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UC2625: UC2625

Youssef Ojeil
Intellectual 355 points
Part Number: UC2625

Hello, 

I am using the UC2625 and i have come across "E/A OUT range" in the datasheet. There seems to be minimal description in the datasheet about what that range refers to or affects. It would be great if someone could clear up my confusion.

Thank You

Youssef

  • 0
    TI__Mastermind 18900 points
    Hello Youssef,

    The team has been notified of your question and should answer relatively soon.

    Best,
    -Cole
  • 0
    TI__Mastermind 18900 points

    Hello Youssef,

    E/A OUT refers to the output voltage range of the Error Amplifier in the UC2625. Note that E/A IN +/- and OUT are just the general inputs and outputs of the amplifier. More info on that below.

    I've attached a screenshot of the description of EA from the datasheet below:

    Keeping the error amplifier general is useful when breaking down the motor drive circuit into a control feedback system.

    For example, we can use the output speed compared with some speed command using the error amplifier to aquire the error between the intended and current speed of a motor. This is a helpful step in developing a feedback path that handles speed regulation (i.e. sees current speed, compares it with a speed command, and determines that the PWM need to run faster or slower to meet the current speed). This kind of operation is discussed in the reference design given below: http://www.ti.com/lit/an/slua106/slua106.pdf

    Best,

    -Cole

  • 0 in reply to Cole Macias
    Intellectual 355 points

    Thanks for getting back to me. I think you might have misunderstood my question. In my design we are feeding back E/A out to E/A IN (-) and sweeping E/A IN (+) from 0V to 4V assuming that the error amp should not work below 0.25V and above 3.5V since that is the range found in the datasheet as shown below.

    But we did not get those results, instead the error amp kept working under 0.25 till about 10mV up to 3.55V. 

    Another issue I am currently running into is the temps at which the device runs. I ordered UC2625DW and in the datasheet it says that they run from -40C to 105C but in the packaging information as well as the link I am about to send it is shown that it runs from -40C to 85C.

    http://www.ti.com/product/UC2625

  • 0 in reply to Youssef Ojeil
    TI__Mastermind 18900 points

    Hey Youssef,

    Okay, yes, sorry for the miscommunication.

    The E/A OUT Range refers to the output limitations of the Error Amplifier. Going to 10mV and 3.55V instead of 0.25V and 3.5V is actually better performance than what is shown in the spec. This is okay and expected.

    When TI puts a spec in the Electrical Characteristics table, TI gaurentees that 6 sigma of the devices (99.7%) should fall within the table. While the particular device that is evaluated may have better performance, another device may not performance as well. With that said, it should fall within the sigma 6 bounds shown in the table you've listed (i.e. the device E/A output range should be less than or equal to 0.25V and greater than or equal to 3.5V). Hopefully this explains the situation.

    For your second question, thank you for bringing this to our attention. It will take some time to confirm but I assume this mismatch could be the difference between junction and ambient temperature (since TI focused more on showcasing junction temperature instead of ambient in the past).

    Hope this answers your question,

    -Cole

  • 0 in reply to Cole Macias
    Intellectual 355 points
    Hey Cole,

    Thank you for replying, your information about E/A out range was helpful and did clear my confusion.

    Concerning my second question, when you get the chance to confirm, would you be able to let me know ?

    Thank you again,

    Youssef
  • 0 in reply to Youssef Ojeil
    Intellectual 355 points

    Hey Cole,

    Another question but this time it is about the SSTART pin. To my understanding SSTART should only be pulled low if VCC drops below threshold or overcurrent is reached. Does that mean that if none of these two conditions are met then SSTART should be sitting at its natural high (which i believe was around 4.5V) ? 

    I also see in your typical application that SSTART has a 51k resistor to VREF and cap to ground, are both the resistor to VREF and cap to ground needed for it to function properly ? Or would the SSTART pin be high without the resistor to VREF if none of the above mentioned conditions were met ?

    Regards, 

    Youssef

  • 0 in reply to Youssef Ojeil
    TI__Mastermind 18900 points

    Hey Youssef,

    After doing some consulting, the Operating Temperature refers to the Junction Temperature (TJ) from -40 to 105C. There is a mistake on TI.com that I'll will signify that the range needs to be fixed. Thank you for your patience. Just note that newer devices refer to ambient temperature instead of junction temperature when they say "operating temperature". When in doubt, check what the datasheet has defined operating temperature.

    Anyways, you are right the SSTART, in normal operation, is pulled up to approximately VREF (5V or 4.5V in your case). In addition, both components are required for operation.

    When a fault occurs (VCC drop out or overcurrent condition), the base/gate of the transistor will be set (note the S in the block diagram) to "1" and which pulls down SSTART to GND. The resistor is needed because the act of pulling down the pin to GND would be shorting VREF to GND in the fault condition (which is not safe). As a result, the resistor limits the current  from VREF during the fault condition but lightly pulls the pin to VREF during normal operation.

    For the capacitor, we need to look at when the transistion occurs from the fault condition to normal operation. When the base/gate of the transistor is set back to 0, which makes the SSTART pin no longer tied to GND, there is a 10uA current source that is fed back into the pin. This current will charge the capacitor and help raise the voltage back to VREF. Without the capcitor, the 10uA has no where to go and the voltage will not rise as expected.

    Hope this helps,

    -Cole

  • 0 in reply to Cole Macias
    Intellectual 355 points
    Hey Cole,

    Thanks again for answering my questions, I just want to clarify one last thing. In my initial application I did not have SSTART connected to VREF yet SSTART was still naturally high when no faults occur. Based on your answer I now know that if i wish to connect it to VREF I would need the cap and resistor, but what if SSTART is not connected at all to VREF ?

    Regards,
    Youssef
  • 0 in reply to Cole Macias
    Intellectual 355 points
    Another question.

    To my understanding when SPEED-IN passes a certain threshold which is around 250mV it should inhibit any change in direction. So if initially I have DIR set to a high and change it to a low with the same input conditions then my outputs should not change state, but they do. Is there something I am missing or misunderstanding when it comes to the SPEED-IN threshold ?

    Thank You and sorry for the multiple replies.