AN2689 datasheet(19/42 Pages) STMICROELECTRONICS

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AN2689

Data line protection

Doc ID 14310 Rev 3

19/42

It becomes then

then

So to determine the time duration (t’) at half value of the max current crossing the Transil,

use the equation:

which reduces to

This produces a duration of t’ = 457 ns.

If now we use this value in the peak pulse power versus exponential pulse duration graph in

Figure 20, we can see that the SM6T27AY Transil is able to withstand more than 100 kW

power dissipation for a 457 ns pulse duration (extrapolation of Figure 20). As the application

can be submitted to an ambient temperature of +85 °C maximum, it is necessary to apply a

derating factor to this value. Figure 22 from the datasheet indicates the derating factor we

must apply.

Figure 22.

Peak power dissipation versus initial junction temperature

Figure 22 shows that the derating factor is about 75% of the maximum power admissible.

Then the device will be able to withstand more than 75 kW for a 457 ns surge duration as

described in this example.

So compared to the application example where PPP = 392 W, the SM6T27AY device is more

than suitable for the application

The SM6T27AY is too powerful with regards to the ISO 10605 test. Then, consider the use

of a less powerful device such as the ESDA25LY shown in Figure 23.

()

Vbr

=exp

-1

)

(

()

Vbr

=exp

)

(

()

exp

Vbr

() 2

'Rd

100

200

300

400

500

600

700

10/1000 µs

100

125

150

175

pp (W)

Tj(°C)

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