• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 639 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2941CT - ON/OFF CURRENT FLOW

Fabio Capozzi
Prodigy 20 points
Other Parts Discussed in Thread: LM2941

Hi, a power supply is built around LM2941CT.  Across 12V supply and ON/OFF pin there is 15K and from this pin to ground is 4.7K. 

Hence the quiescent voltage there should be around 2.86 Volt. The ON OFF pin is then fed through a bat54a to a micro controller pin that when out a logic 0 voltage put the On Off at 0.3 Volt so switching on the regulator.  Out voltage is set at 8.5 Volt through two resistors as directed in the data sheet

This setup works and the output is stable and regulated. My problem is that when the regulator is off the on/off pin voltage is not correct. Initially the voltage with that resistor values was around 4.5 volt and so i decreased the 4.7K setting around 1.5K. Now when power is applied and regulator is shutted off initial voltage is about 2.8 volt and slightly rise up in a few minutes to about 4.5 Volt. Since i have a micro controller at 3 volt this exceeded the specification of the pin and can be dangerous for the micro safety. Shutting down the pin to ground the max current should be limited by 15k resistor at a value not more than 12/15000 = 0.8 mA. Actually the measured current is about 2.8 mA, so it seem that the extra current is yield by LM2941 through on/off pin. I was expecting this to be an input with a max current draw of 100 uA as the data sheet says.

Moreover when supply is applied for long time (in off condition) the on/off point became unstable and voltage rapidly change ranging from 1.6 to 3.3 volt as if there is an oscillation somewhere. However the regulator function is working well, the problem is that a micro controller pin attached to that point put the CPU at risk.

I partially solved the problem clamping the on/off voltage with a 3.0 volt zener but the on/off behavior if quite strange.  i don't know if for some reason the 2941 has been damaged, however the output regulator voltage is good and clean when switched on. 

After supply applied for half an hour the resistance from On/Off pin and Input pin is 3.5k .. when power removed this value slowly rise up to 5.6k and more. 

My question is .. is all this normal and i am mis-using the on/off pin or the part need to be replaced ?

  • 0
    TI__Mastermind 22610 points

    The characteristics of the LM2941 On/Off pin are not fully described in the datasheet.

    The On/Off voltage threshold is approximately 1.2V. Due to the design, when the On/Off pin is voltage below 1.2V it sinks very little current, as the On/Off pin voltage increases above 1.3V the current into the pin will increase.

    When the On/Off pin voltage is at 2.0V the On/Off pin will ~typically~ appear to be (2V/50uA) 40k Ohms to GND.

    That should make your orginal resistor pair (15k and 4.7k) end up with about 2.62V at the On/Off pin, just a bit lower than your calculated 2.86V.

    You report that the actual On/Off pin voltage is "... around 4.5 volt ...".

    The increased voltage value is not expected, not normal, and indicates that the On/Off pin is sourcing current, not sinking current.

    As described, it is likely that the LM2941 On/Off pin has been damaged.

     

     

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Donald, tnx for you explaination. I made some experiments using the same schematic with new LM2941CT  parts and evertything is normal. So, ok .. the part has been damaged.

    Now i have to understand what are the condition for the on/off pin to be damaged in order to avoid it to occurr in the future. A few ideas 

    1) Should the on/off pin be damaged if connected directly to Input terminal ?

    2) Should the on/off pin be damaged if ADJ pin left floating ?

    3) Should the on/off pin be damaged if ADJ pin connected to ground or to Output terminal 

    The part should be protected with current limitation and thermal shutdown so an overcurrent in the output should not be the cause for the on/off pin to be damaged.

    Can you describe briefly the condition that can lead to on/off pin failure ?

    Tnx, Fabio.

  • 0 in reply to Fabio Capozzi
    TI__Mastermind 22610 points

    Q1) ... if connected directly to input terminal ...

    A1) The LM2941 datasheet does not call out any AbsMax, or Operating, limits for the On/Off pin. I believe that when the part was designed the intent was to be compatable with 0V-5V type TTL logic. I have bench tested the LM2941 On/Off pin up to +20V, and have not seen any problems.

    Q2) ... if ADJ pin left floating ...

    A2) There is no connection between ADJ pin and On/Off pin

    Q3) ... if ADJ pin connected to GND or OUT terminal ...

    A3) There is no connection between ADJ pin and On/Off pin

    Most likley cause of On/Off pin failure would be ESD damage.