POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 5V regulator typically

use a 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize the regulator’s output, reduce noise on the supply line,

and improve the supply’s transient response. However, their

presence does not eliminate the need for a local 0.47µF

tantalum bypass capacitance connected between the

LM4913’s supply pins and ground. Do not substitute a ce-

ramic capacitor for the tantalum. Doing so may cause oscil-

lation.

Keep the length of leads and traces that connect capacitors

between the LM4913’s power supply pin and ground as

short as possible. Connecting a 0.47µF capacitor, CB, be-

tween the BYPASS pin and ground improves the internal

bias voltage’s stability and improves the amplifier’s PSRR.

The PSRR improvements increase as the bypass pin ca-

pacitor value increases. Too large, however, increases

turn-on time and can compromise the amplifier’s click and

pop performance. The selection of bypass capacitor values,

especially CB, depends on desired PSRR requirements,

click and pop performance (as explained in the section,

Proper Selection of External Components), system cost, and

size constraints.

MICRO-POWER SHUTDOWN

The LM4913 features an active-low micro-power shutdown

mode. When active, the LM4913’s micro-power shutdown

feature turns off the amplifier’s bias circuitry, reducing the

supply current. The logic threshold is typically VDD/2. The

low 0.1µA typical shutdown current is achieved by applying a

voltage to the SHUTDOWN pin that is as near to GND as

possible. A voltage that is greater than GND may increase

the shutdown current.

There are a few methods to control the micro-power shut-

down. These include using a single-pole, single-throw switch

(SPST), a microprocessor, or a microcontroller. When using

a switch, connect a 100kW pull-up resistor between the

SHUTDOWN pin and VDD and the SPST switch between

the SHUTDOWN pin and GND. Select normal amplifier op-

eration by opening the switch. Closing the switch applies

GND to the SHUTDOWN pin, activating micro-power shut-

down. The switch and resistor guarantee that the SHUT-

DOWN pin will not float. This prevents unwanted state

changes. In a system with a microprocessor or a microcon-

troller, use a digital output to apply the active-state voltage to

the SHUTDOWN pin.

HEADPHONE (SINGLE-ENDED) AMPLIFIER

OPERATION

BTL/SE [Mono (BTL)/Stereo (SE)] Function

Applying a voltage greater than 0.9VDD to the LM4913’s

BTL/SE headphone control pin switches the amplifier’s op-

eration from mono BTL to stereo SE. Applying a voltage less

than 0.55VDD to the LM4913’s BTL/SE headphone control

pin switches the amplifier’s operation from stereo SE to

mono BTL.

Figure 3 shows how to control the LM4913’s headphone

function using four external resistors and a dual-switch ste-

reo headphone jack. External resistors R4 - R6 provide the

control voltages that are applied through the upper head-

phone jack switch. R6 and R7 provide a DC return path for

the SE coupling capacitors.

{Resume here and check other Fig. 3 below.} With no head-

phones connected to the headphone jack, the R5-R6 voltage

divider sets the voltage applied to the BTL/SE pin (pin 7) at

approximately 50mV (comfortably below the 0.55VDD logic-

low threshold). This 50mV tells the LM4913 to select the

signal applied to the MONO-IN input and places the LM4913

in mono BTL operation. When stereo SE operation is de-

sired, both headphone jack switches are opened with a

headphone plug. Opening the lower one allows R5 to apply

VDD to the BTL/SE pin. This switches the amplifier’s inputs

to the stereo signal. Opening the lower one breaks the

connection between AMP4’s output and the BTL speaker,

200617A1

FIGURE 3. Headphone Operation and BTL - SE Mode Switching

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