Lt1576

Hey,

I am using the LT1576 dc-dc buck converter to step down a 9 volts dc to 5v. I used the application circuit below.
My inductor value is 75uh and I used the same components in the application circuit.
I have a problem when I connect a 5ohm 5 watts resistor as a load. The input voltage will drop to 6v and the output voltage will drop to 3v. When I used a 125ohm and 10watts resistor there is no problem.

Thanks!

Are you using a 9V battery for your input voltage? If so, it cannot handle that much current.

Under a 5 Ohm load, your regulator is trying to source 1A, outputting 5 Watts. Assuming an efficiency of 90%, which is generous, the 9V source must provide 5W/0.9 = 5.56W. This means it would need to provide about 0.62A, which is well above what a 9V battery is capable of sourcing without a significant drop in output voltage. The internal resistance will cause a large voltage drop on the output contacts and will give you results like that.

I am using a power supply to get my 9volts input.

What capacitor value are you using on the input? Do you happen to know what the ripple current of that capacitor is?

I’m seeing the datasheet suggest the ripple current rating for that cap needs to be

RIPPLE CURRENT RATING ≥ IOUT/2

Which in your case if you’re outputting 1A would be 500mA.

If you’re seeing the input voltage to the circuit droop, that’s an indication that there’s some sort of unfavorable interaction between the circuit and whatever is supplying it. Any sort of current limiting behavior on the part of the supply would be a likely cause. Another possibility may be some sort of interaction between the control mechanisms of the supply and the circuit, which could be addressed with suitable filtering between the two.

I am using a 10uf capacitor as input capacitor

I was able to figure out one my problem. My power dc- power supply can output 0.5 A. I used a different power supply that can output 3A. Now the problem is my output voltage is not 5volts. The output voltage is 4.6 volts. I think the control circuit in the voltage regulator will keep the output voltage at 5volts.

I
was able to figure out one my problem. My power dc- power supply can output 0.5 A. I used a different power supply that can output 3A. Now the problem is my output voltage is not 5volts. The output voltage is 4.6 volts. I think the control circuit in the voltage regulator will keep the output voltage at 5volts.
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Does the output voltage drop as the load current increases, or is it always 4.6V? If it is always 4.6V, then I would look carefully at your feedback resistor values.

If it drops as load current increases, then it is probably due to some other components and/or your layout. You need to use the right components to get good results. It might be enlightening to see the output on an oscilloscope to see what the waveform looks like.

You should carefully read the application section (pages 8-23) in the datasheet for how to calculate the required specifications for the inductor, as well as the input and output caps. It requires more than just selecting the right inductance and capacitance values – specs like saturation current, ripple current, ESR, component materials, etc. all have an impact. Also, the layout recommendation shown on page 16 should be followed for best results.