General design procedure

AN4209

12/19

DocID023949 Rev 2

2.2.1

Step 1: STRVS preselection

In the absence of transient voltages, the STRVS should act as an open circuit and should

have no effect on normal circuit performance (Criteria 1 in Table 2). The preselection begins

the admissible clamping voltage which includes a safety margin generally 15% below the

absolute rating of the end-component to protect.

At this time, several STRVS may cover both criteria 1 and criteria 2. In the first pass using

the highest rated voltage available is recommended. This generally minimizes the standby

consumption and power dissipation.

The smallest package available is primarily chosen to optimize the solution from the thermal

point of view with minimum loop numbers.

2.2.2

Step 2: Clamping voltage assessment

using curves provided in the datasheet. This point has to be verified in the worst case

should not exceed 125 °C. This step requires a current measurement because the STRVS

current is generally unknown. If so, it is recommended the designer starts the evaluation

with the highest pre-selected rated voltage and with the biggest package available to avoid

a thermal failure. When the peak current can be predicted or simulated, the clamping

voltage can be directly calculated.

decreased by recursion until the lowest pre-selected STRVS can be identified at a cost of a

higher power dissipation and quiescent current consumption.

2.2.3

Step 3: Dissipated power calculation

Power dissipation assessment is requested to evaluate the maximum and average junction

given at 125°C overestimating losses.

By using Equation 6 and/or Equation 10, operating junction temperature of the device can

be computed.

compatible with design rules. Otherwise, the user should return to step 1 or 2 to go over the

available electrical characteristics and package performances of the device. When

dissipated power is too high, a second option is to use STRVS in serial configuration in

order to spread the heat flow between packages.

2.2.4

Step 4: In-situ test

In the last step, we recommend checking the application to ensure that all criteria are

respected with the final selected device.

This step requires a current waveform and lead temperature measurement to calculate the

actual junction temperature and the clamping voltage.