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LM293: LM293DR Comparator Issue in Solar OCP Application

Brian Wang0928
Genius 15290 points

Part Number: LM293

Hi Team, 
 
This concerns a TI LM293DR comparator used in a Solar OCP (Over-Current Protection) application. We have discovered the following issue during testing and request your assistance:
Problem Description:
Customer's designed circuit (see attached plot below) shows that under normal temperature conditions, when Pin 5 voltage is higher than Pin 6 voltage, the output L_OFFINV_S is High. However, when the ambient temperature reaches 60°C, even though Pin 5 voltage remains higher than Pin 6 voltage, the output unexpectedly transitions to Low. Why does the output change to Low at 60°C when Pin 5 is still higher than Pin 6, especially since this temperature is within the operating range and the output should remain High?
Are there any adjustments needed in the circuit design?
 
Circuit1.png
Circuit2.png
Additional Information:
The L_OFFINV_S signal subsequently connects to 12 AND gates, with 12 identical groups as shown in the second circuit diagram.
 
 
Test Results:
At 25°C (see Plot with 25Degree):
 
Pin 5 voltage > Pin 6 voltage
L_OFFINV_S output: High level
Status: Normal operation 
25Degree.jpg
 
 
At 60°C (see plot here):
Pin 5 voltage > Pin 6 voltage (still higher)
L_OFFINV_S output: Low level (unexpected)
Status: Abnormal behavior
60Degree.jpg
 
 
Expected behavior: L_OFFINV_S should remain High when Pin 5 > Pin 6, regardless of temperature within the operating range.
Actual behavior: L_OFFINV_S transitions to Low at 60°C despite Pin 5 still being higher than Pin 6.
Please advise on the root cause and potential circuit modifications.
 
Regards
Brian
  • 0
    TI__Guru**** 174826 points

    Hello Brian,

    IN- blue, 2.5V ref always on, that's fine.

    IN+ red, Good appears to be 0V to 2V step. Abnormal starts above zero and steps up to about 1.5V. So input is not the same, but why?

    OUT green. Starts low, because 5V is 0V or comparator power is on 

    What is yellow? Is 5V and comparator power on throughout the test? What is comparator supply voltage (pin 4,8)?

    Is this a new design, do all samples do this at higher temperature? 

  • 0 in reply to Ron Michallick
    TI__Genius 15290 points

    Hello Ron,

    Thanks. Here please find the Q&A. Please comments. thanks. 

    Q1: The IN+ (red trace) shows what appears to be a normal 0V to 2V step response. However, the abnormal case starts above zero and steps up to approximately 1.5V. The inputs are different - why is this occurring?

    A1: This is caused by different voltage division ratios at the resistors after Pin 7 transitions from LOW to HIGH. Additionally, there's an unusual observation: at 60°C ambient temperature, when L_OCP_HW is LOW, Pin 5 measures approximately 0.3V. This is unexpected.

    Q2: The OUT (green trace) starts low. Is this because the 5V supply is at 0V initially, or because the comparator power is turning on?

    A2: The comparator power supply remains on continuously throughout operation.

    Q3: What does the yellow trace represent? Are both the 5V supply and comparator power on throughout the entire test? What is the comparator supply voltage at pins 4 and 8?

    A3: This signal is a HIGH signal provided directly from the previous stage, as shown in the schematic. It transitions to HIGH during the middle portion of the test. Throughout this process, the power supply remains on, and pins 4 and 8 maintain a stable 12V supply.

    Q4: Is this a new design? Do all samples exhibit this behavior at higher temperatures?

    A4: This is not a new design. We modified the circuit by changing resistor RW514 from 2kΩ to 5.11kΩ. At 60°C ambient temperature, we observe that Pin 5 voltage is higher than Pin 6, yet the output incorrectly pulls LOW.

    Regards

    Brian

  • 0 in reply to Brian Wang0928
    TI__Guru**** 174826 points

     Brian,

    I find some of the data supplied suspect. Nevertheless, I believe I figured out the issue.

    Note that IN- is about 1.13V. For OUT to go high, IN+ must exceed IN-.

    L_OCP_HW input is 2V in  normal operation and a little lower in abnormal operation. Also an issue is that forward voltage of DW51 drops at high temperature. Both of these effects reduce IN+ voltage (before OUT goes high).

    With the components in schematic, IN+ just barely exceeds IN- voltage in the best case. IN+ can't exceed IN- is the worst case. RW511 is far too low a value. 20k is a reasonable minimum, roughly 1.4V drive to IN+. 

    Once output goes high, IN+ will rise to L_OCP_HW voltage.