LMH6739 Datasheet, PDF (9/10 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Video Buffer

Application Section (Continued)
shows a typical configuration for driving a 75Ω Cable. The
amplifier is configured for a gain of two to make up for the 6
dB of loss in R_OUT.
20104102
FIGURE 9. Maximum Power Dissipation
POWER DISSIPATION
The LMH6739 is optimized for maximum speed and perfor-
mance in the small form factor of the standard SSOP-16
package. To achieve its high level of performance, the
LMH6739 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 the need for proper precautions
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 LMH6739 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 ambi-
ent.
To reduce the junction temperature many options are avail-
able. 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 dissi-
pation for the LMH6739:
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 LMH6739 package can dissi-
pate at a given temperature can be derived with the following
equation (See Figure 9):
P_MAX = (150^o – T_AMB)/ θ_JA, where T_AMB = Ambient tempera-
ture (˚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 LMH6739 is protected against electrostatic discharge
(ESD) on all pins. The LMH6739 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 LMH6739 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.
9
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