ADP2301 outputs 2.5V instead of 5V

Hi 

I am trying to make a DCDC converter using the ADP2301. The design is more or less taken from the design tool ADP230x_BuckDesigner_Release.xlsm. 

Down below is a picture of my circuit: 

I was hoping to get 5V as output, but instead i am seeing 2-2.5V with a lot of ripple. (see picture below) 

Here is some more scope pictures for you to understand what is going on. 

At BST pin: 

Enable pin:

Feedback pin: 

And here is the PCB layout. Underneath is a solid GND plane. 

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Can anybody help me out here? I am a bit confused about the feedback pin. From the datasheet the regulation voltage is 0.8V which it is designed to be at 5V. Why am i seeing almost 2V when my output is 2.5V? 

I have no load at the output, does that have an impact? 

Best,

Mathias

  • 0
    •  Analog Employees 
    on May 15, 2017 4:22 PM

    Hello, Mathias.

    Good day. Can you please provide the input/output specifications (Vin range, Vout, Iout)?

  • 0
    •  Analog Employees 
    on May 15, 2017 11:54 PM

    Hello again, Mathias.

    Thanks for providing the i/o specs. Upon checking your schematic and layout, it looks like there is no problem. Can you please double-check the feedback resistors you installed? The FB pin voltage should be 0.8V at any condition, even at no load.

    Also, please probe the SW pin and see if there is switching which should be similar to what is shown on figure 26 of the datasheet. The amplitude of the switching waveform should be equal to your input voltage, while the frequency should be 1.4MHz (since you are using ADP2301) and the duty cycle should depend on your input voltage (duty=Vout/Vin).

  • Hi Migs, 

    Thank you for your time! Much appreciated. 

    This is what I see at pin 6 (SW), which is far from what I expect. 

    Here is the same pin AC coupled: 

    which shows that it is running at 630kHz and not 1.4MHz. 

    I will try and resolder the Feedback resistors. 

    Thanks! 

    Mathias

  • Okay, so I reseated those feedback resistors, which give me some better results. 

    This is the output, which looks perfect. 

    However it only stays like this for approx. 2 seconds, then it begins to oscillate, as seen here: 

    Is this because of poor PCB layout? It switches back and forth between a clean 5V output and that nasty oscillation. 

    And BTW, i only have a 10µF output capacitor not 27µF as it is on the schematic, but I wouldn't think that is the problem..

    Best,

    Mathias

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    Added: 

    I decided to resolder all of it. And now I find that under light load (<250mA) i get a nice and stable 5V output. 

    However when I increase the load to over approx. 250 mA the voltage at the output decreases. 

    In this case I am using a 100ohm resistor as load, which gives a stable output voltage at 1V.

    This is the SW pin: 

    Why can't the regulator source enough current?