Bypass Pin Capacitor

In order to achieve the largest unclipped output voltage

swing, a DC reference voltage for the circuit is derived inter-

nally by a resistor divider from the power supply voltage. To

reduce sensitivity to audio frequency variations on the refer-

ence voltage, a de-coupling capacitor is recommended be-

tween the BYPASS pin and the best available AC ground ref-

erence. This is typically the shield connection from the signal

source. A poor choice is the power ground because consid-

erable current flows from the LM4663 through pins 4,8,17,

and 21 to the power supply return. This current is rich in har-

monic frequencies related to the input audio frequency.

These harmonics can easily couple into the signal path by

sharing power ground conductors with the bypass capacitor

connection. The result is increased THD.

Minimum Load Resistance

As specified in the Absolute Maximum Ratings, the minimum

load resistance supported by the LM4663 at the output of the

filters is 2.5 Ohms. Loads (usually loudspeakers) should be

tested over the audio band to determine whether or not there

are resonant points of diminshed impedance less than this

limit.

Output Filter Components

Table 2 shows suggested standard values of the capacitor

(C) and the inductor (L) used in Figure 1’s output filter for 4

Ω

and 8

Ω loads. The bandwidth for each case is typically

18kHz.

TABLE 2. Output filter component values for different

load resistance.

Load Resistance (

Ω) Capacitor C (µF) Inductor L (µH)

42

22

81

44

Short Circuit Tolerance

The LM4663, when used in the recommended application

circuit, is tolerant of limited duration short circuit connections

between the filtered positive and negative outputs. Shorts

from either output to ground or supply voltage, or from the

unfiltered outputs to any other low impedance node can re-

sult in permanent damage to the IC.

Start-up Current

The LM4663 exhibits abnormally high quiescent current

drain when the supply voltage is below the specified operat-

ing range of 4.5 to 5.5V. It is, therefore, recommended that

the IC be initially powered up in the shutdown mode (SD =

V

supplies is not usually a problem unless the current limit of

have a much lower current capability (such as USB applica-

tions). In these cases power must be applied with shutdown

asserted and switched to the operating mode after power is

present and settled. An external circuit that provides this

condition automatically is given in Figure 2, and is recom-

mended for those current-critical applications.

Printed Circuit Board Layout

Figures 3 through 7 show the layout of a recommended

four-layer PC board that is optimized for the LM4663 and as-

sociated external components. This layout emphasizes short

connections between the power reservoir capacitors near

the power amp outputs and their associated pins. Short con-

nections reduce inductance and EMI radiation from the out-

put switching currents. Also, the distance from the switching

outputs to the filter inductors is similarly minimized to reduce

radiated noise. Table 4 is the recommended circuit’s

bill-of-materials (BOM). Figure 8 is a photograph of the rec-

ommended four-layer PC board.

The board is designed for use with an external 5V supply, 4

Ω

speakers, and 32

Ω headphones. Apply the supply voltage to

the V

channel speaker between the Lout+ and Lout− pads and the

right channel speaker between the Rout+ and Rout− pads.

Connect the left and right headphone speakers to the HPL

and HPR pads, respectively. The board has two sets of HPL

and HPR pads. Use the set found across the top edge for a

headphone jack like that shown in Figure 1. The other set

found along the right edge can be used for a three terminal

headphone jack. Ensure that a speaker’s + terminal is con-

nected to an amplifier’s + output. This preserves the phase

relationship between the left and right channels.

The board accepts two stereo inputs. Apply channel 1’s right

and left input signals to the Rin1 and Lin1 pads, respectively.

Apply channel 2’s right and left input signals to the Rin2 and

Lin2 pads, respectively.

The board’s bottom edge has seven jumpers. From left to

right, these jumpers select micropower shutdown, input

channel, headphone amplifier, Class D amplifier, and head-

phone amplifier when plugging in headphones. Table 3 is a

guide for selecting the various functions.

TABLE 3. These are the functions selected by the

jumpers on the recommended PC board.

Jumper

Function

SD-L

Shorting selects normal operation.

SD-H Jumper is open.

SD-H

Shorting activates shutdown:

micropower operation.

SD-L Jumper is open.

INSEL-L

Shorting selects stereo input 1.

INSEL-H Jumper is open.

INSEL-H

Shorting selects stereo input 2.

INSEL-L Jumper is open.

Class AB

Shorting activates the headphone

amplifier and deactivates the Class D

amplifier. Class D and HP jack

jumpers are open

Class D

Shorting activates the Class D amplifier

and deactivates the headphone

amplifier. Class AB and HP jack

jumpers are open

HP Jack

Shorting allows the act of plugging in

headphones to activate the headphone

amplifier and deactivate the Class D

amplifier. Removing headphones

restores Class D amplifier operation and

deactivates the headphone amplifier.

Class AB and Class D jumpers are

open. (Requires the use of a headphone

jack like that shown in Figure 1.)

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