However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation versus a single-ended amplifier

operating at the same conditions.

P

(2)

Since the LM4895 has bridged outputs, the maximum inter-

nal power dissipation is 4 times that of a single-ended am-

plifier. Even with this substantial increase in power dissipa-

tion, the LM4895 does not require additional heatsinking

under most operating conditions and output loading. From

Equation 3, assuming a 5V power supply and an 8

Ω load,

the maximum power dissipation point is 625mW. The maxi-

mum power dissipation point obtained from Equation 3 must

not be greater than the power dissipation results from Equa-

tion 4:

P

θ

JA

(3)

The LM4895’s

θ

Depending on the ambient temperature, T

surroundings, Equation 4 can be used to find the maximum

internal power dissipation supported by the IC packaging. If

the result of Equation 3 is greater than that of Equation 4,

then either the supply voltage must be decreased, the load

impedance increased, the ambient temperature reduced, or

the

θ

traces near the output, V

lower the

θ

heatsinking allowing higher power dissipation. For the typical

application of a 5V power supply, with an 8

Ω load, the

maximum ambient temperature possible without violating the

maximum junction temperature is approximately 30˚C pro-

vided that device operation is around the maximum power

dissipation point. Recall that internal power dissipation is a

function of output power. If typical operation is not around the

maximum power dissipation point, the LM4895 can operate

at higher ambient temperatures. Refer to the Typical Per-

formance Characteristics curves for power dissipation in-

formation.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor location on both the

bypass and power supply pins should be as close to the

device as possible. A larger half-supply bypass capacitor

improves PSRR because it increases half-supply stability.

Typical applications employ a 5V regulator with 10µF and

0.1µF bypass capacitors that increase supply stability. This,

however, does not eliminate the need for bypassing the

supply nodes of the LM4895. Although the LM4895 will

operate without the bypass capacitor C

may decrease. A 1µF capacitor is recommended for C

value maximizes PSRR performance. Lesser values may be

used, but PSRR decreases at frequencies below 1kHz. The

issue of C

and click and pop performance.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4895 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. In addition, the LM4895 con-

tains a Shutdown Mode pin, allowing the designer to desig-

nate 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 V

GND to set the LM4895 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

Select pin to the same state as the Shutdown Mode pin. For

simplicity’s sake, this is called ’shutdown same’, as the

LM4895 enters shutdown mode whenever the two pins are

in the same logic state. The trigger point for either shutdown

high or shutdown low is shown as a typical value in the

Supply Current vs Shutdown Voltage graphs in the Typical

Performance Characteristics section. It is best to switch

between ground and supply for maximum performance.

While the device may be disabled with shutdown voltages in

between ground and supply, the idle current may be greater

than the typical value of 0.1µA. In either case, the shutdown

pin should be tied to a definite voltage to avoid unwanted

state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry, which pro-

vides a quick, smooth transition to shutdown. Another solu-

tion is to use a single-throw switch in conjunction with an

external pull-up resistor (or pull-down, depending on shut-

down high or low application). This scheme guarantees that

the shutdown pin will not float, thus preventing unwanted

state changes.

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