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UC3856: EA and CS usage

Emrah Yildirim
Prodigy 10 points
Part Number: UC3856
Other Parts Discussed in Thread: UC3846

Hello, first of all I'm sorry that I don't speak enough English.

I am driving 2 mosfets with UC3856. I designed a buck converter with 2 mosfets connected to each other.

I get 20kHz output from A and B outputs with 40kHz synchronous signal and combine these outputs with 2 mosfets and get 40kHz again (for switching with low frequency)

I can control the voltage at the buck converter output by changing the dead time in the sync signal. This is great. No problem.

I am reading the current at the input of the buck converter with the current transformer. That is the Drain end of the mosfets. 

I'm getting a ramp signal. Pretty much the same ramp signals for both 20kHz mosfets.

I enter the ramp signal (0.5V-1.3V) that I read from the current transformer to the CS+ end of the integrated and change the limit values with CLSS.

While the dead time at the A output of the IC is throttled, the B output is not throttled. When I lower the limit further, B output starts to throttle. I think this problem is due to the fact that both signals are not exactly equal. The same goes for the EA pin. I want to keep the output voltage and current constant. 

How should I use the EA pin and CS pin of the IC? Is there a direct relationship between these pins or do they control them separately? I examined the internal schematic of the integrated but could not interpret it. How should the EA connection be made?

Thank you for reading and taking the time.

Emrah Y.

  • 0
    TI__Expert 7900 points

    I'm having a bit of difficulty understanding the connection of UC3856.  First, you have a single UC3856 and 2 MOSFETs, correct?  Second, you are not using the EA pin and applying a voltage to the CLSS to manually control the duty cycles?  Manual control is quite "slow" and this may be part of the problem.

    The UC3856 is an improved version of a family of older controllers, UC3846.  The data sheet of the older controllers has more detail on how to use the part.  Examine Figures 5 and 6 in the UCx847/7 data sheet in the following link.  This figure shows the EA and COMP connections.

    https://www.ti.com/lit/ds/symlink/uc3846.pdf?ts=1628778553927&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUC3846

  • 0 in reply to Eric Reicher
    Prodigy 10 points

    Hello, thanks for your answer.

    Here is my mosfet connection. You can think of the A and B signals coming out of the UC3856 as entering these two mosfets. (of course not directly, there is a driver circuit in between) For CS, I indicated the place where I got the signal with an arrow. But in response to this signal, the duty is not throttled properly. The duty of one of the outputs (A or B) decreases. Then the other starts to decrease.

    But now my second question is more important. What does the EA pin do? And how is it checked? 

    I reviewed the PDF you sent. But I can't use EA in real life. What exactly does this pin control?

  • 0 in reply to Emrah Yildirim
    TI__Expert 7900 points

    You are using the CLSS pin to manually control the current out.  This is supposed to be the current limit (or soft start) pin.  The Error Amp (EA) is supposed to be used to close the loop by sensing the output voltage and controlling the duty cycles of the FETs.  The EA uses the COMP and FB- pins to form a PI type controller.