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LM7800: LM7805 behavior with Comparator LM339

Shrey Gupta
Prodigy 125 points
Part Number: LM7800
Other Parts Discussed in Thread: LM339

Tool/software:

Good morning everyone,

I am currently working with the LM339 comparator and the LM7805 voltage regulator and have encountered an issue. When I connect the output terminal of the LM339 comparator to the input of the LM7805, the voltage at the comparator's output pin drops from 12V to approximately 2.48V. However, when I disconnect the comparator's output from the LM7805, the voltage returns to 12V.

Considering that the LM339 comparator has an open-collector output and the LM7805 receives its input directly from the VCC rail, I am unsure why this behavior occurs.

Could you please help me understand the root cause of this issue? For reference, I have attached images of the simulation setup for your review.

Thank you in advance for your guidance!

Image 01

Image 02

Image 03

Image 04

  • 0
    TI__Expert 6930 points

    Hi Shrey,

    Sorry for the delay in getting back to you. I will answer this from an LDO perspective and also invite the Comparator team to contribute to the answer:

    The LDO has a quiescent current of 5 mA and when the comparator output is connected to the LDO input, the VIN drops sue to a drop across the 10K R2 resistor. Please eliminate R2 and re-simulate to confirm.

    Regards

    Ishaan

  • 0 in reply to Ishaan Shetye
    TI__Guru**** 154086 points

    Hello Shrey,

    You found the problem. R2 provides all the drive to LM7805. It is not enough.

    Open collector means the LM339 output can sink current towards ground. In other words LM399 can make the output voltage lower.  

    Open collector also means the LM339 output can not source any current. In other words LM399 can not make the output voltage higher. 

    Shrey Gupta said:
    Considering that the LM339 comparator has an open-collector output

    What is the goal of the circuit? A full redesigned may be in order.