and out-

put impedance of the charge pump. A C5 value that is too low

results in a loss of current drive, leading to a loss of amplifier

headroom. A higher valued C5 improves load regulation and

lowers charge pump output impedance to an extent. Above

2.2

μF, the R

of C5 and C6 dominate the output impedance. A lower value

capacitor can be used in systems with low maximum output

power requirements.

Charge Pump Hold Capacitor (C6)

The value and ESR of the hold capacitor (C6) directly affects

the ripple on CPV

put ripple. Decreasing the ESR of C6 reduces both output

ripple and charge pump output impedance. A lower value ca-

pacitor can be used in systems with low maximum output

power requirements.

Power Supply Bypassing /Filtering

Proper power supply bypassing is critical for low noise per-

formance and high PSRR. Place the supply bypass capaci-

tors as close to the device as possible. Typical applications

employ a voltage regulator with 10µF and 0.1µF bypass ca-

pacitors that increase supply stability. These capacitors do

not eliminate the need for bypassing of the LM48861 supply

pins. A 1µF capacitor is recommended.

Input Capacitor Selection

The LM48861 requires input coupling capacitors. Input ca-

pacitors block the DC component of the audio signal, elimi-

nating any conflict between the DC component of the audio

source and the bias voltage of the LM48861. The input ca-

pacitors create a high-pass filter with the input resistors R

9

www.national.com