LMH6738 Datasheet, PDF (13/20 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Op Amp

LMH6738
www.ti.com
SNOSAC1E – APRIL 2004 – REVISED MARCH 2013
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 LMH6738 used in a back-terminated 75Ω RGB analog video system (with 2 V_PP output voltage), the
total power dissipation is around 435 mW of which 340 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 140° 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 LMH6738:
1. Calculate the quiescent (no-load) power: P_AMP = I_CC* (V_S) V_S = V⁺-V^−
2. Calculate the RMS power dissipated in the output stage:
– P_D (rms) = rms ((V_S - V_OUT)*I_OUT) where V_OUT and I_OUT are the voltage and current across the external
load and V_S is the total supply current
3. Calculate the total RMS power: P_T = P_AMP+P_D
The maximum power that the LMH6738, package can dissipate at a given temperature can be derived with the
following equation (See Figure 28):
P_MAX = (150º – T_AMB)/ θ_JA, where T_AMB = Ambient temperature (°C) and θ_JA = Thermal resistance, from junction
to ambient, for a given package (°C/W). For the SSOP package θ_JA is 120°C/W.
ESD PROTECTION
The LMH6738 is protected against electrostatic discharge (ESD) on all pins. The LMH6738 will survive 2000V
Human Body model and 200V Machine model events.
Under closed loop operation the ESD diodes have no effect on circuit performance. There are occasions,
however, when the ESD diodes will be evident. If the LMH6738 is driven by a large signal while the device is
powered down the ESD diodes will conduct.
The current that flows through the ESD diodes will either exit the chip through the supply pins or will flow through
the device, hence it is possible to power up a chip with a large signal applied to the input pins. Shorting the
power pins to each other will prevent the chip from being powered up through the input.
Copyright © 2004–2013, Texas Instruments Incorporated
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