Datasheet

LM4938
SNAS245B –FEBRUARY 2005–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
HTSSOP PACKAGE PCB MOUNTING CONSIDERATIONS
The LM4938's HTSSOP (die attach paddle) package (MH) 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, finally, surrounding air. The result is a low voltage audio
power amplifier that produces 2.0W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful
consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4938's
high power performance and activate unwanted, though necessary, thermal shutdown protection.
The MH package must have its exposed DAP soldered to a grounded copper pad on the PCB. The DAP's PCB
copper pad is connected to a large grounded plane of continuous unbroken copper. This plane forms a thermal
mass heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided
PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer
or backside copper heat sink area with 32(4x8) (MH) vias. The via diameter should be 0.012in–0.013in with a
1.27mm pitch. Ensure efficient thermal conductivity by plating-through and solder-filling the vias.
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 LM4938 MH package should be 5in
2
(min) for the same
supply voltage and load resistance. The last two area recommendations apply for 25°C ambient temperature.
Increase the area to compensate for ambient temperatures above 25°C. In systems using cooling fans, the
LM4938MH can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a
2.5in
2
exposed copper or 5.0in
2
inner layer copper plane heatsink, the LM4938MH can continuously drive a 3Ω
load to full power. In all circumstances and conditions, the junction temperature must be held below 150°C to
prevent activating the LM4938's thermal shutdown protection. The LM4938's power de-rating curve in the Typical
Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts
for the HTSSOP are shown in the Demonstration Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mounting a package is available in Texas Instruments'
AN1187.
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 2.1W to 2.0W. 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.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 3, the LM4938 output stage consists of two pairs of operational amplifiers, forming a two-
channel (channel A and channel B) stereo amplifier. (Though the following discusses channel A, it applies
equally to channel B.)
Figure 3 shows that the first amplifier's negative (-) output serves as the second amplifier's input. This results in
both amplifiers producing signals identical in magnitude, but 180° out of phase. Taking advantage of this phase
difference, a load is placed between −OUTA and +OUTA and driven differentially (commonly referred to as
“bridge mode”). This results in a differential gain of
A
VD
= 2 * (R
f
/R
i
) (1)
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