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LTC7840 LTspice model bugs

Category: Hardware
Product Number: LTC7840
Software Version:

I've used the LTC7840 in a design which is currently in manufacturing transfer. Just wanted to list some issues I've noticed with the LTC7840 model. I just re-synced my LTspice install, so I'm using the latest model. These can all be seen with small (or no) modifications to the test fixture circuit. I've attached a modified test fixture which makes some of the issues easier to observe.

1. PLL does not work with external clocks below ~215kHz. Can see this by connecting V2 to the SYNC pin.

2. Model disagrees with the "Current Sense Threshold vs ITH Voltage (ILIM Pin Float)" figure in the datasheet. The datasheet shows roughly y=(x-0.68)*0.104, while in LTspice I see something like y=(x-0.30)*0.086. Can see this without any changes to the test fixture circuit.

3. ILIM pins don't seem to function as described in the datasheet. Setting ILIM to GND sets the current sense threshold near 48mV (23mV in the datasheet). Setting ILIM to 0.3V gives the same behavior as ILIM floating (datasheet says the threshold should be near 50mV). Can see this by uncommenting the .step command for RILIM.

4. Hiccup mode does not seem to function, or at least I have not seen it happen in LTspice, regardless of what I do to it. Per my understanding, it should happen during the test fixture circuit's startup, with no modifications.

5. Soft start doesn't function as expected. During soft-start, the VFB pins should be equal to their respective SS pins (aside from inrush and current limiting), but in the model VFB seems closer to twice SS (but not quite). Also the SS pin current is only 5uA, not 10uA as specified in the datasheet. Can see this without any changes to the test fixture circuit.

Each issue isn't too terrible, but combined they made the LTspice model fairly useless for my application, especially for simulating startup behavior.

Let me know if you have trouble replicating any of these issues.



8666.7840 testfixture modded 08_28_22.asc