Datasheet

LM4880
SNAS103C –NOVEMBER 1995–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4880 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This 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 the supply to provide maximum device performance. By switching the shutdown pin to V
DD
,
the LM4880 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown
pin voltages less than V
DD
, the idle current may be greater than the typical value of 0.7 μA. 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 in
conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground
and enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4880. This
scheme ensures that the shutdown pin will not float which will prevent unwanted state changes.
POWER DISSIPATION
Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to
ensure a successful design. Equation 1 states the maximum power dissipation point for a single-ended amplifier
operating at a given supply voltage and driving a specified output load.
P
DMAX
= (V
DD
)
2
/(2π
2
R
L
) (1)
Since the LM4880 has two operational amplifiers in one package, the maximum internal power dissipation point
is twice that of the number which results from Equation 1. Even with the large internal power dissipation, the
LM4880 does not require heat sinking over a large range of ambient temperatures. From Equation 1, assuming a
5V power supply and an 8Ω load, the maximum power dissipation point is 158 mW per amplifier. Thus the
maximum package dissipation point is 317 mW. The maximum power dissipation point obtained must not be
greater than the power dissipation that results from Equation 2:
P
DMAX
= (T
JMAX
-T
A
)/θ
JA
(2)
For the LM4880 surface mount package, θ
JA
= 170° C/W and T
JMAX
= 150°C. Depending on the ambient
temperature, T
A
, 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
either 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 96°C 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 may be
increased accordingly. Refer to Typical Performance Characteristics 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 Typical Performance Characteristics, the effect of a larger half supply bypass capacitor
is improved low frequency PSRR due to increased half-supply stability. Typical applications employ a 5V
regulator with 10 μF and a 0.1 μF bypass capacitors which aid in supply stability, but do not eliminate the need
for bypassing the supply nodes of the LM4880. The selection of bypass capacitors, especially C
B
, is thus
dependant upon desired low frequency PSRR, click and pop performance as explained in PROPER SELECTION
OF EXTERNAL COMPONENTS, system cost, and size constraints.
10 Submit Documentation Feedback Copyright © 1995–2013, Texas Instruments Incorporated
Product Folder Links: LM4880