EUA5544 datasheet(9/10 Pages) EUTECH

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EUA5544

DS5544 Ver 2.4 Oct.2007

Application Note

Power Supply Decoupling, CS

EUA5544 is a high-performance CMOS audio amplifier

that requires adequate power supply decoupling to

ensure the output total harmonic distortion (THD) is as

low as possible. Power supply decoupling also prevents

the oscillations causing by long lead length between the

amplifier and the speaker.

Applications that employ a 5V regulator typically use a

10µF in parallel with a 0.1µF filter capacitors to stabilize

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

and improve the supply’s transient response. However,

this does not eliminate the need for a local bypass

capacitor connected between the EUA5544’s supply

pins and ground. A bypass capacitor value in the range

of 0.1µF to 1µF is recommended for CS.

Input Capacitor Ci

Amplifying the lowest audio frequencies requires a high

value input coupling capacitor, Ci. A high value

capacitor can be expensive and may compromise space

efficiency in portable designs. In many cases, however,

the headphones used in portable systems have little

ability to reproduce signals below 60Hz. Applications

using headphones with this limited frequency response

reap little improvement by using a high value input

capacitor.

In addition to system cost and size, turn-on time is

affected by the size of the input coupling capacitor Ci. A

larger input coupling capacitor requires more charge to

reach its quiescent DC voltage. This charge comes from

the output via the feedback. Thus, by minimizing the

capacitor size based on necessary low frequency

response, turn-on time can be minimized. A small value

of Ci, 1µF, is recommended.

Bypass Capacitor, CB

Besides minimizing the input capacitor sizes, careful

consideration should be paid to the bypass capacitor size.

The bypass capacitor, CB, is the most critical component

to minimize turn-on pops since it determines how fast

the EUA5544 turns on. The slower the EUA5544’s

output ramp to their quiescent DC voltage (nominally

1/2 VDD), the smaller the turn-on pop.

Output Coupling Capacitors, Cc

Typical single-supply audio amplifiers that drive

single-ended (SE) headphones use a coupling capacitor

on each SE output. This output coupling capacitor

blocks the half-supply voltage to which the output

amplifiers are typically biased and couples the audio

signal to the headphones.

The output coupling capacitor and impedance of the

load form a high-pass filter governed by equation.

fc (highpass)=1/(2

π RLCC)

(4)

For example, a 220µF capacitor with an 32Ω speaker

would attenuate low frequencies below 22Hz. The main

disadvantage, from a performance standpoint, is the load

impedance is typically small, which drives the

low-frequency corner higher degrading the bass

response. Large values of CC are required to pass low

frequencies into the load.

Micro Power Shutdown

The voltage applied to the Shutdown pin controls the

EUA5544 shutdown function. Activate micro-power

shutdown by applying a logic-low voltage to the

Shutdown pin. When active, the EUA5544 micro-power

shutdown feature turns off the amplifier’s bias circuitry,

reducing the supply current.

There are a few ways to control the micro-power

shutdown.

These

include

using

single-pole,

single-throw (SPST) switch, a microprocessor, or a

microcontroller. When using a switch, connect an

external pull-up resistor between the Shutdown pin and

VDD. Connect the switch between the Shutdown pin and

ground. Select normal amplifier operation by opening

the switch. Closing the switch connects the Shutdown

pin to ground, activating micro-power shutdown. The

switch and resistor guarantee that the Shutdown 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

Shutdown pin.

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