• State Resolved
  • Locked Locked
  • Replies 12 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 725 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

Original question:

LM2776 not working??

WEBENCH® Tools/LM2776: LM2776 IC getting hot within few seconds after power up

sekar M
Prodigy 170 points
Part Number: LM2776
Other Parts Discussed in Thread: LMZ14201, LM2902

Tool/software: WEBENCH® Design Tools

I am using LM2776 switching regulator for -5V reference in my circuit. I needed 200 mA output current from this IC for the worst case scenario. So, I decided to use WEBENCH design tool for help.

I have given following inputs to the design tool:

input : +5V

Output : -5V

Current : 200 mA

Ambient temperature : 30°C


The desgin tool created one circuit and I followed it in my circuit design. But when the pcb is turned on, it gets heated as fast as possible and it's test point it delivers around +1.2V instead of negative output.

If I remove this circuit and turn on the board, other sections work flawlessly. When reconnect this circuit, it gets heated and affects it's power source. It's power source is LMZ14201 with +5V configuration. Without LM2776 circuit, LMZ14201 delivers 5V perfectly. If LM2776 is present, it will deliver only 1.96V (approximately).

Please tell me what might be the problem in the existing design. Used circuit is given below. Designed by Webench tool.

Thank you!

  • 0 in reply to sekar M
    TI__Expert 3645 points

    Hello,

    I need more details about your application. which is the load used? How many boards/ICs show this problem? Can you measure the Vin, Iin, Vout, Iout and the voltage across the Cfly (differential probe or subtract with the oscilloscope the voltage at each terminal to gnd)?

  • 0 in reply to Alfonso Furio
    Prodigy 170 points

    Hi, 
             The load for LM2776 is an op-amp LM2902 for sensing purpose, since it needs differential input. What I have measured with LM2776 is,

    Vin = +1. 96V

    Vout = +1.2 V)

    Enable pin is same as Vin. I could not measure  Iout. The package of LM2776 is SOT23 - 6 . I am also having doubt about the marking on IC for pin identification. 

  • 0 in reply to sekar M
    TI__Expert 3645 points

    Hello,
    can you check the voltage between the pins? If the device is damaged you will see the short circuit, you can keep one probe of the multimeter on the Vin pin and subsequently analyze all the other ones. Which is your doubt about the pin identification?

  • 0 in reply to Alfonso Furio
    Prodigy 170 points

    Hi,

                 I have found what causes heating issue. I have wrongly connected the pins due to the confusion between application circuit and pin details. Now it is not getting hot. I have changed new IC too. But now I am getting only -1.4V output for +5V input. I am using a webench design and same packages as suggested by the simulation result.

  • 0 in reply to sekar M
    TI__Expert 3645 points

    Hello,

    using a fresh IC, can you isolate our part on your board and test it? (just connecting a supply at the input and using a resistive load).

    Can you send a pic of your setup?

     

  • 0 in reply to Alfonso Furio
    Prodigy 170 points

    Sorry, the set up cannot be revealed. But I am sure that, the connection is checked numerous times. 
    3 capacitors are good in condition. There is no load given. We are checking the output with multimeter and oscilloscope.

  • 0 in reply to sekar M
    TI__Expert 3645 points

    Hello,

    do you have any update about your issue?

  • 0 in reply to Alfonso Furio
    Prodigy 170 points

    Hello sir,

                          I have developed a circuit separate board and tested. At last, I have found what creates the problem. It is a flying capacitor and an output capacitor. Both have the values of 1 µF and 10 µF respectively as per webench design. What  I did is two things :

    1. Changing a flying capacitor from 1 µF to 0.1 µF

    2. Changing an output capacitor from 10 µF to 1 µF.

    After this, I have got a steady output from IC. Before this change, that flying capacitor gets trouble in charging fully up to the input voltage. For instance, when supplied  4.9V as input, C fly only charged up to  4.5 V only. Usually, this happened at the turn-on time of the circuit.  This created a problem at the output side. At the output pin, I have got only 1.6V as a negative output. So, I decided to change these capacitor values and now it works fine. 

    If anybody knows what causes the flying capacitor not to fully charged at turn-on time, please explain. The input supply to this LM2776 circuit is obtained from the +5V LMZ14201TZ circuit.

    Note: I couldn't implement a webench design fully in my circuit. 

  • 0 in reply to sekar M
    TI__Expert 3645 points

    Hello,

    this problem that you mentioned is related to the impedance during the charge transfer. It can be related to the layout (adding parasitic) or the type of capacitor used. In the d/s in mentioned that is recommended Cout of 10uF for heavy load. What is your load? (webench selection was set at max current )

    Did you follow our recommendations for the layout?

    Which type of capacitors did you use?

     

  • 0 in reply to Alfonso Furio
    Prodigy 170 points

    Hello sir,

                     Yes. As you told, first I designed a circuit for maximum current consumption(200 mA) of our design and I got the circuit design from webench and went ahead with that design. 

                    After I succeeded in generating -5V output, the changed design (15 mA is the real load current ) is given as input to the webench tool but I got the same circuit that the tool generated for 200mA load. 

                  So, I couldn' t find out the real reason behind this issue. Also, I have followed the inputs given in the datasheet specifically for the capacitor's materials. I have used ceramic capacitors of X5R and X7S.   

  • 0 in reply to sekar M
    TI__Expert 3645 points

    Hello,
    did you test always the same IC? I mean the one wrongly connected. As said before, the issue should be related to the impedance path during the charge transfer into the Cfly (and from Cfly to Cout). The factors that can affect this behavior are:
    1) ratio between Cin, Cout, and Cfly
    2) layout
    3) type of capacitor
    If you want, you can check our EVM (has the same Cout and Cfly of your initial application) and it works fine.