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CM8662IP Datasheet(PDF) 5 Page - List of Unclassifed Manufacturers |
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CM8662IP Datasheet(HTML) 5 Page - List of Unclassifed Manufacturers |
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5 / 8 page  CM8662 AUDIO POWER AMP. WITH SHUTDOWN MODE 2003/02/11 Rev. 1.0 Champion Microelectronic Corporation Page 5 APPLICATION INFORMATION Bridge Configuration Explanation As shown in Figure 1, the CM8662 has two operational amplifiers internally, allowing for a few different amplifier configuration. 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 Ri while the second amplifier's gain is fixed by the two internal 10kΩ 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 180 ° . Consequently, the differential gain for the IC is AVD = 2*(Rf/Ri) By dividing the load differentially through output Vo1 and Vo2, and 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. Consequently, 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 which will damage high frequency transducers used in loudspeaker systems, please refer to the Audio Power Amplifier Design section. A bridge configuration, such as the one used in CM8662, also creates a second advantage over single-ended amplifier. 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 permanent 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. Equation 1 states the maximum power dissipation point for a bridge amplifier operating at a given supply voltage and driving a specified output load. PDMAX = 4*(VDD) 2/(2π2RL) (1) Since the CM8662 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the CM8662 does not require heatsinking. From Equation 1, assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is 625mW. The maximum power dissipation point obtained from Equation 1 must not be greater than the power dissipation that results from Equation 2: PDMAX = (TJMAX-TA)/ΘJA (2) For package M08A, ΘJA= 170℃/W and for package N08E, Θ JA= 107℃/W. TJMAX=150℃ for the CM8662. Depending on the ambient temperature, TA, of the system surroundings, Equation 2 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 is greater than that of Equation 2, then wither the supply voltage must be decreased, the load impedance increased, or the ambient temperature reduced. For the typical application of a 5V power supply, with an 8Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 44℃ provided that device operation is around the maximum power dissipation point. Power dissipation is a function of output power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature can be increased. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output powers. Power Supply Bypassing As with any power 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. As displayed in the Typical Performance Characteristics section, the effect of a larger half supply bypass capacitor is improved PSSR due to increased half-supply stability. Typical applications employ a 5V regulator with 10μF and a 0.1μF bypass capacitors which aid in supply stability, but do not eliminate the need for bypassing the supply nodes of the CM8662. The selection of bypass capacitors, especially CB, is thus dependant upon desired PSSR requirements, click and pop performance as explained in the section, Proper Selection of External Components, system cost, and size constraints. Shutdown Function In order to reduce power consumption while not in use, the CM8662 contains a shutdown pin to externally turn off the amplifier's bias circuitry. The shutdown feature turns the amplifier off when a logic high is placed on the shutdown pin. The trigger point between a logic low and logic high level is typically half supply. It is best to switch between ground and supply to provide maximum device performance. By switching the shutdown pin to VDD, the CM8662 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltage less than VDD, the idle current may be greater than the typical value of 0.7μA. In either case, the shutdown pin should be tied to a definite voltage because leaving the pin floating may result in an unwanted shutdown condition. In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which provides a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch that when closed, is connected to ground and enables the amplifier. If the switch is open, then a soft pull-up resistor of 47 kΩ will disable the CM8662. There are no soft pull-up resistors inside the CM8662, so a definite shutdown pin voltage must be applied externally, or the internal logic gate will be left floating which could disable the amplifier unexpectedly. |
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