LM7480-Q1: Iso Pulse Protection - TVS selection

Yassine Damoh

Prodigy 205 points

Part Number: LM7480-Q1

Hello,

Based on the 11.2 TVS Selection for 12-V Battery Systems section of the datasheet, it is mentioned that the SMBJ33CA clamps at 44V/12A during the Pulse1 test.
Is this based on the following formula to estimate the dynamic resistance for pulses longer than 1ms ?

What's the power limit used to verify the design ? both the Peak-Power (for 10us/1000us) and thermal impedance curves show values lower than 12Ax44V = 528W at 2ms pulse (~1.41ms conduction time)
For the 24V system, SMBJ150A for D3 and SMBJ33CA for D4 are recommended. but I don't see how they will cover the load dump pulse, as the clamping voltage (maximum value usually shown on datasheets) add up to more than the maximum pulse for 24V system (202V max). - Maybe for Pulse 3b?

What are the voltage level to be expected for 48V system ?
In some cases, the thermal impedance curve and the temperature coefficient are not provided to calculate the Rdyn and the clamping voltage. is there a better way to estimate the minimum clamping voltage ?

Thanks in advance for your support.

Yassine

over 1 year ago

0 Rakesh over 1 year ago

TI__Guru** 105361 points

Hi Yassine,

Thanks for reaching out.

We will go through your questions and get back in couple of days.

Appreciate your patience here.

Best Regards

Rakesh

0 Rakesh over 1 year ago in reply to Rakesh

TI__Guru** 105361 points

Hi Yassine,

We use below approach to calculate the dynamic resistance. Typically, TVS diode manufacturers provide these information in the data sheet.

In case you have difficulty in getting required data to calculate the Rdyn and the clamping voltage, please refer equivalent power rated TVS diode in ST micro. They usually provide it in their data sheets.

https://www.st.com/content/ccc/resource/technical/document/datasheet/19/78/91/39/07/cd/40/65/CD00001366.pdf/files/CD00001366.pdf/_jcr_content/translations/en.CD00001366.pdf

Expect load dump event, all automotive transients are of low transient power and can be managed with SMB type diodes.

We have not seen such spec / requirements in battery based 48V system.

Best Regards,

Rakesh

0 Yassine Damoh over 1 year ago in reply to Rakesh

Prodigy 205 points

Thanks Rakesh for the reply.

What about pulses above 1ms ? do you have a formula for that ?

Pulse 1 per instance, (assuming 44V clamping voltage for the SMBJ33CA), the conduction time should would be higher than 1ms (approximately 1.4ms based on the approximation above) - Is the dynamic resistance equation still valid in this case ?

The formula below, was taken from an old ST application note for pulses above 1ms

Thanks again,
Yassine

0 Rakesh over 1 year ago in reply to Yassine Damoh

TI__Guru** 105361 points

yes, we can use that equation for >1ms. please refer page 15 in the attachment.

en.CD00181783.pdf

Power management

Power management forum

LM7480-Q1: Iso Pulse Protection - TVS selection