LMH6739

SNOSAD2G – MAY 2004 – REVISED MARCH 2013

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POWER DISSIPATION

The LMH6739 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 LMH6739 consumes an appreciable amount of

quiescent current which cannot be neglected when considering the total package power dissipation limit. The

= ±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

channels).

With the LMH6739 used in a back-terminated 75

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 LMH6739:

1. Calculate the quiescent (no-load) power:

(1)

2. Calculate the RMS power dissipated in the output stage:

(2)

3. Calculate the total RMS power:

(3)

The maximum power that the LMH6739 package can dissipate at a given temperature can be derived with the

following equation (See Figure 29):

to ambient, for a given package (°C/W). For the SSOP package

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.

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