BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the CM8600 has two internal operational amplifiers. 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 20kΩ 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 *(Rf/Ri)
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 CM8600, 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 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 CM8600 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 or from Equation 1.
PDMAX = 4*(VDD)2/(2π2RL)
It is critical that the maximum junction temperature TJMAX of 150°C is not exceeded. TJMAX can be determined from the
power derating curves by using PDMAX and the PC board foil area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of θJA, resulting in higher PDMAX values without thermal shutdown
protection circuitry being activated. Additional copper foil can be added to any of the leads connected to the CM8600. It is
especially effective when connected to VDD, GND, and the output pins. Refer to the application information on the CM8600
reference design board for an example of good heat sinking. 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. Refer to the Typical Performance Characteristics curves for power
dissipation information for different output powers and output loading.
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. Typical applications
employ a 5V regulator with 10μF tantalum or electrolytic capacitor and a ceramic bypass capacitor which aid in supply stability.
This does not eliminate the need for bypassing the supply nodes of the CM8600. The selection of a bypass capacitor, especially
CB, is dependent upon PSRR requirements, click and pop performance (as explained in the section, Proper Selection of
External Components), system cost, and size constraints.
In order to reduce power consumption while not in use, the CM8600 contains shutdown circuitry that is used to turn off the
amplifier’s bias circuitry. In addition, the CM8600 contains a Shutdown Mode pin (LD and MH packages only), allowing the
designer to designate whether the part will be driven into shutdown with a high level logic signal or a low level logic signal.
This allows the designer maximum flexibility in device use, as the Shutdown Mode pin may simply be tied permanently to
either VDD or GND to set the CM8600 as either a "shutdown-high" device or a "shutdown-low" device, respectively. The
device may then be placed into shutdown mode by toggling the Shutdown pin to the same state as the Shutdown Mode pin. For
simplicity’s sake, this is called "shutdown same", as the CM8600 enters shutdown mode whenever the two pins are in the same
logic state. The MM package lacks this Shutdown Mode feature, and is permanently fixed as a ‘shutdown-low’ device. The
trigger point for either shutdown high or shutdown low is shown as a typical value in the Supply Current vs Shutdown Voltage
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