ADP160 output higher than expected

Discussion in 'General Electronics Chat' started by Andrew1234, May 10, 2016.

  1. Andrew1234

    Thread Starter New Member

    Feb 16, 2015
    20
    1
    Hi Forum:

    I am using the ADP160 (www.analog.com/ADP160), specifically the TSOT packaged version with a fixed output voltage of 2.5V. The input to this device is a 5V regulated supply. I am expecting 2.5V at the output of the ADP160, but I am in fact getting about 2.9V. I have checked the input, and this is working fine at a measured 4.96V. Does anyone have an idea of what may cause the voltage to be higher than expected? I would think if there was a path to ground at the output, that the voltage would be lower than expected, or that the part would simply fail. The 2.5V output is being used as a reference voltage to some 5V op amps that can receive a mid-rail reference. Any thoughts on places to look for debug?

    Thanks!
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,308
    884
    Are you sure you're using the appropriate input and output capacitors for this device? Especially for the output capacitor, the datasheet recommends a ceramic capacitor of at least 1 μF with an ESR of no more than 1 Ω to ensure stability. With the wrong capacitors, the device can oscillate at RF frequencies, and the output voltage as read on a meter will be way off. I suggest checking with an oscilloscope to make sure this isn't happening.

    Other than that, the only things I can think of are mis-wiring or a damaged part.
     
  3. Andrew1234

    Thread Starter New Member

    Feb 16, 2015
    20
    1
    I am using a 1uF cap on the input and output, with a 20% tolerance. That should ensure it is above the minimum capacitance shown in table of 0.7uF. However I didn't pay attention to the ESR requirement. Table 2 actually says the min max range is 0.001 ohm to 0.2 ohm. Unfortunately the caps I am using do not specify the ESR on the package -- My meter does allow for an ESR measurement, and I get 1.2 ohm so I'm not even in the ballpark. I'll try ordering 4.7uF caps with an ESR in the specified range and see if that solves my problem. Thanks so much for you suggestion (and I once again have to learn to carefully read the datasheet!)
     
  4. Andrew1234

    Thread Starter New Member

    Feb 16, 2015
    20
    1
    I put a scope on the Vin and Vout rather than a DMM as you suggested. It turns out that the "regulated" input voltage is not the 4.96V as I thought, but is actually oscillating quite a bit. 4.96V is just the RMS value I guess, and so I really don't have a clean DC voltage as an input to the ADP160. The upstream regulation circuit is based on the ADP2302. The circuit I'm using is shown in figure 58 of the datasheet (www.analog.com/adp2302), and I'll be looking carefully at the passives around it to see what the problem is. I do have a 15V input to this part rather than the 12V shown in figure 58, which I assume is okay given the input range of the part.
     
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