Tool/software:
Hi,
I'm looking to use the LM74700D-Q1 in in a 24V automotive system to protect the rest of my system against reverse battery connections.
In the LM74700D-Q1 datasheet, section 10.2.4 "Selection of TVS Diodes and MOSFET for 24-V Battery Protection Applications" the design example selects an SMBJ58A and a SMBJ26A and connects them back to back.
I have read this section many times but I can't see how the LM74700D-Q1 will be properly protected against positive transients as the Absolute Maximum of the Anode pin to ground is 70V. The typical and maximum breakdown voltages of the SMBJ58A are 64.40V and 71.20V respectively. The forward drop of the SMBJ28A is listed as <3.5V. With these devices in series, the forward breakdown of the combined TVS diodes is 67.9-74.7V, depending on tolerance.
Am I missing something or is this not low enough to protect the LM74700D-Q1 anode pin? These voltage numbers are only at 1mA of current where the breakdown voltage is specified. The situation will only get worse with increased TVS current. For example, at the specified fault current for the SMBJ58A, the maximum clamping voltage is 93.6V- well above the Absolute Maximum voltage.
Why did the design example not select a lower voltage TVS in place of the SMBJ58A to properly protect the LM74700D-Q1 and the MOSFET?
I would select the SMDJ40A and SMDJ28A which has a maximum clamping voltage of 64.5V (10/1000μs). Accounting for the forward drop of the SMBJ26A, the total forward drop is now 68V which is below the Absolute Maximum. In the negative transient situation, the maximum clamping voltage would be 45.4+3.5=47.9V (10/1000μs). Would this approach work?
What am I missing here?
