®
LY8890
1.0 Watt Audio power Amplifier
Rev. 1.5
APPLICATION INFORMATION
BRIDGED CONFIGURATION EXPLANATION
As shown in Figure 1, the LY8890 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 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 )
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 LY8890, 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.
Power Dissipation
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged
or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. Since the LY8890 has two operational amplifiers in one package, the
maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power
dissipation for a given application can be derived from the power dissipation graphs of from equation 1.
P
DMAX = 4 X (VDD)2 / (2 π2 RL) ……………………………..(1)
It is critical that the maximum junction temperature TJMAX of 150°C is not exceeded. TJMAX can be
determine from the power derating curves by using PDMAX and the PC board foil area. By adding additional
copper foil, the thermal resistance of the application can be reduced, resulting in higher PDMAX. Additional
copper foil can be added to any of the leads connected to the LY8890.If TJMAX still exceeds 150°C, then
additional changes must be made. These changes can include reduced supply voltage, higher load
impedance, or reduced ambient temperature. Internal power dissipation is a function of output power.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and power supply pins should be as close to the device
as possible.
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
15
