EUA4890
Application Information
Bridged Configuration Explanation
Proper Selection of External Components
The structure of the EUA4890 is basically composed of
two identical internal power amplifiers; the first one si
externally configurable with gain-setting resistors Rin and
Rf (the closed-loop gain is fixed by the ratios of theses
resistors) and the second is internally fixed in an inverting
unity-gain configuration by two resistors of 20kΩ. So the
load is driven differentially through OUTA and OUTB
outputs. This configuration eliminates the need for an
output coupling capacitor.
The EUA4890 is unity-gain stable and requires no
external components besides gain-setting resistors, and
input coupling capacitor and proper bypassing capacitor
in the typical application.
Gain-Setting Resistor Selection (Rin and Rf)
Rin and Rf set the closed-loop gain of the amplifier.
In order to optimize device and system performance, the
EUA4890 should be used in low gain configurations.
The low gain configuration minimizes THD + noise
values and maximizes the signal to noise ratio, and the
amplifier can still be used without running into the
bandwidth limitations. Low gain configurations require
large input signals to obtain a given output power. Input
signals equal to or greater than 1Vrms are available from
sources such as audio codecs.
The differential-ended amplifier presents two major
advantages:
-
The possible output power is four times larger (the
output swing is doubled) as compared to single-ended
amplifier under the same conditions.
-
Output pins (OUTA and OUTB) are biased at the
same potential VDD/2, this eliminates the need for an
A closed loop gain in the range from 2 to 5 is
recommended to optimize overall system performance.
An input resistor (Rin) value of 20kΩ is realistic in most
of applications, and does not require the use of a too large
capacitor Cin.
output coupling capacitor required with
single-ended amplifier configuration.
a
The differential closed loop-gain of the amplifier is
R
Vorms
Vinrms
f
given by Avd=
2×
=
Input Capacitor Selection (Cin)
R
in
The input coupling capacitor blocks the DC voltage at the
amplifier input terminal. This capacitor creates a
high-pass filter with Rin, the cut-off frequency is given by
Power Dissipation
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased
power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. Since the
EUA4890 has two operational amplifiers in one package,
the maximum internal power dissipation is 4 times that of
a single-ended amplifier. The maximum power dissipation
for a given application can be derived from the power
dissipation graphs of from equation1.
1
fc =
2 * ∏ *R * C
in
in
The size of the capacitor must be large enough to couple
in low frequencies without severe attenuation. However a
large input coupling capacitor requires more time to reach
its quiescent DC voltage (VDD/2) and can increase the
turn-on pops.
An input capacitor value between 0.1µ and 0.39µF
P
= 4*(V )2 /(2π2R )
------------(1)
DD L
performs well in many applications (with Rin=22kΩ).
DMAX
Bypass Capacitor Selection (Cby)
It is critical that the maximum junction temperature TJMAX
of 150°C is not exceeded. TJMAX can be determine from
the power derating curves by using PDMAX and the PC
board foil area. By adding additional copper foil, the
thermal resistance of the application can be reduced,
resulting in higher PDMAX. Additional copper foil can be
added to any of the leads connected to the EUA4890.If
TJMAX still exceeds 150°C, then additional changes must
be made. These changes can include reduced supply
voltage, higher load impedance, or reduced ambient
temperature. Internal power dissipation is a function of
output power.
The bypass capacitor Cby provides half-supply filtering
and determines how fast the EUA4890 turns on.
This capacitor is critical component to minimize the
turn-on pop. A 1.0µF bypass capacitor value (Cin=＜
0.39µF) should produce clickless and popless shutdown
transitions. The amplifier is still functional with a 0.1µF
capacitor value but is more susceptible to pop and click
noise. Thus, a 1.0µF bypassing capacitor is recommended.
Power Supply Bypassing (CS)
As with any amplifier, proper supply bypassing is critical
for low noise performance and high power supply
rejection. The capacitor location on both the bypass and
power supply pins should be as close to the device is
possible.
DS4890 Ver 1.7 Dec. 2006
12
EUTECH [ EUTECH MICROELECTRONICS INC ]
