Datasheet

LM4911, LM4911Q
www.ti.com SNAS152M –MAY 2004–REVISED JUNE 2011
POWER DISSIPATION
Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to
ensure a successful design. When operating in capacitor-coupled mode, 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 LM4911/LM4911Q has two operational amplifiers in one package, the maximum internal power
dissipation point is twice that of the number which results from Equation 1. From Equation 1, assuming a 3V
power supply and an 32Ω load, the maximum power dissipation point is 14mW per amplifier. Thus the maximum
package dissipation point is 28mW.
When operating in OCL mode, the maximum power dissipation increases due to the use of the third amplifier as
a buffer and is given in Equation 2:
P
DMAX
= 4(V
DD
)
2
/ (π
2
R
L
) (2)
The maximum power dissipation point obtained from either Equation 1 or Equation 2 must not be greater than
the power dissipation that results from Equation 3:
P
DMAX
= (T
JMAX
- T
A
) / θ
JA
(3)
For package DGS0010A, θ
JA
= 190°C/W; for package NGY0010A, θ
JA
= 63°C/W. T
JMAX
= 150°C for the
LM4911/LM4911Q. Depending on the ambient temperature, T
A
, of the system surroundings, Equation 3 can be
used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 or
Equation 2 is greater than that of Equation 3, then either the supply voltage must be decreased, the load
impedance increased or T
A
reduced. For the typical application of a 3V power supply, with a 32Ω load, the
maximum ambient temperature possible without violating the maximum junction temperature is approximately
144°C provided that device operation is around the maximum power dissipation point. Thus, for typical
applications, power dissipation is not an issue. 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 the Typical Performance Characteristics curves for power dissipation information for lower
output powers.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS
The LM4911/LM4911Q's exposed-DAP (die attach paddle) package (WSON) provides a low thermal resistance
between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the
die to the surrounding PCB copper traces, ground plane, and surrounding air.
The WSON package should have its DAP soldered to a copper pad on the PCB. The DAP's PCB copper pad
may be connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink,
and radiation area. Further detailed and specific information concerning PCB layout, fabrication, and mounting an
WSON package is available from TI's Package Engineering Group under application note AN-1187 (SNOA401).
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important for low noise performance and high power supply
rejection. The capacitor location on the power supply pins should be as close to the device as possible.
Typical applications employ a 3V regulator with 10mF 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
LM4911/LM4911Q. A bypass capacitor value in the range of 0.1µF to 1µF is recommended for C
S
.
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