LMH6733 Datasheet, PDF (17/25 Pages) Texas Instruments – Single Supply, 1.0 GHz, Triple Operational Amplifier

LMH6733
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SNOSAW0D – JANUARY 2007 – REVISED MAY 2013
Power Dissipation
The LMH6733 is optimized for maximum speed and performance in the small form factor of the standard SSOP-
16 package. To achieve its high level of performance, the LMH6733 consumes an appreciable amount of
quiescent current which cannot be neglected when considering the total package power dissipation limit. The
quiescent current contributes to about 40° C rise in junction temperature when no additional heat sink is used (V_S
= ±5V, all 3 channels on). Therefore, it is easy to see that proper precautions need to be taken in order to make
sure the junction temperature’s absolute maximum rating of 150°C is not violated.
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_JMAX is never exceeded due to the overall power dissipation (all 3
channels).
With the LMH6733 used in a back-terminated 75Ω RGB analog video system (with 2 V_PP output voltage), the
total power dissipation is around 305 mW of which 220 mW is due to the quiescent device dissipation (output
black level at 0V). With no additional heat sink used, that puts the junction temperature to about 120° C when
operated at 85°C ambient.
To reduce the junction temperature many options are available. Forced air cooling is the easiest option. An
external add-on heat-sink can be added to the SSOP-16 package, or alternatively, additional board metal
(copper) area can be utilized as heat-sink.
An effective way to reduce the junction temperature for the SSOP-16 package (and other plastic packages) is to
use the copper board area to conduct heat. With no enhancement the major heat flow path in this package is
from the die through the metal lead frame (inside the package) and onto the surrounding copper through the
interconnecting leads. Since high frequency performance requires limited metal near the device pins the best
way to use board copper to remove heat is through the bottom of the package. A gap filler with high thermal
conductivity can be used to conduct heat from the bottom of the package to copper on the circuit board. Vias to a
ground or power plane on the back side of the circuit board will provide additional heat dissipation. A combination
of front side copper and vias to the back side can be combined as well.
Follow these steps to determine the maximum power dissipation for the LMH6733:
1. Calculate the quiescent (no-load) power:
P_AMP = I_CC X (V_S)
where
• V_S = V⁺-V^−
(1)
2. Calculate the RMS power dissipated in the output stage:
P_D (rms) = rms ((V_S - V_OUT) X I_OUT)
where
• V_OUT and I_OUT are the voltage and the current across the external load
• 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 LMH6733, package can dissipate at a given temperature can be derived with the
following equation (See Figure 41):
P_MAX = (150°C/W– 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 SSOP package θ_JA is 120°C/W
(4)
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