5V to 3.3V + 1.2V  or  12V to 3.3V to 1.2V ? - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: May 19, 2011, 01:48:31 pm
content: I need to power an FPGA (Cyclone IV) which has 1.2V for VCCINT, 2.5V for PLL and analog and I'm going to use 3.3V for all I/Os. I want to use the LM2833Z for the 3.3V and 1.2V rail but it accepts a maximum input voltage of 5.5V. I was planning on powering it from 12V since 12V adapters are dime a dozen but that's too much for the LM2833Z. I believe the 2.5V will not be loaded too high so I will power that from a linear regulator from the 3.3V rail. So I was thinking there are some options:1. Get a 5V adapter instead and run both the 1.2V and the 3.3V converters from this input voltage directly.2. Get another chip for the 3.3V rail that can handle 12V and run the 1.2V converter from the 3.3V rail.3. Put down a 5V converter on the board for powering both the 1.2V and 3.3V converter. Seem unnecessary though.With option 2, the sum of the 1.2V and 3.3V rail will be limited to what the 3.3V converter can deliver (I was thinking of something like the LM2676).

author: Psi
author_rank: Super Contributor
timestamp: May 19, 2011, 03:02:59 pm
content: 5v adaptors are almost as common as 12v adaptors.Its probably the easiest option.

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 19, 2011, 03:09:14 pm
content: Yeah, unless there is a good reason to have a 12v source (such as an automotive application), go with native 5V.Otherwise, you are putting a switching DC to DC converter to get down to 5ish volts or losing 7V x Current Draw of Project in power, for a linear 12v to 5v conversion. Neither are a great option. One can introduce a good bit of noise and the other a good bit of heat.If you find some 6V adapters, you can poor man them down with some forward biased diodes dropping 0.6V a piece.

author: shadewind
author_rank: Frequent Contributor
timestamp: May 19, 2011, 03:31:59 pm
content: You're probably right. There are converters that can do 3.3V from 12V though (most cannot do 1.2V from 12V apparently) but that seems a bit convoluted as well.

author: deephaven
author_rank: Frequent Contributor
timestamp: May 20, 2011, 03:43:44 pm
content: I use an LM26400 which provides 1.2V and 3.3V and a linear reg off the 3.3V for 2.5V. This works well with either a 12V or 5V input.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 20, 2011, 05:14:38 pm
content: coincidentally, am studying a circuit with HT73XX on it. 1.8V,...,3.3V,...,5V from 12V supply, quite small in size and capacity though, 250mA, not sure if its enuf for your fpga.http://www.datasheetarchive.com/HT7333-A-datasheet.html

author: shadewind
author_rank: Frequent Contributor
timestamp: May 20, 2011, 07:16:33 pm
content: Quote from: deephaven on May 20, 2011, 03:43:44 pmSince this is an FPGA dev board for personal use, it's hard to know the power consumption beforehand. When using Altera's Early Power Estimator tool, I calculated the theoretical maximum current draw (max resource utilization with maximum toggle rate on the maximum clock frequency) on the 1.2V rail to be approximately 3A, that's why I chose the LM2833Z. But maybe it's foolish to use such an extreme maximum? I don't want to limit myself in what I can do (I am a beginner in this field) but maybe I'll never hit that limit?