AA4838
Agamem Microelectronics Inc.
PRELIMINARY
AUDIO POWER AMPLIFIER
extends below 30Hz.
• OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE
The AA4838 contains circuitry that minimizes turn-on and shutdown transients or “clicks and
pops”. For this discussion, turn-on refers to either applying the power supply voltage or when
the shutdown mode is deactivated. While the power supply is ramping to its final value, the
AA838’s internal amplifiers are configured as unity-gain buffers. An internal current source
changes the voltage of the BYPASS pin in a controlled, linear manner. Ideally, the input and
outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers
remains unity until the voltage on the BYPASS pin reaches 1/2 VDD. As soon as the voltage
on the BYPASS pin is stable, the device becomes fully operational. Although the BYPASS pin
current cannot be modified, changing the size of CB alters the device’s turn-on time and the
magnitude of “clicks and pops”. Increasing the value of CB reduces the magnitude of turn-on
pops. However, this presents a tradeoff: as the size of CB increases, the turn-on time
increases. There is a linear relationship between the size of CB and the turn-on time. Here
are some typical turn-on times for various values of CB:
C
T
2ms
20ms
44ms
94ms
200ms
0.01µF
0.1µF
0.22µF
0.47µF
1.0µF
• DOCKING STATION INTERFACE
Applications such as notebook computers can take advantage of a docking station to connect
to external devices such as monitors or audio/visual equipment that sends or receives line
level signals. The AA4838 has two outputs, Right Dock and Left Dock, which connect to
outputs of the internal input amplifiers that drive the volume control inputs. These input
amplifiers can drive loads of >1kΩ(such as powered speakers) with a rail-to-rail signal. Since
the output signal present on the RIGHT DOCK and LEFT DOCK pins is biased to VDD/2,
coupling capacitors should be connected in series with the load when using these outputs.
Typical values for the output coupling capacitors are 0.33µF to 1.0µF. If polarized coupling
capacitors are used, connect their "+" terminals to the respective output pin, see Figure 2.
Since the DOCK outputs precede the internal volume control, the signal amplitude will be
equal to the input signal’s magnitude and cannot be adjusted. However, the input amplifier’s
closed-loop gain can be adjusted using external resistors. These 20k resistors (RFR, RFL)
are shown in Figure 2 and they set each input amplifier’s gain to -1. Use Equation 7 to
determine the input and feedback resistor values for a desired gain.
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©Copyright Agamem Microelectronics Inc.
www.agamem.com.tw
2008/8/26
AGAMEM MICROELECTRONICS INCOPERATION RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE
TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. AGAMEM DOES NOT ASSUME ANY
LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCTS OR CIRCUIT DESRIBED HEREIN; NEITHER
DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
AGAMEM [ AGAMEM MICROELECTRONIC INC. ]
