Current source design (tricky?)

[Ken Smith]

In article <d107rt011g9@drn.newsguy.com>,
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Ken Smith wrote...

Mine's even simpler:

. (O)----o-- Rs --o----S D----------(O)
. | | _|__|_
. e\| | ---- Q2
. |------' |
. /| Q1 G
. | 2N711 |
. '-------------o---- Rg ---- gnd

It only needs about 0.4V to regulate.

Hmm, maybe even less, but hey, no fair using Ge transistors.

Why not. I've got about 100 of them.

--
--
kensmith@rahul.net forging knowledge

 

[Jamie]

John Woodgate wrote:

I read in sci.electronics.design that John Larkin <jjSNIPlarkin@highTHIS
landPLEASEtechnology.XXX> wrote (in <34m631dbl759hd9g8vmhd119ku9mbap6je@
4ax.com&gt about 'Current source design (tricky?)', on Sat, 12 Mar 2005:


The National datasheet calls it Miller Effect, but that's sort of an
ancient blanket term for any sort of capacitive feedback.


Yes, and in that sense FB is right, but in the strict sense, Miller
effect occurs between output and input ports of a single active device.

So everybody is wrong. Or right.
thanks for clearing that up John.

i really didn't want to inject on to this but, i do see there
are others that fully understand what is happening here.
i find that FB has some good points about many things and from what
i can see exhibit's a very wide knowledge base! most likely from
experience, even though he does get carried
away with voicing his opinion about a couple Others here :_

That aboy FB, keep up the good work!

 

[Jamie]

Winfield Hill wrote:

Ken Smith wrote...

Mine's even simpler:

. (O)----o-- Rs --o----S D----------(O)
. | | _|__|_
. e\| | ---- Q2
. |------' |
. /| Q1 G
. | 2N711 |
. '-------------o---- Rg ---- gnd

It only needs about 0.4V to regulate.


Hmm, maybe even less, but hey, no fair using Ge transistors.


i have some brand new one's still in packages!

 

[Ken Smith]

In article <5h_Yd.50990$BX6.1154@fe06.lga>,
Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
[...]

i remember the days of using an incandescent lamp!

There used to be a "constant current" tube that was really a specially
tricked out light bulb.



--
--
kensmith@rahul.net forging knowledge

 

[Jamie]

Ken Smith wrote:

In article <5h_Yd.50990$BX6.1154@fe06.lga>,
Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
[...]

i remember the days of using an incandescent lamp!


There used to be a "constant current" tube that was really a specially
tricked out light bulb.



Yeah, well thats a new one to me.

i remember seeing some of the first
Mac solid state Hi-fi audio gear..
germanium transitor power amps with a
incandescent bulb in the bias circuit
to help get a better linear response on the
on the final stages of the amp!

 

[John Woodgate]

I read in sci.electronics.design that Jamie <jamie_5_not_valid_after_5_P
lease@charter.net> wrote (in <TH_Yd.50994$rj7.22881@fe06.lga&gt about
'Current source design (tricky?)', on Sun, 13 Mar 2005:

Ken Smith wrote:

In article <5h_Yd.50990$BX6.1154@fe06.lga>,
Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
[...]

i remember the days of using an incandescent lamp!


There used to be a "constant current" tube that was really a specially
tricked out light bulb.



Yeah, well thats a new one to me.
i remember seeing some of the first
Mac solid state Hi-fi audio gear..
germanium transitor power amps with a
incandescent bulb in the bias circuit
to help get a better linear response on the
on the final stages of the amp!



I suspect that was a form of short-circuit protection, but I've never

seen an amplifier using it.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Larkin]

On Sun, 13 Mar 2005 09:50:58 +0000 (GMT), Tony Williams
<tonyw@ledelec.demon.co.uk> wrote:

In article <d0v57j0p90@drn.newsguy.com>,
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

. simple low-drop-out current source
.
. (O)----+-- Rs --+----S D----------(O)
. | 0.1 | _|__|_
. | R1 ---- Q2
. e\| | |
. |------+ G
. /| Q1 | |
. | R2 |
. | | |
. '--------+----+--- Rg ---(O) gnd

[snip]

Replace R1 with a schottky diode, and run R2 to ground. Or buy an
opamp.

John

 

[Jim Thompson]

On 9 Mar 2005 14:47:39 -0800, fxalpha@gmail.com wrote:

Hi,

I'm trying to design a power current source. It has to have some
features that make it a bit of a challange to design (for me at
least...)
it has to do the following:

- supply about 2-3 amps (no need for programmability, it can be preset
to a value when constructed).
- have a low minimum voltage drop, of 1 Volt or less (0.5V would be
better). That is, it should work even if Vcc=12V and the load needs 11V
to keep the preset current. of course, it should also work with a
higher Vcc.
- keep a reasonably constant current for both changes in the load, and
(the bigger problem) for a changing Vcc.
- the load is returned to ground, so the source has to be on the "high"
side.
- NO need for good precision, +/-5% is more than ok. the conditions are
DC so bandwidth isn't an issue as well.


[snip]

See...

Newsgroups: alt.binaries.schematics.electronic
Subject: SED Question: Current source design (tricky?) - LowDropCS.pdf
Message-ID: <vn7931dht3mjesq77l8tg70p4qtpc6j4gn@4ax.com>

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Ken Smith]

In article <sq++cgCbCINCFwcd@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:

I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <d11rpa$m5o$2@blue.rahul.net&gt
about 'Current source design (tricky?)', on Sun, 13 Mar 2005:
In article <5h_Yd.50990$BX6.1154@fe06.lga>,
Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
[...]
i remember the days of using an incandescent lamp!

There used to be a "constant current" tube that was really a specially
tricked out light bulb.


Well, yes, for sufficiently basic differences. A 'barretter' has an iron
wire filament in a hydrogen-filled envelope.

Yes, thats the device I remember. I had to hunt high and low to find one
for a radio I was fixing.

--
--
kensmith@rahul.net forging knowledge

 

[Terry Given]

Ken Smith wrote:

In article <sq++cgCbCINCFwcd@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:

I read in sci.electronics.design that Ken Smith
kensmith@green.rahul.net> wrote (in <d11rpa$m5o$2@blue.rahul.net&gt
about 'Current source design (tricky?)', on Sun, 13 Mar 2005:

In article <5h_Yd.50990$BX6.1154@fe06.lga>,
Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
[...]

i remember the days of using an incandescent lamp!

There used to be a "constant current" tube that was really a specially
tricked out light bulb.



Well, yes, for sufficiently basic differences. A 'barretter' has an iron
wire filament in a hydrogen-filled envelope.


Yes, thats the device I remember. I had to hunt high and low to find one
for a radio I was fixing.

It kinda makes one wonder just where thermionic technology would be
today, if it had the same $$$ spent on it as Si did.

I read a neat paper a few years back in an IEEE professional comic,
wherein it was shown that Si cant beat TE at high power levels, due to
the operating temperature ranges. If only we could *actually* buy static
induction transistors, thyristors etc. *sigh*

Cheers

 

[Tony Williams]

In article <d11n0302397@drn.newsguy.com>,
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Tony Williams wrote...
Assuming the MOSFET is operating in the triode region,
what Vg-s swing is required to hold Id within <5% when
Vd-s changes by 2V (as set by the 12-14V Vs range)?

[snip]

With a big FET like the IRF4905, we're into the constant-current
"saturated" region if Vds is greater than about 500mV at Id = 2A
(this big FET is happy conducting well over 100A, if given a
little Vds to work with, so a low 2A is getting down into its
high-gain subthreshold region). We can estimate the FET's
transconductance to be about 10S at 2A and Vds, above 0.5 to 1V
(datasheet fig 1), so the FET's source-follower gain, G = gm RL
/ (1 + gm RL), would be above 0.5, which isn't too bad...

I see it. You evaded the problems of working in the triode
region by deliberately choosing the appropriate FET, even
though it looks like outrageous overkill.

Yes, going for current overkill on these low-voltage drop
c-c sources looks the obvious design choice now.
It reduces the work the amplifier has to do, which helps
in things like transient PSRR, etc.

Thanks.

--
Tony Williams.

 

[Larry Brasfield]

"Fred Bloggs" <nospam@nospam.com> wrote in
message news:4232DFB9.3020107@nospam.com...

Larry Brasfield wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:qypYd.8841$1S4.942601@news.xtra.co.nz...
Larry Brasfield wrote:
...
A number of op-amps on the market today are
very tolerant of capacitive loading because they
have a feature whereby that loading causes the
gain-bandwidth of the part to drop, almost in
proportion to the loading, such that the extra
pole remains far enough above the unity gain
crossover frequency that stability is preserved.
The LM8261 suggested by Mr. Hill is a good
example of this class.

I have been bitten quite badly by a similar "feature" in
the LM6134 (its a slew-rate modification).

The feature I mentioned above works by causing the
effective value of an internal capacitance to increase.
So it changes both the linear small-signal response
(less GBW) and the slew limiting (slower).

Nah- you're full-o-shyte! The capacitive loading decreases the effective internal capacitance due to Miller effects because it
reduces the gain.

You are the one who is full of it, Fred.

The way the feature works, typically, is by means of
a capacitor placed between the output and the "gain
node", the internal node where current sources develop
the device's voltage gain. The output buffer, under light
load conditions, bootstraps this capacitor so that it does
not much load the gain node relative to the AC grounded
integrating capacitor. When a capacitive load is present,
the increased drop across the buffer output impedance
increases the current that must be supplied to the "gain
node" for any given dV/dt. This represents an increase
in the effective capacitance loading the "gain node", and
serves to reduce the gain bandwidth while tending to
leave the excess poles unchanged.

Miller effect has very little to do with it. In fact, Miller
effect, to the small extent it occurs, acts only to slightly
move some internal excess pole(s) out a little bit when
a capacitive load is present.

You, or at least anybody capable of learning something,
can see a description of this feature's operation and a
schematic in the datasheet for Linear Technology's LT1812.
Start at:
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1009,C1022,P1838

[More blather founded on spite and ignorance cut.]
....

who has never built anything in his life.

Your ability to discern such a falsehood from such a
distance with so little evidence marks you.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Fred Bloggs]

- supply about 2-3 amps (no need for programmability, it can be preset
to a value when constructed).
- have a low minimum voltage drop, of 1 Volt or less (0.5V would be
better). That is, it should work even if Vcc=12V and the load needs 11V
to keep the preset current. of course, it should also work with a
higher Vcc.
- keep a reasonably constant current for both changes in the load, and
(the bigger problem) for a changing Vcc.
- the load is returned to ground, so the source has to be on the "high"
side.
- NO need for good precision, +/-5% is more than ok. the conditions are
DC so bandwidth isn't an issue as well.

You should be safe doing this:

View in a fixed-width font such as Courier.

..
.. -----------------REGULATOR MODULE-------------------
.. | (RM) |
.. | +----------------------+---+ |
.. | | | | PMOS | 2.5A
.. | | I-> | | +-------+ | IL
..VBATT>--------+----/\/\----+---------------------|s d|---->
.. | | Rs 100m | | | | | |
.. | / / | | | g | |
.. | 2k 1% 2k 1% | / +-------+ |
.. | / / | 2k | |
.. | \ \ | / | |
.. | | | | \ | |
.. | + +---+ | | | 220 | |
.. | | | | +-||---|---+---/\/\----+ |
.. |Vref ---/ | | | 22n | | |
.. | // \ -+ / | | | |
.. | --- 200\<-+ +---|--+ |
.. | - | TL431 / | |\ | | | |
.. | |(NTE999) \ +---|-\ | | |
.. | | | | >---+ | |
.. | +------------|------|+/ === |
.. | | | |/ | 100n |
.. | | | LM741 | | |
.. | / / (NTE941)+------+ |
.. | 2k 1% 2k 1% | |
.. | / / | |
.. | \ \ | |
.. | | | | Vref |
.. | +------------+---------+ IL= ---- |
.. | | Rs |
.. -------------------|--------------------------------
.. N
..
..
.. calibration: disconnect load,
..
.. jumper TL431 K-A to short,
..
.. adjust 200R pot for 0.000V at '741 (+)-(-)
..
.. w/ batt applied.
..
..
..
.. TRANSIENT PROTECTION
..
..
.. FAST BLO +--RM---+
.. 5A | |
.. VBATT>---~-----+--------+------------BATT IL---->>--+
.. | | | | |
.. +---+ |+ +---N---+ |
.. 24V | - | === | |
.. TVS |/^ | |35WVDC | |
.. | - | |1000U | +-----+
.. +---+ | | |LOAD |
.. | | | +-----+
.. | | | |
.. +--------+----------------+ |
.. | ---
.. / ///
.. 22 1W
.. / METAL
.. \ OXIDE
.. 1n4007 |
.. +---|<|-----+
.. ---
.. ///

 

[Fred Bloggs]

Fred Bloggs wrote:
scratch that- slight scale factor problem:
View in a fixed-width font such as Courier.

..
.. ----------------------REGULATOR MODULE-------------------
.. | (RM) |
.. | +---------------------------+---+ |
.. | | | | PMOS | 2.5A
.. | | I-> | | +-------+ | IL
..VBATT>--------+----/\/\---------+---------------------|s d|---->
.. | | Rs 100m | | | | | |
.. | / / | | | g | |
.. | 2k 1% 2k 1% | / +-------+ |
.. | / / | 2k | |
.. | \ \ | / | |
.. | | | | \ | |
.. | + +---+-/\/\-+ | | | 220 | |
.. | | | 1k | | +-||---|---+---/\/\----+ |
.. |Vref ---/ | | | | 22n | | |
.. | // \ -+ / / | | | |
.. | --- 100 200\<-+ +---|--+ |
.. | - | TL431 / / | |\ | | | |
.. | |(NTE999) \ \ +---|-\ | | |
.. | | | | | >---+ | |
.. | +-----/\/\-+------|------|+/ === |
.. | | 910 | |/ | 100n |
.. | | | LM741 | | |
.. | / / (NTE941)+------+ |
.. | 2k 1% 2k 1% | |
.. | / / | |
.. | \ \ | |
.. | | | | Vref |
.. | +-----------------+---------+ IL=----- |
.. | | 10xRs |
.. ------------------------|--------------------------------
.. N
..
..
.. calibration: disconnect load,
..
.. jumper TL431 K-A to short,
..
.. adjust 200R pot for 0.000V at '741 (+)-(-)
..
.. w/ batt applied.
..
..
..
.. TRANSIENT PROTECTION
..
..
.. FAST BLO +--RM---+
.. 5A | |
.. VBATT>---~-----+--------+------------BATT IL---->>--+
.. | | | | |
.. +---+ |+ +---N---+ |
.. 24V | - | === | |
.. TVS |/^ | |35WVDC | |
.. | - | |1000U | +-----+
.. +---+ | | |LOAD |
.. | | | +-----+
.. | | | |
.. +--------+----------------+ |
.. | ---
.. / ///
.. 22 1W
.. / METAL
.. \ OXIDE
.. 1n4007 |
.. +---|<|-----+
.. ---
.. ///

 

[Ken Smith]

In article <DN6Zd.9321$1S4.995531@news.xtra.co.nz>,
Terry Given <my_name@ieee.org> wrote:
[....]

I read a neat paper a few years back in an IEEE professional comic,
wherein it was shown that Si cant beat TE at high power levels, due to
the operating temperature ranges. If only we could *actually* buy static
induction transistors, thyristors etc. *sigh*

It is starting to look like SITs are just one of those things that never
make it into general use. The IGBT has eaten up some of its market but
not that much. The one device that really seems missing from the spectrum
is something with a Vgs(off) of about -2V and that can handle 600V and.or
50A.


--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <423597C1.7070008@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
[....]

You should be safe doing this:

I suggest adding a capacitor between the points I've marked "A" and "B" in

the modified drawing below. I suggest the value as 22nF. Imagine a
positive step arrives on the VBATT of the existing circuit. The output of
the LM741 will only step positive by 1/2 that amount so the other 1/2
appears as a step in the Vgs of the MOSFET (ignoring its capacitance).

Unless I've over looked something, the added 22nF will reduce the step to
near zero and match the time constant of the other 22nF so the whole
effect is brought near zero.

View in a fixed-width font such as Courier.

.
. -----------------REGULATOR MODULE-------------------
. | (RM) |
. | +----------------------+---+ |
. | | | | PMOS | 2.5A
. | | I-> | | +-------+ | IL
.VBATT>--------+----/\/\----+---------------------|s d|----
. | A | Rs 100m | | | | | |
. | / / | | | g | |
. | 2k 1% 2k 1% | / +-------+ |
. | / / | 2k | |
. | \ \ | / | |
. | B | | | \ | |
. | + +---+ | | | 220 | |
. | | | | +-||---|---+---/\/\----+ |
. |Vref ---/ | | | 22n | | |
. | // \ -+ / | | | |
. | --- 200\<-+ +---|--+ |
. | - | TL431 / | |\ | | | |
. | |(NTE999) \ +---|-\ | | |
. | | | | >---+ | |
. | +------------|------|+/ === |
. | | | |/ | 100n |
. | | | LM741 | | |
. | / / (NTE941)+------+ |
. | 2k 1% 2k 1% | |
. | / / | |
. | \ \ | |
. | | | | Vref |
. | +------------+---------+ IL= ---- |
. | | Rs |
. -------------------|--------------------------------
. N
.
.
--

--
kensmith@rahul.net forging knowledge

 

[Tony Williams]

In article <d146rq08jo@drn.newsguy.com>,
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

It wasn't a form of "outrageous overkill" that drove my MOSFET
selection, although it helped in the issue you raised, but rather
an effort to find a part that could better handle 3A * 15V = 30W
max, in the event someone used the design with a shorted load.

45W.

An apparent overkill is common when one uses modern power
MOSFETs, designed for switching use, in linear applications
where one wants to reduce the burden on the heatsink. The
IRF4905 is rated at 200W (for impossible 25C case temp), but
this means it has a nice low 0.77 C/W junction-to-case thermal
resistance. After adding in say 0.33 C/W for an insulator, the
1.1 C/W total means we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

In normal operation the MOSFET must be able to
dissipate (15-11)*2.5 Watts. Outside of that it may
be useful to implement a current shutdown/turndown,
either as a foldback (based on the 11 ohm load line),
or temperature sensed (based on a switching PTC).

--
Tony Williams.

 

[Fred Bloggs]

Ken Smith wrote:

In article <423597C1.7070008@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
[....]

You should be safe doing this:


I suggest adding a capacitor between the points I've marked "A" and "B" in
the modified drawing below. I suggest the value as 22nF. Imagine a
positive step arrives on the VBATT of the existing circuit. The output of
the LM741 will only step positive by 1/2 that amount so the other 1/2
appears as a step in the Vgs of the MOSFET (ignoring its capacitance).

Unless I've over looked something, the added 22nF will reduce the step to
near zero and match the time constant of the other 22nF so the whole
effect is brought near zero.



View in a fixed-width font such as Courier.

.
. -----------------REGULATOR MODULE-------------------
. | (RM) |
. | +----------------------+---+ |
. | | | | PMOS | 2.5A
. | | I-> | | +-------+ | IL
.VBATT>--------+----/\/\----+---------------------|s d|----
. | A | Rs 100m | | | | | |
. | / / | | | g | |
. | 2k 1% 2k 1% | / +-------+ |
. | / / | 2k | |
. | \ \ | / | |
. | B | | | \ | |
. | + +---+ | | | 220 | |
. | | | | +-||---|---+---/\/\----+ |
. |Vref ---/ | | | 22n | | |
. | // \ -+ / | | | |
. | --- 200\<-+ +---|--+ |
. | - | TL431 / | |\ | | | |
. | |(NTE999) \ +---|-\ | | |
. | | | | >---+ | |
. | +------------|------|+/ === |
. | | | |/ | 100n |
. | | | LM741 | | |
. | / / (NTE941)+------+ |
. | 2k 1% 2k 1% | |
. | / / | |
. | \ \ | |
. | | | | Vref |
. | +------------+---------+ IL= ---- |
. | | Rs |
. -------------------|--------------------------------
. N
.
.

That addition definitely makes for an improved transient response- by a
factor of 50x from what I can see. The big 1000u across the regulator
limits the current perturbations to under 5% when subjected to things
like that +30/-300V 20u/30u transient, and the 22 ohm resistor makes for
a nice low pass attenuating Volts-transient to mV-transient applied to
the circuit and on a time scale it can follow. Since we can't put a
component in series with the main current path, the MOSFET is on its
own, with the body diode saving it during negative transients, but no
protection from positive transients. The part numbers suggested thus far
are shy on Vbr,dss.

 

[Winfield Hill]

Tony Williams wrote...

... we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

Ahh, but the unit may well be bolted to a slab of metal.


--
Thanks,
- Win

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill <hill_a@t_rowland-
dotties-harvard-dot.s-edu> wrote (in <d16orb0285k@drn.newsguy.com&gt
about 'Current source design (tricky?)', on Tue, 15 Mar 2005:

Tony Williams wrote...

... we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

Ahh, but the unit may well be bolted to a slab of metal.


....at 150 C when the engine is running, on a hot day.

--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk