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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I've made two boost converters, using the below chips;
one uses this, while the other uses
I've noticed that whenever I plug in a...load, like, say, my ipod, the IC will start clicking, and the clicking is mimicked in the screen brightness of my ipod touch. This ...ripple, is preventing the ipod from charging corrrectly, and actually discharges it (which is bad, for obvious reasons) (also, yes, I have resistors across the data input/outputs of the USB port, as per instructions for making IPOD chargers)
They both are made to run off of two AA's, (though, I'm thinking 3 may be more realistic for the TI IC one)
When adding another load like 50 ohms across the regulator, it stops the flickering, and the ipod charges normally. However, for obvious reasons, this is wasteful of power, and with two AA's, that's really not a good thing. (
Also, another note, both have a 5 volt linear regulator (7805) on their outputs, as I've made the output voltage of them adjustable with a potentiometer. (don't want to kill any USB devices :) ) The TI IC one has a roughly 200 uH inductor (litz) wrapped on a powered iron core from a PC supply. The other one has a 100 uH inductor with standard magnet wire wrapped around a yellow powdered iron core. The TI one has 470 uF filter capacitance on the output, while the LM3578 one has 1000 uF.
Is there some way to get rid of this nasty flickering/ripple issue, asides from adding a wasteful min load?
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Um, perhaps you should not be using your ipod to test such a device, but that aside, you could use a diode in series with the inductor to prevent it from discharging your device. I'd get rid of the 7805 as it is probably being degraded when the input drops below the output as you've described. You can put a 5.1V zener diode in parallel to the load to help regulate but you should really be using the feedback loop to get the proper voltage.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
But Scotchtapelord, the problem isn't the output voltage; It's fine, it's just the ripple that's being the issue. For some reason that causes the ipod to "think" it's charging, when in actuality it's doing quite the opposite.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Ripple is fluctuation of output voltage, so it is your problem. I don't see how the design could be sinking current from what you are charging, assuming you followed the asynchronous designs in the datasheet. What is the unloaded output voltage?
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
I think I figured out the problem; It's going into discontinuous mode, because I guess the ipod touch is too light of a load for the regulator... I need a bigger inductor, which apparently will fix the issue.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Forty - The input-output voltage relationship changes when moving into discontinuous mode, and it can be difficult for simple controllers to handle both cases.
Inducktion, if you have this capability with your chips, you can try increasing the switching frequency which will tend to push you back into continuous mode. However, when the ipod is done charging the current will drop and it may go discontinuous again so you may wish to add a minimum load, or perhaps current sensing with a load that will switch on when you want to maintain voltage under low load conditions.
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Boost Converters' issues
