# LTC3350: Discharge/charge current limitations in LTSpice Simulation different than calculated

Hi,

After dimensioning our LTC3350 design, we started the verification phase. One step is simulating it with LTSpice, where we observe an unexpected behaviour:

We have choosen Rsnsc to be 1.5 mOhm. This leads - according to datasheet - to an Ichg(max) of

32 mV / 1.5 mOhm = 21.33 A, which can be considered as charge current.

The incuctor current - considered as maximum discharge current - can be calculated as Ipeak = 58 mV / Rsnsc = 38.66 A

Now when we simulate the design, we observe a limit of 16 A (neither 38.66 A nor 21.3 A) while discharging.

Changing Inductivity, Cc capacitor, Compensation, Icap-filtering adding and removing and even output voltage reduction does not change anything regarding the discharge current limit. It stays exactly constand, as if the Rsnsc had higher resistance. But if we reduce the Rsnsc to 1 mOhm, the limitation increases to ~22 A. Further reducing of Rsnsc brings the design in our range where it works, hence it's not limited due to anything else. But just changing Rsnsc without knowing what's wrong and later ending with another problem is not how we work.

The charge current is also limited otherwise than expected: to 16.5 A instead of 21.33 A. (Independent of anything, even when no load is connected and independent whether or not the input shunt resistor Rsnsi is reduced.)

We use LTSpice XVII(x64) (17.0.27.0). May the LTC3350 spice model be wrong somehow? (We only want to share the complete schematic with Analog devices, so if you need it to reproduce, let me know where to send.)

Here's the behaviour with 1 mOhm (R6 is Rsnsc):

Best regards,

Michael

Parents
• Hi,

In between, I have verified the demo 3350.asc spice design and observe the exact same behaviour: (We just changed Csupercap from 1 mF to 5 mF to avoid charge current limitting due to input current limit.)

The charge current is about 1.3 times lower than calculated with the Rsnsc of 6 mOhm
We would expect a max charge current of: 0.032 V / 0.006 Ohm = 5.33 A.

But the charge current - when simulated and configured Csupercap to 5 mF - is limited to ~4.1 A.

Here's are design and simulation result:

• Hello,

from your schematic you have 16mOhm between VSNSP and VSNSN, this sets the overall(=system) current limit towards (32mV/16mOhm)=2A. The current for the output load (12V/10Ohm=1.2A) is prioritized versus the charge current, hence charging current will be reduced further on. Have you checked this? The available charge current depends on the available charge input power: (0.8A*12V)/vcap.

• Dear Markus,

Do you have any news regarding this? Is there a Betaversion we can test it?

If not, when do you expect the issue will be fixed?

Thank you and best regards,
Michael

• Hello Michael,

no news yet, the issue was reported to the model group and I think they're working on it. No info or visibility on their timeline. We have to be patient.

kind regards

Markus

• Hi Michael, Markus,

LTspice model fix is done. Updated model should be live after up to 24 hours. Please do "Sync Release" in LTspice later. Thanks!

• Hello Fil,

confirmed and done. Thank You!

• Hi Fill, Hi Markus,

Thank you. We're still testing. As you remember (my first post) the issue was not only the charge current. We're still running different simulations under different circumstances. For now everything looks fine. I will update once we have verified everything carefully.

Best regards,

Michael