Datasheet

LM4923, LM4923LQBD
SNAS211E –JULY 2004–REVISED MAY 2013 www.ti.com
Best thermal performance is achieved with the largest practical copper heat sink area. In all circumstances and
conditions, the junction temperature must be held below 150°C to prevent activating the LM4923's thermal
shutdown protection. Figure 24 in the Typical Performance Characteristics shows the maximum power
dissipation versus temperature. Example PCB layouts are shown in the Demonstration Board Layout section.
Further detailed and specific information concerning PCB layout, fabrication, and mounting an WQFN package is
available from Texas Instruments's package Engineering Group under application note AN1187.
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 4Ω LOADS
Power dissipated by a load is a function of the voltage swing across the load and the load's impedance. As load
impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and
wire) resistance between the amplifier output pins and the load's connections. Residual trace resistance causes
a voltage drop, which results in power dissipated in the trace and not in the load as desired. This problem of
decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load
dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide
as possible.
Poor power supply regulation adversely affects maximum output power. A poorly regulated supply's output
voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output
signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the
same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps
maintain full output voltage swing.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifer, whether the amplifier is bridged or
single-ended. Equation 2 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
) Single-Ended (2)
However, a direct consequence of the increased power delivered 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
DMAX
= 4 * (V
DD
)
2
/ (2π
2
R
L
) Bridge Mode (3)
Since the LM4923 has bridged outputs, the maximum internal power dissipation is 4 times that of a single-ended
amplifier. Even with this substantial increase in power dissipation, the LM4923 does not require additional
heatsinking under most operating conditions and output loading. From Equation 3, assuming a 5V power supply
and an 8Ω load, the maximum power dissipation point is 625mW. The maximum power dissipation point obtained
from Equation 3 must not be greater than the power dissipation results from Equation 4:
P
DMAX
= (T
JMAX
- T
A
) / θ
JA
(4)
The LM4923's θ
JA
in an NGP0008A package is 140°C/W. Depending on the ambient temperature, T
A
, of the
system 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
JA
reduced with
heatsinking. In many cases, larger traces near the output, V
DD
, and GND pins can be used to lower the θ
JA
. The
larger areas of copper provide a form of heatsinking allowing higher power dissipation. 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 62°C provided 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 LM4923 can operate at higher ambient temperatures.
Refer to the Typical Performance Characteristics curves for power dissipation information.
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