®
LY8209
3.3W/CH Stereo Class D Audio Power Amplifier
Rev. 1.2
APPLICATION INFORMATION
Input Resistors (Ri) and Gain
The LY8209 have two internal amplifier stages. The pre-amplifier gain is externally configurable, while the
total gain is internally fixed. The closed-loop gain of the pre-amplifier gain is set by selecting the Rf
(Rf=150KΩ) to Ri while the total gain is fixed at 4x. So the input resistors (Ri) set the gain of the amplifier
according to the equation.
Pre-Amplifier Gain = (Rf / Ri) x 2
Total Gain = [(Rf / Ri ) x 2] x 2
AVD = 20 x log [4 x (Rf /Ri )]
The resistor matching is very important in the amplifiers. Balance of the output on the reference voltage
depends on matched ratio of the resistors. CMRR, PSRR, and cancellation of the second harmonic distortion
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 LY8209 to limit noise injection on the high-impedance nodes. For optimal performance the gain
should be set to 4 V/V or lower. Lower gain allows the LY8209 to operate at its best,
For example
Table 1. Typical Total Gain and AVD Values
Rf (KΩ)
Ri (KΩ)
150
150
2
150
75
150
50
150
25
150
15
Pre AMP. Gain
Total Gain
AVD (db)
4
6
12
20
4
8
12
24
40
12.04
18.06
21.58
27.60
32.04
Input Capacitors (Ci)
The LY8209 using fully differential source, So the input coupling capacitors are required. The input capacitors
and input resistors form a high-pass filter with the corner frequency(fc), determined in the equation.
fc = 1 / ( 2πRi Ci )
The value of the input capacitor is important to consider as it directly affects the bass (low frequency)
performance of the circuit. Speakers in wireless phones cannot usually respond well to low frequencies, so
the corner frequency can be set to block low frequencies in this application. Equation is reconfigured to solve
for the input coupling capacitance.
Ci = 1 / ( 2πRi fc )
If the corner frequency is within the audio band, the capacitors should have a tolerance of ±10% or better,
because any mismatch in capacitance causes an impedance mismatch at the corner frequency and below.
Lyontek Inc. reserves the rights to change the specifications and products without notice.
5F, No. 2, lndustry E . Rd. IX, Science-Based Industrial Park, Hsinchu 300, Taiwan
TEL: 886-3-6668838
FAX: 886-3-6668836
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