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.

  • Resolved

LM5145: LM5145 Standby Mode has output of 1.2V

Part Number: LM5145

Hi experts,

Using LM5145 EVM, when controller enters standby mode, there is a approximately 1.2V Vout in over 40s. And I am also wandering why there is a standby mode in almost every buck controllers which are with UVLO. The corresponding pic is shown as below: in which

Red line indicates UVLO;

Blue line indicates VIN;

Sky-blue line indicates Vout.


My questions are:

1. Why controller must feature Standby Mode? Is it because VCC needs time to rise?

2. Why there is a 1.2V output with rise time RC similar to Cout*(Rfb1+Rfb2)?

Thank you very much for your help!

Regards,

Wenhao

  • In reply to Wenhao Wu:

    Wenhao,

    Please also share a picture of the back of the board.

    I'll be receiving EVMs shortly. Once I do I'll recreate your tests with your setup.

    -Sam

  • In reply to Samuel Jaffe:

    Hi Sam,

    I will take the pic later i have access to lab. Thank you ahead!

    Wenhao
  • In reply to Wenhao Wu:

    Wenhao,

    I tried to reproduce this in the lab and got similar results until I realized I incorrectly assumed which pin is pin 1. I thought it was the first on the long side but it's actually the one on the edge closest to the dot. I was tying RT to GND instead of EN. Once I tied EN to GND I found regular operation - very low input current and no output.

    Please check that you're pulling the correct pin to GND.

    -Sam

  • In reply to Samuel Jaffe:

    Hi Sam,

    Sorry i did not understand what you meant. Did you actually mean i should tie EN to GND with a connecter? On the other hand i think it could pull down EN making the device stay in shut-down mode. Thank you for you answering! Do you still need my experimental setup now? ihave already taken a pic of back of the setup.

    Wenhao
  • In reply to Wenhao Wu:

    Wenhao,

    You said you see an output of 1.2V when the device enters standby mode. This mode would be entered by connecting EN to GND. Is this how you entered standby?

    I do not need the picture if this is resolved.

    -Sam
  • In reply to Samuel Jaffe:

    Hi Sam,

    I did not connect EN to GND. As I can understand in datasheet. 5145 will only enter standby mode when VEN is between 0.6 to 1.25V. So i just let it be, i changed nothing of EVM. Naturally, EN is supposed to connect to VIN via a resistor dividor. If nothing changed to EVM but just set VIN=10V, 1.2V output is still there. Sorry to ask you again. Could you please try if just connect VIN and VOUT, GND and nothing changed to EVM, in this case, test output? If you have a 1.2V output, then you recreate it, please tell the cause. Thank you very much!

    Wenhao
  • In reply to Wenhao Wu:

    Wenhao,

    I understand now. I've been able to reproduce this in the lab. I will investigate and get back to you.

    -Sam

  • In reply to Samuel Jaffe:

    Hi Sam,

    Thank you that you have recreated the issues. Looking forward to your reply. 

    Wenhao

  • In reply to Samuel Jaffe:

    Wenhao,

    This voltage is due to the ILIM current source. This is typically around 100uA which will charge the output caps but taper off due to the current drain from the feedback resistors and other components.

    You can add a minimum load or ensure the EN voltage does not linger in this state if this characteristic is unacceptable for your design.

    -Sam
  • In reply to Samuel Jaffe:

    Hi sam,

    Now i understand the reason and i will inform my team about this issue. Another question please, can this standby output issue be solve by using Rsense mode? On the other hand this will increase the total cost and size because large sensing resistor needed. Need your confirm. Thank you!

    Wenhao

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.