Using an Arduino Mega GPIO pin to enable MAX1771 DC/DC converter

Question

TL;DR: DC/DC booster does not work, and MOSFET seems stuck.

I have a 12V fan that I wish to power with a 7.4V battery via a MAX1771 DC/DC converter chip (datasheet in hyperlink). The fan is rated at 0.06A.

I am utilising the 12v preset output, bootstrapped circuit to power my fan. My SHDN pin is connected to a GPIO pin on the Arduino Mega such that when I write digital HIGH, it is 'off', and when I write digital LOW, it is 'on'. The Arduino is also powered by the 7.4 V battery and shares common ground with the MAX1771.

The issues I am facing are these:

1. I implemented the example circuit in the datasheet (Fig 2a). I followed the datasheet as closely as I could, with the exception of Rsense (I soldered 30 1.2 ohm resistors in parallel to give 0.04 ohms), and using 3 100uF aluminium electrolyte capacitors in parallel to give 300uF for C4, and the substituting the MOSFET for a BUK9575. Im afraid I cannot remember the names of the other types of capacitors/inductors, but their values are the same, and are polarised as per the datasheet specifications.

   The first time I put everything together, it seemed to work fine, perhaps a little loss (output was approx 11.68V). But once I shifted all the components over to a new breadboard (yeah it was breadboarded for display purposes) and trimmed the components so they fit snugly in the breadboard (trying to be neat and tidy here) without any extra long leads, the converter suddenly loses its functionality, so the output is now at 6.8 volts instead of the previous 11.68 volts.

2. Failing that, I decided to see if regardless if the converter was not able to step up my voltage, I figured if I could at least enable/disable the converter all wouldn't be lost. So I created a code in Arduino 1.6.9 IDE that tells my GPIO pin thats connected to the SHDN pin on the MAX1771 to go LOW for one second, and HIGH for another second continously. Tested the code out on an LED and it blinks fine. However, doing this for the MAX1771 causes it to stay permanently on. The MTD was a MOSFET recommended by datasheet. I substituted it with one I had on hand for testing

So these are my 2 issues. I would like to ask what could be the cause of what made my converter unable to boost, and why my arduino is unable to disable the max1771.

Research: Ceramic and tantalum capacitors were recommended by the datasheet, but since the circuit worked with regular ceramic and aluminium electrolyte capacitors, I figured its not such a big deal.

Also, I figure maybe its because my arduino's HIGH and LOW signals aren't clean enough. Should I add additional caps to filter the Arduino's GPIO?

Is my MOSFET sensitive enough to changes in gate voltage? I hope I did not blow it by accident.

Answer 1

Firstly the shut down operation - the MAX1771 is a boost converter and when "off", the MOSFET is also off and so the incoming voltage is transferred through to the output via the inductor and 1N5819 diode. So you will see a little less than the incoming supply at the output. It's not a shutdown feature as you expect - i.e. it doesn't turn the output off; it just turns the chip off.

Regarding the failure to boost (given that it once did), either you have wired the new circuit up incorrectly or you have unwittingly destroyed the chip or MOSFET.