LMH6552 Datasheet, PDF (29/38 Pages) Texas Instruments – 1.5-GHz Fully Differential Amplifier

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LMH6552
SNOSAX9I – APRIL 2007 – REVISED JANUARY 2015
11.3 Thermal Considerations
The WSON package is designed for enhanced thermal performance and features an exposed die attach pad
(DAP) at the bottom center of the package that creates a direct path to the PCB for maximum power dissipation.
The DAP is floating and is not electrically connected to internal circuitry. Compared to the traditional leaded
packages where the die attach pad is embedded inside the molding compound, the WSON reduces one layer in
the thermal path.
The thermal advantage of the WSON package is fully realized only when the exposed die attach pad is soldered
down to a thermal land on the PCB board with thermal vias planted underneath the thermal land. The thermal
land can be connected to any power or ground plane within the allowable supply voltage range of the device.
Based on thermal analysis of the WSON package, the junction-to-ambient thermal resistance (θ_JA) can be
improved by a factor of two when the die attach pad of the WSON package is soldered directly onto the PCB
with thermal land and thermal vias are 1.27 mm and 0.33 mm respectively. Typical copper via barrel plating is 1
oz, although thicker copper may be used to further improve thermal performance.
For more information on board layout techniques, refer to Application Note 1187 Leadless Lead Frame Package
(LLP). This application note also discusses package handling, solder stencil and the assembly process.
11.4 Power Dissipation
The LMH6552 is optimized for maximum speed and performance in the small form factor of the standard SOIC
package, and is essentially a dual channel amplifier. To ensure maximum output drive and highest performance,
thermal shutdown is not provided. Therefore, it is of utmost importance to make sure that the T_JMAXof 150°C is
never exceeded due to the overall power dissipation.
Follow these steps to determine the maximum power dissipation for the LMH6552:
1. Calculate the quiescent (no-load) power:
P_AMP = I_CC* (V_S)
where
• V_S = V⁺ - V^−. (Be sure to include any current through the feedback network if V_OCM is not mid-rail.)
(1)
2. Calculate the RMS power dissipated in each of the output stages:
P_D (rms) = rms ((V_S - V^+OUT) * I^+OUT) + rms ((V_S − V^−_OUT) * I^−_OUT)
where
• V_OUT and I_OUT are the voltage and the current measured at the output pins of the differential amplifier as if they
were single ended amplifiers and V_S is the total supply voltage
(2)
3. Calculate the total RMS power:
P_T = P_AMP + P_D
(3)
The maximum power that the LMH6552 package can dissipate at a given temperature can be derived with the
following equation:
P_MAX = (150° – T_AMB)/ θ_JA
where
• T_AMB = Ambient temperature (°C)
• θ_JA = Thermal resistance, from junction to ambient, for a given package (°C/W)
• For the SOIC package θ_JA is 150°C/W
• For WSON package θ_JA is 58°C/W
(4)
NOTE
If V_CM is not 0V then there will be quiescent current flowing in the feedback network. This
current should be included in the thermal calculations and added into the quiescent power
dissipation of the amplifier.
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