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Startup Circuit for CMOS current source

Hi,

Is there a simple way to implement a startup circuit for a CMOS (IC) current source, such that after the current source starts up, the current flow in the startup circuit is not only diverted away from the current source but rather the startup circuit is completely shut off (draws 0 current) after the current source has started up.

The cmos current source is of the " four (2P 2N) transistors in a loop with a resistor in the source of one of the transistors " type. Supply voltage changes from 4.5V to 5V. This is for a low power application.

I have seen Kevin Aylwards website where there is an example of this kind (for a bandgap reference) but it uses depletion mode devices. Unfortunately i dont have these devices available in the process.

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Any advice is much appreciated.

Thanks QQ

Reply to
QQ
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Without a depletion mode device I do not believe it is possible to have start-up current go _completely_ to zero.

But you _can_divert_ the start-up current away from your reference to reduce power supply sensitivity.

Do you have specifications for desired reference current and total current consumption?

Keep in mind that a reference that uses only CMOS devices sucks for accuracy. It'll be "flat" versus supply, but not constant versus temperature or device process corner variations.

I usually charge a fee for this kind of question ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Hi Joerg,

I have poly resistors. What do you suggest?

Thanks QQ

Joerg wrote:

Reply to
QQ

Hello QQ,

No poly resistors either?

Regards, Joerg

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Reply to
Joerg

Hi Jim,

Thanks for your advice, I was beginning to think this problem was harder than I initially thought too.

The current source does not need to be very accurate. It is used for bias.

There are no required specifications, this is just a thought exercise. For that reason (and that I cant afford it :) I cannot pay you for your expertise - sorry.

QQ

Reply to
QQ

I was just joking... provide some specs and I'll show you how tomorrow afternoon.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Um... use a capacitor?

John

Reply to
John Larkin

Sheeeesh, John, That's as bad as a digital guy using one-shots ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Well, how slow can a power supply come up?

John

Reply to
John Larkin

Hello QQ,

Maybe you could replace the depletion mode device with it to supply the start-up current.

Regards, Joerg

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Reply to
Joerg

Hi Jim,

As for the specification - current in the startup circuit less than 1nA

- after the current source has stabilized would be good. Supply varies only from 4.5V to 5V - I am hoping for the switching off of the startup circuit to not be too sensitive to power supply voltage

- incase the supply range were wider.

Thanks QQ

Reply to
QQ

Takes some VERY BIG resistors.

What is the bias current value supposed to be?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Nah. One-shots are OK as long as the timing requirements are sloppy enough that a factor of, say, 3 either way makes no difference (even when single-stepping with an ICE).

_Tuning_ one-shots, however, is always an exercise in turd-polishing.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

I like HC123's. Nice schmitt inputs, retriggerable, tolerate big timing resistors, complementary outs. Logic doesn't have to be synchronous, it only has to be right.

John

Reply to
John Larkin

I despise transient-type start-ups... very risky, very bad performance with jerky ramp-up PSU's.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Hello Jim,

Can analog guys use one-shots then?

I did on my last design. And on most of them before that. But no 555,

123 or stuff like that. Plain old 74HC14. Do I have to stand in the corner now and be ashamed?

Regards, Joerg

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Reply to
Joerg

[snip]

Show me ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

I'm curious--why would you bother auto-tuning a one-shot? Sounds a little bit like a turd car wash.....

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

"The modified circuit also uses a depletion mosfet..." In my entire lifetime I've encountered processes that had a depletion-mode mosfet only twice.

Many CMOS processes do not include a PNP device.

I regularly do start-up circuits, but the start-up current doesn't go completely to zero. But it CAN easily be diverted so that it has no effect on circuit operation.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

QQ's responses seem to imply pondering rather than real application... I've yet to see even a vague specification.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

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