They are mostly only spec'd at the rated standoff voltage or the unipolar ones at max allowed working reverse voltage.
AFAIK engineers had to often stack them in telco apps to fulfill the leakage requirement.
-- Regards, Joerg http://www.analogconsultants.com/
How about a resistor from regulator input to ground? ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
After sorting on output (12V and adjustable), input over 25V, and cases with enough pins, there's 9 -- which in DigiKey parlance means 3.
3 is "a number", but it's not a large enough number for me.-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
I'm trying to cut the current consumption from the regulator. When the line is at 25V it's powering a bunch of stuff. "4V" is really just a diode drop from a LiPo cell (yes, it's not really 4V) after a boost converter has been shut down. I'm looking at stuff I can easily turn off.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
I guess my real question is how much can I trust data sheets on this one, and what pitfalls (like temperature!) should I watch out for.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
Max is the worst case but that is for 25C in this example. For the whole temperature range you usually have to write to the manufacturer. I had to do this a lot over the decades. If this is for a product and a critical parameter get it in writing, such as via email.
If you just have to maintain a low enough voltage below a certain input voltage you could consider a circuit-side bleeder resistor to ground.
-- Regards, Joerg http://www.analogconsultants.com/
Regulator type? Load under normal conditions? ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm looking for work... see my website.
So what's wrong with turning off the regulator? Regulators are often designed to be turned off for exactly this reason. Why ignore all those devices and instead kludge something with a Zener?
-- Rick C
You clearly didn't do the search right. Digikey doesn't make it easy to search for output voltage. By selecting *only* 12 volts for the output you eliminated some huge percentage of the potential units. "Adjustable" is also much more exclusive than you need. Try going to the TI site which has (or at least had) much better search capabilities allowing the input max and min as well as the output max and min to be specified separately. Or contact some FAEs and tell them what you are looking for. They often know their product line much better than the search engine.
If you think there are only 3 linear regulators that suit your needs then you clearly are not familiar with the market.
-- Rick C
You mean all three of them that are listed in DigiKey? Out of the hundreds of 3-terminal ones available?
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
In other application (semi-precision voltage clamp), I've found that the reverse leakage is not nearly as low as would be desirable.
I fixed that by inverting the transistors, which reduced leakage considerably, while increasing dynamic range (limited by Vceo instead of Vebo).
The extra bias current required to maintain the clamp 'reference' (due to low hFE) wasn't a problem in that case.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Read my other post. If you can't find more than 3 devices you don't understand how to find parts.
-- Rick C
Heat one up and see what the leakage does.
+1!
IME, there are more with enables than without. Most don't have an accurate threshold but many do, also.
Switch ground? ;-)
You could interpose a PFET between the Li BATT and the boost to kill the ba ttery drain entirely. The low voltage hefty current types are numerous and cheap in quanitity. Fairchild FDS9934C or Vishay Si9939 or Si2329DS or ... gazillions in stock all over the place.
Zener diodes start looking a bit like resistors at the lower voltages. High efficiency illumination LEDs are much better if the exact voltage doesn't matter.
Recent regulators with UVLO and battery monitors will do this better than anything else. They have a low duty cycle polling interval for UVLO so that the resistors setting the threshold don't waste power. The feature is primarily for keeping LiPo batteries safe during storage so the drain is extremely low.
-- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
If you might be taking 100 mA through the zener, it'll have to be a 1W rated unit; power semiconductors are usually intended for up to 150C temperatures, and won't have low leakage limits.
So, could you use a transistor follower and a low current zener, maybe with a collector resistor to share the heat load?
7V Zener to base of 1A NPN pass transistor, The pulldown on the base (to ground) will sink maybe 5 mA when powered (18V), and at 0.4V (base turned off) would still handle 100 uA, Small zeners are often spec'ed at 100nA leakage.
And if the regulator of choice has no ENABLE pin, then what?
-- Wescott was obviously referring to a three terminal series regulator, so your rsponse is irrelevant. As to your damnation of 6.8 volt Zeners, 6.8 volts is pretty close to where, with the specified reverse current through the diode, the Zener's tempco goes away, so why would you think 6.8 volt Zeners suck? John Fields
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