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UC1825: what is definition for UC1825 start up current?

Samuel Kerem
Prodigy 70 points
Part Number: UC1825
Other Parts Discussed in Thread: UC2825

Is that a current consumed while the input voltage is below undervoltage threshold (e.g. 8V)? Then, wouldn't be the proper name as: IC disabled current?

Or, this is the current that appears as the input voltage crosses 8V and the device is enabled? 

For example, here is the setup.

The voltage source is V_src=30V. This particular IC has the startup current 1mA. there is a 20K resistor between the voltage source and UC1825 pin 15 (V_In).

Thus, there is (30V-1mA*20K) =10V @ V_In (pin 15). That is above the undervoltage threshold, so the device must start, but the operational current is forced to ~1mA. Which features are operational in this condition?

  • 0
    TI__Guru 72955 points

    Hi,

    It is the current right before the start-up. UC1825 startup current max is 2.5mA. You need to get that much current when VCC reaches 9.6V. IC supply current after startup needs up to 33mA.

  • 0 in reply to Hong Huang
    Prodigy 70 points

    Please answer the following 2 questions:

    1.  If I provide 2.5ma, and the voltage is 10V, are ANY features of the device are operational (for example: the output at low impedance? the ref voltage is available? The clock starts?)

    2. Is this "start up" current -- just the device quiescent current at the fully disabled state?

  • 0 in reply to Samuel Kerem
    TI__Guru 72955 points

    Hi,

    The start current is to start the IC and after start the current demand increases to specified. Start means the IC start to work and with pulses sending out and all functions are available but the current needs to increase to proper level specified by the datasheet. If the current stays at 2.5mA, the VCC will reaches UVLO off.

  • 0 in reply to Hong Huang
    Prodigy 70 points

    Have you worked with that IC?  

  • 0 in reply to Samuel Kerem
    TI__Guru 72955 points

    Hi,

    It looks the issue is resolved. I will close this thread. If you have additional questions, please open a new thread.

  • 0 in reply to Hong Huang
    TI__Guru 72955 points

    Hi,

    You asked what the startup current means, I have already answered your question. If you have additional questions, please start a new thread. If you cannot my explanation, please indicate where. So far I am sending either so need to close this thread.

    Please note simply reject does not help to resolve your true issue,

  • 0 in reply to Hong Huang
    Prodigy 70 points

    This issue is not resolved. I am trying to figure out if you worked with the part (please answer yes or no) or just relying on the datasheet. There are mistakes in the datasheet. (e.g. page 8) Are you sure the pulses appear on the output? I dont see them. And if pulses do appear as you said, for how long?

  • 0 in reply to Samuel Kerem
    TI__Guru 72955 points

    Hi,

    We are making support based on the datasheet and other resources we have and experiences, and not necessarily to use a device to make support as there are many in common from device to device.

    No I did not use this particular device. If you think something is wrong on datasheet you can ask. 

  • 0 in reply to Hong Huang
    Prodigy 70 points

    Look at page 8. www.ti.com/lit/gpn/UC2825 . Design Example: 50W, 48V to 5V DC to DC Converter - 1.5MHz Clock Frequency

    The output on the right (+) is shortened to gnd on the left Vin negative. 

  • 0 in reply to Samuel Kerem
    TI__Guru 72955 points

    Hi,

    It looks the concern is the secondary GND symbol should be different from the primary. But sometimes, a designer drew that by convenience yet not correct.

    I am not sure why this design tie VIN(-) to Vo(-). But as this design should be an isolated design, and even this connection is allowed in practice. But again, I am not sure why this design needs such connection. Also, it may be just a simple drawing error.