Hello:
We've used two of the LMZ34002 chips on our board and fabricated three identical boards. We're using these to generate -12V from 24V but also using the inhibit feature, on one board we are achieving -12V with no issue but on the other two boards we have no output unless we input 20V instead of 24V.
I think a likely culprit of this issue is the addition of a level shifter to use the inhibit feature. The level shifter is implemented exactly as recommended in the datasheet (Pg 19) using the exact same suggested chip, then the -12V configuration is also implemented exactly as shown on Pg15 (Figure 34). However, Fig.34 does not show a level shifter so will the addition of this require component values need to change?
We've confirmed the thermal layout is correct, tested components and confirmed they are as advised and we've even found the schematic for the LMZ34002 (LMZ34002EVM-001) for comparison but see no obvious errors. The INH pin is being pulled down by something so we increased R10 to 64K to match the eval schematic but to no avail.
Do you have any suggestions?
Our Layout:
LMZ34002EVM-001 Layout:
