using bus switch instead of voltage regulator?

[[email protected]]

Hello,

I need to get 3.3v from a 5v input to power an FPGA. Since I'm already
using bus switches to convert the input lines from 5v to 3.3v, I was
thinking about doing to same for VCC and not using a voltage regulator
in the design. These are the parts I use:

http://www.idt.com/?genID=QS3125

According to the datasheet, the bus switch can put out 120mA max. My
FPGA is a LatticeXP LFXP3 which uses 90mA max on startup:

http://www.latticesemi.com/lit/docs/datasheets/fpga/xp_data_sheet.pdf

It seems that I could use these bus switches as voltage regulators if I
wanted to. Is it a good idea though?

 

[John Larkin]

Hello,

I need to get 3.3v from a 5v input to power an FPGA. Since I'm already
using bus switches to convert the input lines from 5v to 3.3v, I was
thinking about doing to same for VCC and not using a voltage regulator
in the design. These are the parts I use:

http://www.idt.com/?genID=QS3125

According to the datasheet, the bus switch can put out 120mA max. My
FPGA is a LatticeXP LFXP3 which uses 90mA max on startup:

http://www.latticesemi.com/lit/docs/datasheets/fpga/xp_data_sheet.pdf

It seems that I could use these bus switches as voltage regulators if I
wanted to. Is it a good idea though?

Bad idea! The bus switch would be a very bad regulator, equivalent to
a fet source follower with drain and gate both at +5.

Use a real voltage regulator.

John

 

[[email protected]]

Absolutely not. Use a nice little switching regulator module from TI,
like the PT5100 series. Very efficient.

 

[Mac]

Absolutely not. Use a nice little switching regulator module from TI,
like the PT5100 series. Very efficient.

Why don't you quote the post you are replying to?

And why recommend a switching regulator for a 90 mA max load?

The LM1117 might be a good choice.

If the average current is expected to be steady, the OP might consider
a shunt regulator like the TL431.

--Mac

 

[John Larkin]

Why don't you quote the post you are replying to?

And why recommend a switching regulator for a 90 mA max load?

The LM1117 might be a good choice.

The 1117 is a great little "MDO" regulator, cheap and very rugged. I
tried to destroy one by applying illegal potentials to various leads
and it didn't mind at all.

It does need a big (22uF or so) output cap to be stable.

We use them for FPGA supplies a lot, switchers rarely.

John

 

[JeffM]

The 1117 is a great little "MDO" regulator, cheap and very rugged.

I tried to destroy one by applying illegal potentials to various leads
and it didn't mind at all.
John Larkin

Do you get the idea that every time a new component
becomes a candidate for use in a product
that Larkin rushes down to that lab to make sure he's in on this?
http://groups.google.com/group/sci....x-their-rated-abs-max-gate-voltage+power-fets

I once saw a job ad for a "Non-Destructive Tester".
Think they would have hired John?

 

[John Larkin]

Do you get the idea that every time a new component
becomes a candidate for use in a product
that Larkin rushes down to that lab to make sure he's in on this?
http://groups.google.com/group/sci....x-their-rated-abs-max-gate-voltage+power-fets

I once saw a job ad for a "Non-Destructive Tester".
Think they would have hired John?

I did an 8-channel load cell/strain gauge signal conditioner...

http://www.highlandtechnology.com/DSS/V385DS.html

that needs a separate remote-sense power supply for each load cell. I
used an LM1117 with its adjust node driven by an opamp, basically
using it as a power amplifier. Under sundry fault conditions, the
various pins could be forced into all sorts of unnatural polarities
and levels, so I tried abusing an 1117 to see if any of the feasible
states might break it. None did.

But right, I do like to torture tiny, defensless parts. *Electronic*
parts, I mean.

John

 

[Mac]

The 1117 is a great little "MDO" regulator, cheap and very rugged. I
tried to destroy one by applying illegal potentials to various leads
and it didn't mind at all.

Heh. "MDO." I like that. It IS a bit of a stretch to call it an LDO, but
it is better in that regard than an LM117.

It does need a big (22uF or so) output cap to be stable.

Yeah.

We use them for FPGA supplies a lot, switchers rarely.

I used a switcher for an FPGA, but only for the 3.3V i/o banks (The
external supply is 5V). For the core voltage I used an LDO, with the 3.3V
as an input.

The total power consumption at 5V is almost an amp, so I think the
switcher was a good idea.

John

--Mac