Datasheet

LM4898, LM4898MMBD
SNAS216E –MAY 2003–REVISED APRIL 2013 www.ti.com
Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and
amplifier share the same PCB layer, a nominal 2.5in
2
(min) area is necessary for 5V operation with a 4Ω load.
Heatsink areas not placed on the same PCB layer as the LM4898 should be 5in
2
(min) for the same supply
voltage and load resistance. The last two area recommendations apply for 25°C ambient temperature. In all
circumstances and conditions, the junction temperature must be held below 150°C to prevent activating the
LM4898’s thermal shutdown protection. The LM4898’s power derating curve in the TYPICAL PERFORMANCE
CHARACTERISTICS shows the maximum power dissipation versus temperature. Further detailed and specific
information concerning PCB layout, fabrication, and mounting an WSON package is available from Texas
Instruments package Engineering Group under application note AN-1187 (SNOA401).
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 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. For example, 0.1Ω
trace resistance reduces the output power dissipated by a 4Ω load from 1.4W to1.37W. 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 amplifier, 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 LM4898 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 LM4898 does not require additional
heatsinking under most operating conditions and output loading. From Equation 3, assuming a 5V power supply
and an 8. 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 LM4898’s θ
JA
in an DGS0010A package is 190°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 theθ
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 30°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 LM4898 can operate at higher ambient temperatures. Refer to
the TYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation information.
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