OPA551 Op Amp w/Single Supply?

[Rileyesi]

I would like to use the Texas Instruments' op amp OPA551 (Burr Brown, actually)
with a single supply rather than the dual that seems to be called for in the
data sheet.

Here is a link to the data sheet http://focus.ti.com/lit/ds/symlink/opa551.pdf

However, I am using the op amp in unity gain to boost the current capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is 200 mA).
The reference voltage is a steady 5 VDC. The OPA551 data sheet says that the
signal must be at least 2 volts from the rail. I could not find any specifics
in the data sheet, but couldn't I use a 0-12 VDC supply and deliver a 5 volt
output? That would make the output at least 5 volts from the supply rail.

Makes sense to me...but I just want to be sure. (Worked well in bench testing,
by the way). I tried contacting TI directly, but with no answer as of yet.

As a side note, I am also using the OPA547 that has a capacity of 500 mA
continuous output for another application. That data sheet does address the
use of a single supply (i.e. you can use one), but the chip is overkill for my
needs and is about twice the cost of the OPA551.

Thanks.

 

[Tim Wescott]

Rileyesi wrote:

I would like to use the Texas Instruments' op amp OPA551 (Burr Brown, actually)
with a single supply rather than the dual that seems to be called for in the
data sheet.

Here is a link to the data sheet http://focus.ti.com/lit/ds/symlink/opa551.pdf

However, I am using the op amp in unity gain to boost the current capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is 200 mA).
The reference voltage is a steady 5 VDC. The OPA551 data sheet says that the
signal must be at least 2 volts from the rail. I could not find any specifics
in the data sheet, but couldn't I use a 0-12 VDC supply and deliver a 5 volt
output? That would make the output at least 5 volts from the supply rail.

Makes sense to me...but I just want to be sure. (Worked well in bench testing,
by the way). I tried contacting TI directly, but with no answer as of yet.

As a side note, I am also using the OPA547 that has a capacity of 500 mA
continuous output for another application. That data sheet does address the
use of a single supply (i.e. you can use one), but the chip is overkill for my
needs and is about twice the cost of the OPA551.

Thanks.

Operational amplifiers in general don't know ground from Adam. They
have their common-mode input range that you can't violate without weird
things happening, and they have their overhead voltages beyond which
they won't drive. As long as the thing is getting sufficient voltage
and you're feeding it the right input voltages it should work fine.

The only thing that makes an op-amp "single-supply" is that it will take
common-mode to one rail or another and will drive close to the same rail.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Winfield Hill]

Tim Wescott wrote...

Rileyesi wrote:
I would like to use the Texas Instruments' op amp OPA551 (Burr Brown,
actually) with a single supply rather than the dual that seems to be
called for in the data sheet.

http://focus.ti.com/lit/ds/symlink/opa551.pdf

I am using the op amp in unity gain to boost the current capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is
200 mA). The reference voltage is a steady 5 VDC. The OPA551 data
sheet says that the signal must be at least 2 volts from the rail. ...

Operational amplifiers in general don't know ground from Adam. ...

Right. Rileyesi, you may run into a bit of trouble pulling 150mA out
of the OPA551, dropping 12V to 5V, creating 1W of heat dissipation.
The junction temp will increase by an unsafe 100C over nearby warmed-up
air in the miniDIP package, and the SO-8 is little better. The DDPak-7
version (OPA551FA) may work, but only if you have a rather large PCB
heat-sink area, and only if you can find them stocked someplace.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Fred Bloggs]

Rileyesi wrote:

However, I am using the op amp in unity gain to boost the current capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is 200 mA).
The reference voltage is a steady 5 VDC.

That's called a power supply, you friggin MORON-

 

[Tim Wescott]

Winfield Hill wrote:

Tim Wescott wrote...

Rileyesi wrote:

I would like to use the Texas Instruments' op amp OPA551 (Burr Brown,
actually) with a single supply rather than the dual that seems to be
called for in the data sheet.

http://focus.ti.com/lit/ds/symlink/opa551.pdf

I am using the op amp in unity gain to boost the current capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is
200 mA). The reference voltage is a steady 5 VDC. The OPA551 data
sheet says that the signal must be at least 2 volts from the rail. ...

Operational amplifiers in general don't know ground from Adam. ...


Right. Rileyesi, you may run into a bit of trouble pulling 150mA out
of the OPA551, dropping 12V to 5V, creating 1W of heat dissipation.
The junction temp will increase by an unsafe 100C over nearby warmed-up
air in the miniDIP package, and the SO-8 is little better. The DDPak-7
version (OPA551FA) may work, but only if you have a rather large PCB
heat-sink area, and only if you can find them stocked someplace.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

Could you use a pass transistor following the op-amp, or do you need to
sink current as well as source it? If you use an emitter follower after
your op-amp you can significantly more current out of the thing -- using
a power MOSFET is even better except that you'll need to really reduce
your bandwidth and pay close attention to overhead.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tim Wescott]

Fred Bloggs wrote:

Rileyesi wrote:


However, I am using the op amp in unity gain to boost the current
capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is
200 mA). The reference voltage is a steady 5 VDC.


That's called a power supply, you friggin MORON-

So if I use a 78L05 instead of a high-precision reference followed by a
power amp it'll be able to sink current and source it while holding the
output steady at 5.00V, _and_ it'll be as precise as a precision reference?

Wow. I never knew.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Rileyesi]

However, I am using the op amp in unity gain to boost the current
capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is
200 mA). The reference voltage is a steady 5 VDC.


That's called a power supply, you friggin MORON-


So if I use a 78L05 instead of a high-precision reference followed by a
power amp it'll be able to sink current and source it while holding the
output steady at 5.00V, _and_ it'll be as precise as a precision reference?

Wow. I never knew.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Thanks, Tim. Your reply was ALOT more polite than what I had in mind!

By the way, you hit my problem right on the head. I can't rely on the
precision of an off the shelf power supply for my application.

 

[Adam Seychell]

Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<c87isi0s2v@drn.newsguy.com>...

of the OPA551, dropping 12V to 5V, creating 1W of heat dissipation.
The junction temp will increase by an unsafe 100C over nearby warmed-up
air in the miniDIP package, and the SO-8 is little better. The DDPak-7
version (OPA551FA) may work, but only if you have a rather large PCB
heat-sink area, and only if you can find them stocked someplace.

Thanks,
- Win

The SO-8 and miniDIP packages of the OPA551/2 are not capable of
getting useful current & voltage capabilities out of this IC. I wonder
why manufactures sometimes design IC with unrealistic power
capabilities that are severely limited by package dissipation. The
DDPak-7 will get you into safe temperatures provided its heat sinked
in a non-standard way. How does one clamp aluminum heatsinks to a SMD
device like this ?

Adam

 

[Tim Wescott]

Rileyesi wrote:

However, I am using the op amp in unity gain to boost the current
capacity of a
voltage reference to about 150 mA max (the capacity of the op amp is
200 mA). The reference voltage is a steady 5 VDC.


That's called a power supply, you friggin MORON-


So if I use a 78L05 instead of a high-precision reference followed by a
power amp it'll be able to sink current and source it while holding the
output steady at 5.00V, _and_ it'll be as precise as a precision reference?

Wow. I never knew.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com




Thanks, Tim. Your reply was ALOT more polite than what I had in mind!

By the way, you hit my problem right on the head. I can't rely on the
precision of an off the shelf power supply for my application.

Did you see my reply to Win's post? It's quite common to use an emitter
follower after an op-amp if you just need a 1-quadrant regulator.
Emitter followers have very good frequency response and a voltage gain
slightly less than one, so it's hard to upset the stability of your op
amp with one. Furthermore with the right pass transistor you can use a
fairly wimpy op-amp and still get 200mA out of your pass transistor.
Zetex advertises some transistors with very nice specs, but you could
probably get by with just about anything that'll dissipate the power.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Rileyesi]

Did you see my reply to Win's post? It's quite common to use an emitter
follower after an op-amp if you just need a 1-quadrant regulator.
Emitter followers have very good frequency response and a voltage gain
slightly less than one, so it's hard to upset the stability of your op
amp with one. Furthermore with the right pass transistor you can use a
fairly wimpy op-amp and still get 200mA out of your pass transistor.
Zetex advertises some transistors with very nice specs, but you could
probably get by with just about anything that'll dissipate the power.

Tim,

Yes I did see your reply and I think that is what I am doing currently. I got
the circuit out of the datasheet for the reference (LT1460-5 by Linear
Technologies go to

http://www.linear.com/pdf/14605fs.pdf

for the datasheet). I am using the circuit listed on page 12 of 12 that shows
a boosted current output. The problem is that I'm finding in the field that I
need to deliver more than 100 mA, even though my bench testing shows otherwise.
Also, there is the little problem of two techs that reversed the positive and
ground on the output of the supply (i.e. downstream of the transistor) and
cooked it! My thought is that the op-amp may be able to survive that condition
better than the transistor.

Thanks for all the advice. I;m a mechanical engineer by degree and am self
taught in electronics.