Playing with IC’s: Part 2
In my previous article, “Playing with IC’s”, I was checking the Fiti #FR9882 IC to see if it may have been the cause of the Cisco router quitting on us. The IC is an 18V 2A 340KHz Synchronous Step-Down DC/DC Converter (SOP-8). For a 3.3V output, the input can be between 5 – 12V. Even when I used a 4.5V input, the IC had an output of a tad over 4V. And when I raised the voltage up to 12V, the IC blew. According to the datasheet the IC should handle 18V and currents up to 2A. My Power Supply was set to a 0.5A max to limit the current.
Here is the Datasheet:
So I bought some new IC’s from a Chinese company on AliExpress and they finally got here. They were not mailed in ESD protective packaging or with any ESD protection at all, so I don’t know if they are even good IC’s. They were just thrown into a small plain plastic baggie. I don’t know if I’ll ever buy anything from them again.
But we will see if any of the IC’s are good. According to the Datasheet, the IC should output 3.3V with an input of between 5 – 12V when set up as the top schematic on page 2. My breadboard is set up to the top schematic, and now I also have a 26.1K 1% resistor for R1 that the schematic calls for. So I don’t have to use the 82K & 39K resistors in parallel like I did in the first article. Everything else is the same as the first article – except the IC, of course.
I believe I’m just wasting my time with these IC’s, but I will go ahead and try them in the hopes that at least one of them survived.
As you can see in the photo, the 26.1K 1% Resistor is well within specs.
Here are a couple photos of me setting up the breadboard.
The new IC is on the breadboard. It would be better to put the circuit on a proper PCB, but the breadboard should be ok to test the IC.
This time I’m going to start off with a 5V input and limit the max current to 0.5A. Since we are only lighting an LED in this circuit, 0.5A should be good enough for the IC to work properly and keep the current lower in case of a short or some other problem. Knowing how the IC’s were shipped to me, there is no telling what will happen when I apply power to the IC. According to the schematic in the Datasheet, the IC should output 3.3V. I will be using my new Uni-T UT-139C DMM to measure the supply voltage to the input of the IC, and measure the voltage of the IC output with my Fluke 87V.
The IC was outputting about 3.4V and the LED was lit, and before I could get the camera and take a photo, it stopped working! I didn’t hear a pop or anything. I don’t know if the IC was bad, got hit with static electricity during shipment, or if I hooked something up wrong. I looked the breadboard over good and couldn’t find anything arcing. Everything was connected correctly also.
I hope that there is at least one IC in the bag that is good. Soldering all the leads to them all is just too much trouble. So I ordered some tiny breakout boards from SparkFun for the IC. It will be a lot easier to work with and less heat transfer to the IC pins. Swapping IC’s will be much easier and less stress on the pins.
The breakout board is on the breadboard and it has continuity to the pins. So, all is well – so far. I removed the switch from the circuit to make things easier.
As you can see in the photos, the breakout board is on the breadboard and the IC is only outputting around 2.28V. Raising the input voltage raises the output voltage. I decided to try another IC and see what it will output.
I changed the IC with another one and it is outputting 2.16V. I started thinking that maybe one LED was not enough load for the IC. So I added some more LED’s because I don’t have an electronic load to use.
That didn’t work either. So I started experimenting with different caps with different capacitances with not much change in output at all. I also started adding various caps for the optional caps that are marked C7 & C8 on the schematic, with no luck.
I decided to remove the IC from the breakout board and solder on another one. Because of the way the IC’s were shipped to me, it is possible that they were all damaged and I’m just wasting my time. But I have 7 left so I may as well try them and hope that one of them will work like the datasheet says it should. Of course I don’t expect them to output a perfect 3.3V on a breadboard, but it should be close enough to test.
Here is a picture of the LED board.
This is actually the 5th IC. I forgot to take a photo of the last IC, but it was only outputting about 0.8V and I removed it straight away. As you can see in this photo, the IC is only outputting about 13mA. I double checked the continuity from the breadboard away from the breakout board pins to the top of the pins of the IC’s to make sure I didn’t get any false readings, and the continuity is good. You can see it in the next photo. I double checked all connections also and everything is good – it has to be the IC’s that are the problem. I also added the 1N4148 diode, D1 in the Fig.3 schematic, and it made no difference. The datasheet says it needs to be in the circuit as an external boost diode when using an input of 5V to help improve the efficiency. I left it in the circuit even though it seemed to make no difference.
It’s not looking good folks: but usually luck would have it that the last IC might be the good one. It would be nice for this experiment if at least one of them survived shipment.
This is the 6th IC and you can see it outputting only 115mA. The boost diode is still in the circuit. Changing caps doesn’t make much difference. I also dropped the load back to one LED with not much change in the output of the IC.
This is the 7th IC. Changing caps and the aforementioned changes didn’t make enough difference to the output to matter. Of course, I rechecked continuity to the IC pins and connections, and all was good. I will check the last 3 IC’s after I have gotten some rest.
I’ll just skip to the last IC. As you can see it is not working right either. I believe the IC’s are damaged. If the problem was just that the IC’s were on a breadboard instead of a PCB, you would think their outputs would at least be consistent. But they were all over the place – some outputting mA’s, some around 1V, and a few about 4V.
I started turning up the input voltage to see what the last IC would do and the output started rising. Then it shorted around 10V and the Amps on the power supply jumped up to the max setting of 0.48A as you can see in the next photo.
After I took this photo the IC was getting hot, so I removed the power from the circuit and decided to end the article. Sometimes things just don’t work out. If the shipper would have protected the IC’s in ESD packaging, It would have made for a better article – that’s what I believe anyway.
Robert Calk is a hobbyist from the USA that loves learning electronics and device repair.
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