APA0710/0711
Application Descriptions (Cont.)
Output SE/BTL Operation (for APA0710 only)
The value of Ci is important to consider as it directly
affects the low frequency performance of the circuit.
Consider the example where Ri is 100kW and the
specification calls for a flat bass response down to
40Hz. Equation is reconfigured as follow :
source or a resistor divider network or the mono head-
phone jack with switch pin as shown in Application
Circuit.
vo+
1
(3)
Ci=
2pRifC
1kW
VDD
Consider to input resistance variation, the Ci is 0.04mF
Control
100kW
so one would likely choose a value in the range of
Pin
0.1mF to 1.0mF.
SE/BTL
100kW
Headphone
Afurther consideration for this capacitor is the leakage
path from the input source through the input network
(Ri+Rf, Ci) to the load.
Jack
Figure 2: SE/BTL input selection by phonejack plug
In Figure 2, input SE/BTL operates as follows :
Thisleakagecurrent creates aDC offset voltage at the
input to the amplifier that reduces useful headroom,
especially in high gain applications. For this reason a
low-leakage tantalum or ceramic capacitor is the best
choice. When polarized capacitors are used, the
positive side of the capacitor shouldface the amplifier
input in most applications as theDC level there is held
at VDD/2, which is likely higher that the source DC
level. Please note that it is important to confirm the
capacitor polarity in the application.
When the phonejack plug is inserted, the 1kW resistor
is disconnected and the SE/BTL input is pulled high
and enables the SE mode.
When this input goes high level, the Vo- amplifier is
shutdown causing the speaker to mute. The Vo+
amplifier then drives through the output capacitor (CC)
into the headphone jack.
Whenthere isno headphoneplugged into the system,
the contact pin of the headphone jack is connected
from the signal pin, the voltage divider set up by
resistors 100kW and 1kW. Resistor 1kW then pulls
low the SE/BTL pin, enabling the BTL function.
Effective BypassCapacitor, Cbypass
As other power amplifiers, proper supply bypassing is
critical for low noise performance and high power
supply rejection.
Input Capacitor, Ci
The capacitors located on the bypass and power
supply pins should be as close to the device as
possible. The effect of a larger half supply bypass
capacitor will improve PSRR due to increased half-
supply stability. Typical application employ a 5V
regulator with 1.0mF and a 0.1mF bypass as supply
filtering. Thisdoesnot eliminatethe need for bypassing
the supply nodes of the APA0710/1. The selection of
In the typical application an input capacitor, Ci, is
required to allow the amplifier to bias the input signal
to the proper DC level for optimum operation. In this
case, Ci and the minimum input impedance Ri form a
high-pass filter with the corner frequency determined
in the follow equation :
1
(2)
FC(highpass)=
2pRiCi
Copyright ã ANPEC Electronics Corp.
Rev. A.5 - Oct., 2005
21
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
