Diodes PAM8902H - Manual (Page 7)

PAM8902H
Document number: DSxxxxx Rev. 1 - 0
7 of 12
www.diodes.com
December 2012
© Diodes Incorporated
PAM8902H
A Product Line of
Diodes Incorporated
Application Information
Select Boost Convertor Output Voltage
Customer can use V
SET
pin to set boost converor output voltage between 8V, 12V and 17.5V. V
SET
pin configuration table as below:
V
SET
Pin
Configuration
Min Max PVCC Voltage
Audio Amplifier Maximum
Output Voltage
Connect to AVDD AVDD – 0.5V AVDD 17.5V 11V
RMS
(V
PP
= 31.1V)
Floating 1V AVDD – 1V 12V 5V
RMS
(V
PP
= 22.6V)
Connect to GND GND 0.5V 8V 5V
RMS
(V
PP
= 14.1V)
Input Resistance (R
I
)
The input resistors (R
I
= R
IN
+ R
EX
) set the gain of the amplifier according to Equation 1 when anti-saturation is inactive.
G = 20 Log [12.8*R
F
/(R
IN
+
REX
)] (dB)
G
SET
R
IN
R
F
G SET = V DD
36.5k Ω 122.6kΩ
G SET = Floating 59k Ω 100kΩ
G SET = GND 82k Ω 77.4kΩ
Where R
IN
is a 77.4K Ω internal resistor, R
EX
is the external input resistor, R
F
is a 122.6K Ω internal resistor. Resistor matching is very important
in fully differential amplifiers. The balance of the output on the reference voltage depends on matched ratios of the resistors. CMRR, PSRR, and
cancellation of the second harmonic distortion diminish if resistor mismatch occurs. Therefore, it is recommended to use 1% tolerance resistors
or better to keep the performance optimized. Matching is more important than overall tolerance. Resistor arrays with 1% matching can be used
with a tolerance greater than 1%.
Place the input resistors very close to the PAM8902H to limit noise injection on the high-impedance nodes. For optimal performance the gain
should be set to lower. Lower gain allows the PAM8902H to operate at its best, and keeps a high voltage at the input making the inputs less
susceptible to noise. In addition to these features, higher value of R I minimizes pop noise.
Input Capacitors (C
I
)
In the typical application, an input capacitor, C
I
, is required to allow the amplifier to bias the input signal to the proper DC level for optimum
operation. In this case, C
I
and the minimum input impedance R
I
form is a high-pass filter with the corner frequency determined in the follow
equation:
C R 2
1
f
I I
C π
=
It is important to consider the value of C I as it directl y affects the low frequency performance of the circuit. For example, when R I is 150k Ω and
the specification calls for a flat bass response are down to 150Hz. Equation is reconfigured as followed:
IFC
I
R 2
1
C
π
=
When input resistance variation is considered, the C
I
is 7nF, so one would likely choose a value of 10nF. A further consideration for this
capacitor is the leakage path from the input source through the input network (C
I
, R
I
+ R
F
) to the load. This leakage current creates a DC 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 should face the amplifier input in
most applications as the DC level is held at V
DD
/2, which is likely higher than the source DC level. Please note that it is important to confirm the
capacitor polarity in the application.