TPA6205A1_15 Datasheet, PDF (15/28 Pages) Texas Instruments – 1.25-W MONO FULLY DIFFERENTIAL AUDIO POWER AMPLIFIER WITH 1.8-V INPUT LOGIC THRESHOLDS

TPA6205A1
www.ti.com .............................................................................................................................................................. SLOS490B – JULY 2006 – REVISED JUNE 2008
Therefore,
p Ǹ2 PL R_L
h_BTL +
4 VDD
(11)
Table 2. Efficiency and Maximum Ambient
Temperature vs Output Power in 5-V 8-Ω BTL
Systems
Output
Power
(W)
0.25
Efficiency
(%)
31.4
Internal
Dissipation
(W)
0.55
Power
From
Supply
(W)
0.75
Max
Ambient
Temperature
(°C)
62
0.50
44.4
0.62
1.12
54
1.00
62.8
0.59
1.59
58
1.25
70.2
0.53
1.78
65
Table 2 employs Equation 11 to calculate efficiencies
for four different output power levels. Note that the
efficiency of the amplifier is quite low for lower power
levels and rises sharply as power to the load is
increased resulting in a nearly flat internal power
dissipation over the normal operating range. Note that
the internal dissipation at full output power is less
than in the half power range. Calculating the
efficiency for a specific system is the key to proper
power supply design. For a 1.25-W audio system with
8-Ω loads and a 5-V supply, the maximum draw on
the power supply is almost 1.8 W.
A final point to remember about Class-AB amplifiers
is how to manipulate the terms in the efficiency
equation to the utmost advantage when possible.
Note that in Equation 11, V_DD is in the denominator.
This indicates that as V_DD goes down, efficiency goes
up.
A simple formula for calculating the maximum power
dissipated, P_Dmax, may be used for a differential
output application:
2 V²
DD
PD _max + _{p2 RL}
(12)
P_Dmax for a 5-V, 8-Ω system is 634 mW.
The maximum ambient temperature depends on the
heat sinking ability of the PCB system. The derating
factor for the 2 mm x 2 mm Microstar Junior™
package is shown in the dissipation rating table.
Converting this to θ_JA:
Θ_JA
+
1
Derating
Factor
+
1
0.0088
+
113°CńW
(13)
Given θ_JA, the maximum allowable junction
temperature, and the maximum internal dissipation,
the maximum ambient temperature can be calculated
with the following equation. The maximum
recommended junction temperature for the
TPA6205A1 is 125°C.
T_A Max + T_J Max * Θ_JA P_Dmax
+ 125 * 113(0.634) + 53.3°C
(14)
Equation 14 shows that the maximum ambient
temperature is 53.3°C at maximum power dissipation
with a 5-V supply.
Table 2 shows that for most applications no airflow is
required to keep junction temperatures in the
specified range. The TPA6205A1 is designed with
thermal protection that turns the device off when the
junction temperature surpasses 150°C to prevent
damage to the IC. Also, using more resistive than 8-Ω
speakers dramatically increases the thermal
performance by reducing the output current.
PCB LAYOUT
For the DRB (QFN/SON) and DGN (MSOP)
packages, it is good practice to minimize the
presence of voids within the exposed thermal pad
interconnection. Total elimination is difficult, but the
design of the exposed pad stencil is key. The stencil
design proposed in the Texas Instruments application
note "QFN/SON PCB Attachment" (SLUA271)
enables out-gassing of the solder paste during reflow
as well as regulating the finished solder thickness.
Typically the solder paste coverage is approximately
50% of the pad area.
In making the pad size for the BGA balls, it is
recommended that the layout use solder-
mask-defined (SMD) land. With this method, the
copper pad is made larger than the desired land area,
and the opening size is defined by the opening in the
solder mask material. The advantages normally
associated with this technique include more closely
controlled size and better copper adhesion to the
laminate. Increased copper also increases the
thermal performance of the IC. Better size control is
the result of photo imaging the stencils for masks.
Small plated vias should be placed near the center
ball connecting ball B2 to the ground plane. Added
plated vias and ground plane act as a heatsink and
increase the thermal performance of the device.
Figure 35 shows the appropriate diameters for a 2
mm × 2 mm MicroStar Junior™ BGA layout.
It is very important to keep the TPA6205A1 external
components very close to the TPA6205A1 to limit
noise pickup. The TPA6205A1 evaluation module
(EVM) layout is shown in the next section as a layout
example.
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