Author Topic: 1.8 volts 2.5 volts what for ?  (Read 3047 times)

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Offline ez24

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1.8 volts 2.5 volts what for ?
« on: May 29, 2015, 08:16:19 pm »
Hi

There is a topic on this forum and one comment on it was "it does not have 1.8 volts".  In Art of Electronics Ed 2 there is a statement regarding what a power supply should have:  "2.5 volts" (among other voltages)

Why do people think 1.8 and 2.5 volts are necessary in a power supply and what are they used for  ie what kind of circuit or components?

My ps will have an adjustable voltage and fix 3.3 v, 5v and 12v.  Just wondering if I should also have fixed 1.8 and/or 2.5 volts or another adjustable voltage.

thanks


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Offline suicidaleggroll

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Re: 1.8 volts 2.5 volts what for ?
« Reply #1 on: May 29, 2015, 08:21:54 pm »
1.8 is becoming increasingly common with new digital logic devices.  It started with 5, moved to 3.3, and is in the process of moving to 1.8.  I'm not sure what the draw of 2.5 is.  About 75% of my designs use 3.3v, about 20% use 1.8v, and the remaining 5% are 5v.
« Last Edit: May 29, 2015, 08:23:40 pm by suicidaleggroll »
 

Offline c4757p

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Re: 1.8 volts 2.5 volts what for ?
« Reply #2 on: May 29, 2015, 08:31:27 pm »
You're not going to be breadboarding 1.8V parts, though, no need for an external power supply to supply that. Just stick a regulator on the board. Reduces the risk of accidentally blowing something up, too.
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Offline bobcat

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Re: 1.8 volts 2.5 volts what for ?
« Reply #3 on: May 29, 2015, 08:55:39 pm »
To reduce power consumption, often complex devices will have a core that operates at 1.8 or 2.2 or 2.5 volts. The I/O operates at 3.3v or 5V. Two power supplies are required for operation. Usually it is best to provide a local (on board) regulator for the core voltage. Some chips even have a core voltage regulator built in.
 

Online mariush

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Re: 1.8 volts 2.5 volts what for ?
« Reply #4 on: May 29, 2015, 09:20:06 pm »
SDRAM works at 3.3v
DDR 1 RAM works at 2.5v
DDR 2 RAM works at 1.8v (default JEDEC voltage) but some manufacturers at the beginning used up to 2.1v
DDR 3 RAM works at 1.5v (default JEDEC voltage) but up to 1.65v was often used (Intel chipsets and processors had trouble handling more than that so most manufacturers restricted themselves to using maximum that value)

A lot of 8bit microcontrollers (think PIC, attiny etc) work fine at 3.3-3.6v with all functions enabled, can work with 2.5v at reduced frequencies (but still all features) and can work at 1.8v but potentially with some features inactive.

SD cards (to the best of my knowledge) work with voltages between 2.0v and 3.6v (or at least some spec says, other documents online say 1.1v-3.6v) so for example if you want to load some code from a SD card or do some data logging to a SD card, you may want to simplify a design and use same voltage for both a microcontroller and a SD card (so you'd use 2.5v , 2.8v, 3.3v)

Otherwise, what everyone else said.

 

Online Howardlong

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Re: 1.8 volts 2.5 volts what for ?
« Reply #5 on: May 29, 2015, 09:32:02 pm »
It's not at all unusual, even five years ago I did a USB stick design with 1.2, 1.8, 3.3 and 5V domains, and there were actually two 3.3V domains as it was mixed signal. Five regulators in one USB stick!

Thankfully chips with multiple power domains are increasingly incorporating on chip regulators, but often they're linear so for battery powered stuff it's sometimes necessary to provide an SMPS.

For battery powered stuff, prolonging battery life is a key product differentiator so as part of the design, innevitably lower voltage devices are often chosen.

Frequently particularly on modern digital devices the operating voltage range is fairly wide, so operating at a lower voltage than, say, 3.3V, is part of the give and take of the engineering design. For example I'm working on an ARM Cortex M4 based design at 2.5V simply because that's the lowest voltage I can get away with with all the devices on that power domain, and therfore the lowest power consumption and longest battery life. If it was a USB bus powered device, I'd almost certainly be running it at 3.3V.
 

Offline Richard Crowley

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Re: 1.8 volts 2.5 volts what for ?
« Reply #6 on: May 29, 2015, 09:45:00 pm »
To reduce power consumption, often complex devices will have a core that operates at 1.8 or 2.2 or 2.5 volts.
Lower operating voltage also enables higher clock speeds. It takes a lot less time to drag a node up and down between logic levels at 1.8V than at 3.3V or 5V

The original question makes almost no sense without some kind of context.
WHY should a power supply "have: 2.5 volts (among other voltages)"
It is almost a complete non-sequitur.
 

Offline con-f-use

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Re: 1.8 volts 2.5 volts what for ?
« Reply #7 on: May 29, 2015, 10:39:24 pm »
The original question makes almost no sense without some kind of context.
WHY should a power supply "have: 2.5 volts (among other voltages)"
It is almost a complete non-sequitur.
And the question should answer itself: "Because there are many chips that use it."
 

Offline SL4P

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Re: 1.8 volts 2.5 volts what for ?
« Reply #8 on: May 30, 2015, 03:06:57 am »
To reduce power consumption, often complex devices will have a core that operates at 1.8 or 2.2 or 2.5 volts.
Lower operating voltage also enables higher clock speeds. It takes a lot less time to drag a node up and down between logic levels at 1.8V than at 3.3V or 5V

The original question makes almost no sense without some kind of context.
WHY should a power supply "have: 2.5 volts (among other voltages)"
It is almost a complete non-sequitur.
Thanks Richard - You beat me to it!
I was wondering when someone was going to bring that in... the primary reason for lower supplies appearing... slew rate!
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Offline LaurentR

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Re: 1.8 volts 2.5 volts what for ?
« Reply #9 on: June 02, 2015, 12:57:09 am »
Modern fabrication processes use core voltages <1V.
With special thick oxide I/O transistors and a bit of trickery you can do 1.8V and 2.5V I/O, but anything higher is complicated.

http://www.tsmc.com/english/dedicatedFoundry/technology/28nm.htm
 

Offline timb

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Re: 1.8 volts 2.5 volts what for ?
« Reply #10 on: June 02, 2015, 01:02:07 am »

It's not at all unusual, even five years ago I did a USB stick design with 1.2, 1.8, 3.3 and 5V domains, and there were actually two 3.3V domains as it was mixed signal. Five regulators in one USB stick!

Thankfully chips with multiple power domains are increasingly incorporating on chip regulators, but often they're linear so for battery powered stuff it's sometimes necessary to provide an SMPS.

For battery powered stuff, prolonging battery life is a key product differentiator so as part of the design, innevitably lower voltage devices are often chosen.

Frequently particularly on modern digital devices the operating voltage range is fairly wide, so operating at a lower voltage than, say, 3.3V, is part of the give and take of the engineering design. For example I'm working on an ARM Cortex M4 based design at 2.5V simply because that's the lowest voltage I can get away with with all the devices on that power domain, and therfore the lowest power consumption and longest battery life. If it was a USB bus powered device, I'd almost certainly be running it at 3.3V.

The new TI MSP432 has a built-in LDO and DC-DC, giving you the choice of a smaller footprint (no need for an inductor) or better power management.


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