EUA5544
Application Note
Output Coupling Capacitors, Cc
Power Supply Decoupling, CS
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.
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.
The output coupling capacitor and impedance of the
load form a high-pass filter governed by equation.
fc (highpass)=1/(2π 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.
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.
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.
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.
There are a few ways to control the micro-power
shutdown. These include using
a
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
V
DD. 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.
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.
DS5544 Ver 2.4 Oct.2007
9
EUTECH [ EUTECH MICROELECTRONICS INC ]
