LTC7003 current limit not activating

I designed a board which uses the LTC7003 and an N-channel MOSFET to control a power output. I have set the current limit to approximately 3.23 A by using a 0.007 Ohm current sense resistor and a 45.3 kOhm resistor on the ISET pin. There is a 0.33 uF capacitor on the TIMER pin which should give a 4.6 ms fault time. I am finding that the LTC7003 does not detect an overcurrent fault even with the output load set to over 4 Amps.

I have used this LTC7003 design on a previous board and it functioned properly. The biggest difference between these two boards is that the new board design uses a different regulator to power the PYLD_PWR net and I have seen that this net is quite electrically noisy, with about 90 mV ripple on the nominal 28 VDC signal and much larger peaks due to switching noise. Could the additional noise be causing issues in the LTC7003?

I have probed the ISET and IMON pins while a 4 A load was present and saw that the voltage at IMON was greater than the voltage at ISET, indicating that the LTC7003 does see that the load current is greater than the threshold. However, the voltage on the TIMER pin remains at 0 V. I have verified that there is not a short from the TIMER pin to ground or any adjacent pins.

I have attached a screenshot of the schematic. Does anybody have any ideas of why the LTC7003 is not detecting an overcurrent fault? I have tried populating the filter capacitor between SNS+ and SNS- pins but saw no effect on the behavior.

  • +1
    •  Analog Employees 
    on Jan 27, 2021 7:38 PM 29 days ago

    The current limit is a bit higher than expected at room. However, the delta Vth tolerance is basically mV around the nominal set point (not percent) so lower Iset voltages are worse. I would consider leaving Iset open, using a 10mohm sense R and resistor from SNS+ to SNS- for best accuracy.

  • I did not realize that the tolerance on the current sense threshold voltage was millivolts. I had used the smallest sense resistor possible to reduce wasted power effectively also reduced the precision of the current limit. However, even testing with a load that gave a current sense voltage of 7 mV over the setpoint did not trip the current limit.

    I reduced the switching noise on the rail powering the LTC7003 and flowing through the sense resistor and the current limit began to activate. I have also increased the sense resistor to 10 mOhm per your recommendation and am now seeing the current limit activate about 400 mA above the setpoint. This is still higher than expected but is acceptable in this application.

    Thank you for your help!