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LM3488: IC is creating a short, in an attempt to prevent a short...

Jeremy Jay
Prodigy 10 points
Part Number: LM3488

I am developing a circuit that receives anywhere from 7V-9V and outputs 12.6V where Imax = 4A. I exported a design from WEBENCH and tried to match it exactly, but had to replace some of the components that were unavailable with similar components.

 

When I apply 7V-9V without a load, I measure 12.6V as expected. The issue is that when driving a motor, there is an inrush current which the LM3488 detects as a short. In order to avoid shorting across the load, the LM3488 enables the MOSFET on the DR pin, which seems to creates a real short from Vin to Gnd.

 

I tried to solve this issue by replacing Rsense with a 0 ohm resistor, and after that didn't work I removed Rfilt to disable the current sense function. Removing Rfilt fixed the shorting problem, but then the output voltage dropped to ~10V instead of 12.6V. After that I tried decreasing the resistance on Rcomp and Rsync, by stacking 2 resistors in parallel. Unfortunately that didn't change anything.

Do you have any suggestions for what to try? I attached the schematic and bill of materials for reference.

Thanks!

BOM.csvLM3488Test.pdf

  • 0
    TI__Genius 17640 points

    Hi Jeremey,

    Thank you very much for using the LM3488, and sorry to hear that you are having troubles. 

    The LM3488 is a peak current mode controller, and Rsense is required in order for the IC to function. Rfilt and Cfilt are used to reduce noise seen at the ISEN pin, so by removing Rfilt you may trigger current limit early, which may reduce your output voltage. The purpose of Rcomp and Ccomp is to adjust the frequency response of the converter, such that it adequately respond to transients. Adjusting Rsync would also not do much since that primarily controls your switching frequency. 

    The first thing I would like to ask is if you are driving the motor using a motor driver to drive your motor. It seems like you are driving a motor directly with the dc-dc converter, which would seem to give some issues.

    Thanks,

    Richard