AN2689 datasheet(24/42 Pages) STMICROELECTRONICS

24 / 42 page

Supply rail protection

AN2689

24/42

Doc ID 14310 Rev 3

Figure 30.

MT5510 output current in short circuit

In Figure 30 the current waveform shape corresponds to the pulse 2 of the ISO 7637-2

standard delivered by the SCHAFFNER NSG5500 generator when terminations are short

circuited.

As can be seen in Figure 30, the time constant is 5.4 µs duration when the generator

outputs are short circuited. When:

therefore

The current surge duration is below 20 µs. So, as explained previously when the duration is

below 20 µs, consider the Rd<20µs to be the same as Rd20µs

From the equations in Section 4.3 on page 23, VCL = 35.54 V and IPP = 7.22 A, which

means the residual over voltage will be consistent with the admissible maximum voltage of

the module to protect.

4.3.2

Power dissipation determination

On the other hand the peak power dissipation in the Transil is:

From the previous calculation in Section 3.3.2 on page 19, the current duration at Ip/2 that

will cross the Transil during the surge is given by the equation:

and the time constant is

t = 3.8 µs

Ipp = 23.5

At I = 0.5 Ipp, t = 0.7

then

= 5.4 µs

Ipp

→

µs

⇒

µs

nom

µs

Ipp

Vbr

Vcl

Rd20 µs = 0.143

Ppp

Ipp

Vcl

Ppp

256

⇒

() 2

'Rd

RsC

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