Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 64 subscribers
  • Views 878 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.

incorrect and unstable output of LP2992

Billyzhao
Prodigy 40 points
Other Parts Discussed in Thread: LP2992

I am now using LP2992AIM5-3.3 as my 3.3V regulator. However, I found that the output of the regulator would first reach 2.8V and then drop down to 2.4V slowly in both cases of disconnecting with other devices and connecting with others(about 50mA load current). 

I have used the recommendation caps selection of the datasheet( 1uF to input, 4.7uF to output and 0.01uF to bypass, all are ceramic). Besides, I have done some tests: 

1. increase the bypass cap from 0.01uF to 0.1uF; 

2. increase the output cap from 4.7uF to 10uF, or parallel two 4.7uF to the output; 

3. increase the input cap from 1uF to 10uF; 

But the result is the same as mentioned above. So what would be the problem?

One more thing: even disconnecting with other devices, it seems that the regulator consumes not less than 10mA current. Is it expected?

Forget to mention that: the On/OFF pin is connected to the Vin directly.

Thank you very much!

Billy

  • 0
    TI__Mastermind 22610 points

    " ... even disconnecting with other devices, it seems that the regulator (LP2992AIM5-3.3) consumes not less than 10mA current. Is it expected? ..."

     No, this is not expected, or correct.

    Under otherwise ~normal~ operation conditions, the expected current consumption with no output load should be less than 125 micro-amps.

    There is nothing in your description that points to any obvious problem, other than the no load que current being excessive.

    If output voltage is low, it's might be possible that bypass pin might have some leakage path to ground ... solder flux, leaky capacitor, or ???

    With no load que current being grossly high, there is a possibility that the device might be damaged, somehow.

    What is your input voltage?

  • 0 in reply to Donald Jones
    Prodigy 40 points
    Hi，Jones
    Thank you very much for your reply. The input voltage is 12V.

    I find that when I supply 12V, the regulator is very very hot, and the output voltage is about 2.5V. But when I decrease the input voltage to about 9V, the regulator cools down, the output voltage becomes 3.3V and the current changes to be about 0.22mA with no load.

    I suspect that maybe the regulator can not handle 12V input voltage, however, the data sheet indeed said that the input range ups to 16V. So I still can not find the solution. What do you think?

    By the way, the footprint of the regulator is SOT23-5.

    Thank you
    Billy
  • 0 in reply to Billyzhao
    TI__Mastermind 22610 points

    "... the regulator cools down, the output voltage becomes 3.3V ..."

    Sounds like 'Thermal Shutdown' behavior

    It's not the higher input voltage, per se. It's more likely the increased dissipation in the SOT23 package due to the increased voltage differential across the device (i.e. Vin - Vout).

    The SOT23 / DBV package thermal resistance (Theta j-a) is rated at 174.2 °C/W.

    That means that for every 100 milli-watts dissipated in the package, the internal (junction) temperature will want to rise 17.4°C above ambient.

    Going through the math shows that with Vin= 12V, Vout= 3.3, Theta ja= 174.2 °C/W, Tambient= 25 °C, and Tj(max)= 125 °C, you will be limited to about 66 milli-amps of load current. Part will be hot (i.e. 125 °C) , but not more than it can handle.

    Unfortunately this datasheet does not include the standard discussion of thermal dissipation considerations, so I will point to a generic document:

          Linear Regulator Design Guide For LDOs (SLVA118A)

          http://www.ti.com/lit/an/slva118a/slva118a.pdf

    See section 4.

  • 0 in reply to Donald Jones
    Prodigy 40 points
    Hi, Jones

    Thank you for your document!

    It seems that it is due to the thermal shut down. But even in the case of no load( having disconnected the regulator with other devices physically), the regulator is still very hot and outputs 22mA. Do you know why?

    Besides, as your calculation shows that, the regulator can handle up to 66mA output current. But why cannot it handle that 22mA current?

    Thank you
    Billy
  • 0 in reply to Billyzhao
    TI__Mastermind 22610 points

    " ... in the case of no load ... the regulator is still very hot and outputs 22mA ..."

    If the input pin is drawing 22mA with no output load attached, that is clearly not correct behaviour. The fact that the part get hot is secondary.

    I have no explanation.

    Device may be damaged, or there is something else happening that has not revealed itself.

     

  • 0 in reply to Donald Jones
    Prodigy 40 points
    I use another set of LP2992 and find that 22mA has gone. So I think the IC I used before was damaged.

    Anyway, thank you very much for your help!