Hey all, new here and new to designing and making PCB’s. I am hoping you will be able to help me out.
I am trying to make a boost converter to take the output from a LI-Poly 3.7V nominal voltage to a 5V output. I am using a Maxim Max1709 IC for the boost conversion. I have drawn the schematic (based off of the manufacturer’s example circuit) and the PCB layout using DipTrace. Moved all the traces around to make them nice and pretty and then used the laser printer toner transfer method to etch the PCB.
After getting all the components soldered to the board, I have continuity across my battery connector which looks to be tripping the short circuit protection on the battery protection circuit. So I am not getting any output. I have made three different boards and the only one I have had some sort of luck with was the first one. On that board I actually go output voltage for a while and then everything when dead. The first board was not designed using DipTrace. I designed it by hand.
Please see the attached images of my schematic and PCB layout. Electronics is not my forte, so I must be screwing up something simple. The component called “Atom” is where the load will be applied. The M1 component is a Soic-8 Mosfet that I am hoping to use as a touch switch.
Any and all help/comments would be greatly appreciated. Thanks in advance.
One thing that immediately stands out is that you’ve used fairly narrow traces. In any SMPS there are going to circuit paths that carry a lot of current. These should be pretty wide and as short as possible. I’m not sure that’s your problem but it definitely IS a problem.
Are you sure that’s a max1709? The data sheet shows it in a 28 pin package and it has an internal 10A MOSFET.
My traces are 0.75mm wide. I used a trace calculator that said that should be good for 2 amps. My application is going to be pulling about 1.5 amps at the 5V.
Maxim makes a 16 pin version also. Below is the schematic from Maxim that I was basing my design off of.
It’s not just DC current you need to watch for. There’s a lot of AC switching going on. You are trying to reduce the size and inductance of high-current switching loops. You also need to keep that current off of sensitive pins like the feedback pin. Look at the Maxim data sheet, especially the section on page 12 - Layout Considerations.
Based on your layout, I would never have guessed that you followed the datasheet instructions on trace width and length. In particular, the inductor is connected to the chip through a rather circuitous route of 3 traces/2 vias. It should be jammed right up against the chip. Watch your return ground paths as well - they need the same treatment. I’d tear it up and start over. If you need to use vias, more than one per layer transition is common.
The eval kit layout is pretty hardcore about the DS instructions. Length is just as important as width. Many datasheets show the high current paths which help me visualize the layout. Note also that the large copper areas are for dissipating heat as well as carrying current.
By the way, a 1A SMPS will have some current paths that are much higher than 1A. It makes sense - duty cycle comes into play. This is why, for example, the inductor is usually spec’d higher than the power supply current.
