LM2622 DC DC boost help

Discussion in 'The Projects Forum' started by letsbully, Apr 5, 2012.

  1. letsbully

    Thread Starter Member

    Mar 23, 2012
    34
    1
    Ok, so I made the following circuit with some modifications:

    [​IMG]

    I changed the value of R1 is 22.68K so the output voltage measures 5.06v. Everything is fine on paper and when using a probe to detect voltage, current, etc. In practice however, the voltage drops significantly when a load is attached (from 5.06v to 1.82v at times). I'll list the components I used. I'll be grateful if someone can help me figure out the culprit. I've tried substituting components that I thought were affecting the circuit without much in terms of results.

    Components:
    L: 10 uH (2.6A, 2.7A saturation, radial type)
    Cc: 2200pF (2.2nF) 100V Ceramic
    Rc: 24K 2 watt 5% metal oxide
    Rfb1: 22K 2 watt 5% metal oxide + 680 2 watt 5% metal oxide in series (22.68K)
    Rfb2: 7.5K 2 watt 5% metal oxide
    Cin: 22uF 63V Aluminum Electrolytic
    Cout: 22uF 63V Aluminum Electrolytic
    D: 40V 3A Schottky 490mV@2A forward voltage

    The only thing I have not tried so far was to replace the Cin and Cout capacitors with a different type of capacitor. There is a note in the LM2622 datasheet about using tantalum or ceramic capacitors in Cout. Could that be the only culprit? I am trying to connect a USB device to the Vout node. The USB device never powers on (the USB device is functional because I have a different circuit with which it works fine at 5.06v with the same USB socket used in this setup). The Vin node has adequate power for this USB device. So, what's the deal?
     
    Last edited: Apr 11, 2012
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I think you have the right value of resistors for the 5 volts out. I didnot see much about the shdn(pin 3) on the data sheet, it did say maximum of 7V and active low. I would conect pin 3 to pin 6 that will insure the IC is turned on. Sometimes an open pin will float near low. That could be the problem.
     
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  3. letsbully

    Thread Starter Member

    Mar 23, 2012
    34
    1
    I attempted to do what you suggested. The results were pretty much the same. With the USB device connected, the voltage drops from 5.07v to 3.95v and the USB device refuses to charge (when I attempted to charge the same device with a step-down converter, I was successful each time. The voltage did drop to around 4.5v when the device was connected).

    Increasing the input voltage by 1.5v does nothing to remedy the voltage drop, which measures at 3.95v under load despite the voltage increase. With a step-down converter I mentioned above, the USB device was steadily pulling 230mA. The LM2622 should be able to provide more than enough.

    I am using AA batteries as my source for this booster, and a "lantern" type 6v battery for the step-down mentioned previously (to make sure the USB device and/or socket I'm using are in fact functioning and not just playing games with me).

    Any other thoughts? Could I have mistakenly zapped the chip when handling it? Doesn't seem plausible because it does produce the right voltage without a load.
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    It is possible that something is wrong with the IC. I am concerned about the AA batteries, if you are using alkaline AA they have a nominal rating of 2.1Ah but that is with a drain of about 80mA. Is the input voltage dropping when you apply the load to the circuit?
    The data sheet did mention ESR of the capacitors. I wonder of the output capacitor with a 63V rating has a low ESR. I am not sure if that is affecting the output. The data sheet also mentions pin 8 needing to be grounded under the chip or shielded from emi. You might contact TI techinical help on thier websit.
     
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  5. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    Can you monitor the voltage at pin 5 with an oscilloscope?
     
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