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EDGE818

Octal 18V Pin Electronics Driver/window Comparator

Semtech Corporation 

Edge818

TEST AND MEASUREMENT PRODUCTS

Application Information (continued)

Power Dissipation/Thermal Considerations

The Edge818 is specified to operate with a die junction

temperature, Tj, of up to 125˚C. The theoretical junction

temperature is calculated as follows:

Tj = Tc + θjc x Ptotal

Where Tj = The Theoretical Junction Temperature

of the Edge818 [˚C]

Tc = The Case Temperature of the

Edge818[˚C]

θjc = The Thermal Impedance of the

Edge818 (junction to top center of

case)[˚C]

Ptotal = The Total Power Dissipation of the

Edge818 [W]

In order to maximize the reliability and operating lifetime

of the Edge818, the junction temperature of the device

should be minimized. It can be seen from the equation

above that the junction temperature of the Edge818 is

both a function of its case temperature and the total power

dissipation of the device. Therefore, one can minimize

the junction temperature of the Edge818 by minimizing

the case temperature and the overall power dissipation of

the device.

relatively large power savings when using the device is to

minimize the power supply levels that are used for a

particular application. (Note that varying power supply

levels may have an effect on device propagation delays

and driver output impedance.) For illustrative purposes,

this approach to power savings is evaluated on the following

application:

Example:

The Edge818 is used to generate 3.3V output swings

on all 8-channels simultaneously under the following

conditions:

• VL = 0V

• VH = 3.3V

• f = 25 MHz

• Zload = 1kΩ||80pF

Under the conditions above, the power dissipation of

the Edge818 is as depicted in Table 1.

VCC[V]

12

VEE[V]

–3.3

VCC–VEE[V]

15.3

Pdiss[W]

3.4

Tj[˚C]

77

The case temperature of the Edge818 can be controlled

through the use of some source of external cooling to

regulate the case temperature (i.e. forced air). A heat

sink can also be attached to the Edge818 in order to

maximize the efficiency and increase the overall heat

capacity of the external cooling used in an application.

7.5

–4.6

12.1

2.2

59

Table 1. Comparison of Edge818 Power Dissipation

and Junction Temperature at f = 25 MHz,

Ta = 25 ˚C, Airflow = 300 LFPM

A heat sink can be attached to the top of the device, and/

or additional cooling can be attained through the bottom

of the device (i.e. into a copper plane on the PCB or a

heat sink attached to the device through a hole in the

PCB). This will significantly decrease the effective thermal

resistance between the case of the Edge818 and the

cooling mechanism being used.

The total power dissipation of the Edge818 can also be

minimized, but is ultimately dependent upon the

requirements of the application. One way to attain a

Note that by reducing the power supply levels in the

application depicted above, a power savings of 1.2W

was realized (and Tj was reduced by 18˚C).

The power dissipation (and hence Tj) of the Edge818 is

directly proportional to its operating frequency. This is

illustrated in Figures 4 and 5.

 2004 Semtech Corp. / Rev. 5, 8/18/04

7

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