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Trying to power an AR9331 router using a power supply have made Answered


I am trying to power an AR9331 router using a power supply I have made. The AR9331 requires a 3.3V supply. My power supply takes 12V as input, then converts it into 5V using a LM2576 step down voltage regulator and then a LM1117 regulator to convert 5V to 3.3V. The output is at the desired 3.3V level and is ripple free. I have verified this using a scope. The problem is that if I use this power supply the AR9331 boots and then hangs. If I try booting the same AR9331 with a lab power supply set at 3.3 V it boots perfectly.

I learned about inrush current and tried to measure it using the following setup:

(see the attached 1)

Since the resistor is 1 ohm the potential across it equals the current.

The next figure shows the potential across the 1 ohm resistor:

(see the attached 2)

The inrush current is about 1A, whereas the normal operating current is 250mA.

I tried two solutions,

1. Tried a current limiting resistor between my 12V lab supply and the LM2576 input. This works. The router boots perfectly.

2. Tried lowering the lab supply to 9V and connecting it to the LM2576 input. without any current limiting resistor. This also works. I suspect the problem is inrush current. Any suggestions?

One interesting observation is that if I power my power supply with 9V or even down to 5.5V the router boots just fine. This has been happening repeatedly during my experiments. This is confusing me completely since the LM2576 is designed for input voltages well above 12V (see datasheet: http://www.kynix.com/uploadfiles/pdf2286/LM2576D2T-012.pdf)



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