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ucc28700: USB Power Supply Specifications

Hagan McBride
Intellectual 260 points
Part Number: UCC28700

I am building a charger for mobile devices and I want the input to be universal 85-265V and the output to be 5V 2.1A. I have my circuit designed and at the output I am getting 5.7VDC at no load, 5.2V at 0.5A, and 5.0V from 1A to 2.1A. With a cable attached I am getting 5.7 at no load, 5.0V at 0.5A, 4.8V at 1A and 4.6V at 2.1A. I am wondering what the normal USB charging specs are and if I am within spec? I am concerned about the no load voltage and the cable drop voltage.  

  • 0
    TI__Guru 74115 points

    BC1.2 states the output voltage is between 4.75V and 5.25V in steady state across input voltatge range and load range (no load to full load). 

  • 0 in reply to Hong Huang
    Intellectual 260 points
    So how can I adjust the no load voltage to get it down without changing the voltages with a load? Also that spec is at the output of the circuit right? not at the end of a cable?
  • 0 in reply to Hagan McBride
    TI__Guru 74115 points
    The voltage is at a charging port so the values are at the cable end to plug into a phone or tablet computer. A cable needs to be specified to know how much voltage drop would be. Then UCC28700 can be programmed to compensate the voltage drop so to make the cable end voltage within 4.75V and 5.25V.

    On the no load voltage 5.7V, does the design have a pre-load at output? Usually a preload such as 1k-ohm resistor can bring this value down. You may need to measure the swtiching frequency at no load. The minimum frequence is typica 1kHz. After reaches this value, if the input and the output energy cannot be balanced, overvoltage can occur, i.e., the output is out of regulation, such as 5.7V as seen.

    Another possible approach to get the no load output voltage back to the regualtion is to try to increase the current sensing resistor value a bit so each switching cycle, the energy delivered from the primary to the secondary becomes smaller than the minimum switching frequency becomes ok to achieve the output voltage regulation.
  • 0 in reply to Hong Huang
    Intellectual 260 points
    Thanks for the advice. I will first try adjusting my preload resistor and if that does not work I will adjust my current sense resistor.
  • 0 in reply to Hong Huang
    Intellectual 260 points
    I was able to get the voltage regulation correct at the output, however now I am still working on getting the voltage drop down across the cable. Thanks for your help