®
LY8897
2.7 Watt Audio Power Amplifier
Rev. 1.1
APPLICATION INFORMATION
Bridged configuration explanation
As shown in Figure 1 and 2, the LY8897 has two operational amplifiers internally, allowing for a few different
amplifier configurations. The first amplifier’s gain is externally configurable, while the second amplifier is
internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by
selecting the ratio of Rf to RIN while the second amplifier’s gain is fixed by the two internal 20kꢀ resistors.
Figure 1 and 2 shows that the output of amplifier one serves as the input to amplifier two which results in both
amplifiers producing signals identical in magnitude, but out of phase by 180°. Consequently, the differential
gain for the IC is
AVD= 2 X ( Rf / RIN ) ………………………………………………………………(1)
By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to
as “bridged mode” is established. Bridged mode operation is different from the classical single-ended
amplifier configuration where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides
differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output
power is possible as compared to a single-ended amplifier under the same conditions.
This increase in attainable output power assumes that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing excessive clipping, please refer to the Audio Power
Amplifier Design section.
A bridge configuration, such as the one used in the LY8897, also creates a second advantage over single
-ended amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage
exists across the load. This eliminates the need for an output coupling capacitor which is required in a single
supply, single-ended amplifier configuration. Without an output coupling capacitor, the half-supply bias across
the load would result in both increased internal IC power dissipation and also possible loudspeaker damage.
Input Capacitors (Ci)
The LY8897 input capacitors and input resistors form a high-pass filter with the corner frequency, fc,
determined in equation Equation 2.
1
fc = ----------------………………………………………………………………(2)
2πRiCi
Equation 3 is reconfigured to solve for the input coupling capacitance.
1
Ci = -----------------…………………………………………………………….(3)
2πRifc
For example
In the table 1 shows the external components. Rin in connect with Cin to create a high-pass filter.
Table 1. Typical Component Values
Reference
Description
Note
Ri
Ci
20Kꢀ
1% tolerance resistors
0.47uF 80%/–20% non polarized
0.1uF
0.22uF
0.33uF
corner frequency 79.57Hz 36.17Hz 20.4Hz 16.93Hz
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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