LM4981 datasheet(13/18 Pages) NSC

13 / 18 page

Application Information (Continued)

down pin to the same state as the Shutdown Mode pin. For

simplicity’s sake, this is called "shutdown same", as the

LM4981 enters shutdown mode whenever the two pins are

in the same logic state. The trigger point for either shutdown

high or shutdown low is shown as a typical value in the

Supply Current vs Shutdown Voltage graphs in the Typical

Performance Characteristics section. It is best to switch

between ground and supply for maximum performance.

While the device may be disabled with shutdown voltages in

between ground and supply, the idle current may be greater

than the typical value of 0.1µA. In either case, the shutdown

pin should be tied to a definite voltage to avoid unwanted

state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry, which pro-

vides a quick, smooth transition to shutdown. Another solu-

tion is to use a single-throw switch in conjunction with an

external pull-up resistor (or pull-down, depending on shut-

down high or low application). This scheme guarantees that

the shutdown pin will not float, thus preventing unwanted

state changes.

AUTOMATIC STANDBY MODE

The LM4981 features Automatic Standby Mode circuitry

(patent pending). In the absence of an input signal, after

approximately 12 seconds, the LM4981 goes into low cur-

rent standby mode. The LM4981 recovers into full power

operating mode immediately after a signal, which is greater

than the input threshold voltage, is applied to either the left or

right input pins. The input threshold voltage is not a static

value, as the supply voltage increases, the input threshold

voltage decreases. This feature reduces power supply cur-

rent consumption in battery operated applications. Please

see also the graph entitled Representation of Automatic

Standby Mode Behavior in the Typical Performance Charac-

teristics section.

To ensure correct operation of Automatic Standby Mode,

proper layout techniques should be implemented. Separat-

ing PGND and SGND can help reduce noise entering the

LM4981 in noisy environments. Auto Standby mode works

best when output impedance of the audio source driving

LM4981 is equal or less than 50 Ohms. While Automatic

Standby Mode reduces power consumption very effectively

during silent periods, maximum power saving is achieved by

putting the device into shutdown when it is not in use.

OUTPUT TRANSIENT (’CLICK AND POPS’)

ELIMINATED

The LM4981 contains advanced circuitry that virtually elimi-

nates output transients (’clicks and pops’). This circuitry

prevents all traces of transients when the supply voltage is

first applied or when the part resumes operation after coming

out of shutdown mode.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATION

The LM4981’s exposed-dap (die attach paddle) package

(LD) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane, and surrounding air.

The LD package should have its DAP soldered to a copper

pad on the PCB. The DAP’s PCB copper pad may be con-

nected to a large plane of continuous unbroken copper. This

plane forms a thermal mass, heat sink, and radiation area

However, since the LM4981 is designed for headphone ap-

plications, connecting a copper plane to the DAP’s PCB

copper pad is not required. The DAP on the LM4981 should

be connected to GND to ensure correct functionality.

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4981’s performance requires properly se-

lecting external components. Though the LM4981 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 perfor-

mance. 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

affected by the value of the flying capacitor (C

C). A larger

valued C

C (up to 3.3uF) improves load regulation and mini-

mizes charge pump output resistance. Beyond 3.3uF, the

switch-on resistance dominates the output impedance for

capacitor values above 2.2uF.

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

output capacitor (C

SS).

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 LM4981 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 (C

inA and CinB in Figure 1). A high

value capacitor can be expensive and may compromise

space efficiency in portable designs. In many cases, how-

ever, the speakers used in portable systems, whether inter-

nal or external, have little ability to reproduce signals below

150Hz. Applications using speakers with this limited fre-

quency response reap little improvement by using high value

input and output capacitors.

Besides affecting system cost and size, the input capacitor

has an effect on the LM4981’s click and pop performance.

The magnitude of the pop is directly proportional to the input

capacitor’s size. Thus, pops can be minimized by selecting

an input capacitor value that is no higher than necessary to

meet the desired −3dB frequency.

As shown in Figure 1, the internal input resistor, R

i and the

input capacitor, C

i, produce a -3dB high pass filter cutoff

frequency that is found using Equation (3). Conventional

headphone amplifiers require output capacitors; Equation (3)

can be used, along with the value of R

L, to determine to-

wards the value of output capacitor needed to produce a

–3dB high pass filter cutoff frequency.

i-3dB =1/2

πR

iCi

(3)

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