iCE40 Eval board 3.3v->2.5v via diode dropout, or is it clamped internally? - Page 1
Forum section: electronics / projects

author: millerb
author_rank: Regular Contributor
timestamp: May 24, 2013, 09:01:55 am
content: The iCE40LP1K's non-volatile config memory block has a 2.5V requirement. The evaluation board for the LP1K supplies this via feeding 3.3V through a schottky diode. With my meter I measure 3.3V at the anode of this diode, and 2.5V at the cathode. Schematic is below:This diode has a forward of 0.36V @ 100mA. I measure 0.21V on my meter which specifies its test current is ~0.8mA.Absolute maximum rating for NVCM is 3.6V. Recommended operating voltage is 2.3V - 3.46V except for one case which I will never use, and I don't believe is an option for this board anyway (programming NVCM), and in that case the max recommended is 3.0V. Idle current draw isn't specified in the documentation, but the holes in the table specify units of microamps. Peak startup supply current is 7.7mA.I don't understand why I'm reading 2.5V at the cathode of this diode. An 800mV drop doesn't make any sense. Is there something I don't know about diodes, or perhaps something I've missed? Only thing I can think of is that it's internally clamped. Is that a common thing? I'd expect to read like 3V or 3.1V, or somewhere in that neighborhood with this circuit. I'm on the fence about whether or not I should save a few cents and stuff a diode with a better drop in my design to get close to the nominal value or play it safe and plop an LDO in there just for that one thing.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 24, 2013, 11:34:02 am
content: I wonder if the part shown on the schematic is actually what's fitted - a standard diode would be a better fit for the required drop

author: millerb
author_rank: Regular Contributor
timestamp: May 24, 2013, 09:00:03 pm
content: Quote from: mikeselectricstuff on May 24, 2013, 11:34:02 amTurns out I was getting a bad reading. Not enough space to get a good connection on either side of this tiny little diode with the meter probe. I took a look at the board design file and there are two plated vias where you can easily probe on the top of the board.With a better meter, I'm measuring 3.271V at the anode and 3.192V at the cathode. The mechanical dimensions of the diode match the specs in the data sheet. So I'm getting ~80mV drop. This probably is the part listed in the BOM.Still at a loss as to why they choose this diode. There is no shortage of diodes that would get you closer to the nominal value of 2.5V for less money and with the same dimensions. It's within spec, but why would someone not try to get closer to the nominal value?All that being said, this package is annoying.If it weren't for that giant heat pad being so close to the inner row, I'd have a shot at wiring it up and doing some tests. It might still be possible, but I've decided to ditch this part and use the 144 pin TQFP HX4K device.The LP1K 84QFN had the right price, features and number of IO for my project, but the lack of PLL was a little disappointing.These iCE40 devices seem to fill a gap between the CPLD and FPGA offerings from Altera and Xilinx. Too bad the packages are so difficult to work with.Take a look at the eval board for the LP1K-84QFN:Is there any way you could get the cost down to a reasonable price for a low volume run with a setup like this? Filled and capped vias, 4 layer board. Traces are 8mils. 9 mil vias with 4mil annular ring.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 24, 2013, 09:55:47 pm
content: Quote from: millerb on May 24, 2013, 09:00:03 pmThat's just nasty - all the unreworkability of a BGA with added large pad to float it off the board if there's too much paste.