Diode drop to step down

When 3.3V supplies were new you could get precision two-terminal series devices that would drop some defined voltage. I don't know if you could get the voltage you wanted, but you might look. It doesn't seem that an LDO -- even on a teeny board with a couple of caps -- is going to make much more of a lump in a cable than a pair of series diodes.

TLV431 will drop a very stable 1.24V...

That'd get him below his desired 3.8V. It seems that most of the series 'precision zener' type things want to regulate at bandgap type voltages and not below.

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Thanks all for quick responses. The current drawn by the board is 1A peak. I am sorry for not mentioning it in the original post. Looks like TLV431 doesn't support this much current.

-M

Actually I don't understand the 3.8V to 4.1V requirement... weird numbers! ...Jim Thompson

As an aside, I sometimes use a 7.5 volt zener, in the forward direction, to drop 3.3 volts down to 2.5 for Vccaux in an FPGA. It's right on the money.

John

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quoted text -

Though we are powering the board with DC supply the board actually expects LiIon battery. So the voltage range of 3.7-4.2V. But we would like to have 0.1V margin to take care of internal battery voltage measurement errors.

-M

As long as you insure there's enough current through the diodes that they're operating well beyond their knee regions, AFAIK it should be fine (...you'll presumably have some cap anyway that'll take care of any small transient bobbles).

An "amplified diode" might be a bit more elegant? Perhaps a few pennies cheaper too, if that matters.

---Joel

What's the relationship between the forward voltage drop of a Zener and its Zener voltage? Higher Zener voltage -> higher forward voltage? (I mean, I'm assuming you ended up with a 7.5V Zener and not, e.g., 5.1V or 9V or similar based on some empirical measurements...)

I tested the ones we had in stock. I think I recall the higher voltage zeners having more forward drop. I'm getting right on 2.5 volts, but Vccaux on these Spartan chips isn't very critical.

I did encounter some consternation, but being President has its benefits.

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At 1 amp I think you are going to get more than 0.65 volts dropped by the 1N4007. (Closer to 1 volt??) Is that a problem? The Schottky will do something similar.

George H.

Have you looked at an LM317 and two resistors? It may be marginal with only 1 volt drop. It would work well if you had another source of DC, not necessarily regulated.

Fred

I had a similar problem in my logic analyzer. I couldn't find large 5V memory modules. I used two BY127 diodes in series to get below 4V and mounted about 10 additional 10uf capacitors on the SIMM modules. If you read the diode specs carefully you'll see there is quite a big difference in the voltage drop of a diode due to temperature and current.

OTOH powering a CPU and memory through diodes may be pushing things to far. Besides that I'm wondering why the board does not have power regulators for the CPU and the memory. 3.8 to 4.1 V seems way to high for DDR anyway. Usually it is 2.5V or less and you need a vref which tracks the power supply lines.

I'd assume there's regulator on board, sounds a lot like something that is supossed to be power by a Li-ion battery

-Lasse

If the board was designed for battery input it may do just fine on 5V, particularly if it has a switching supply. You could either try it, or ask the manufacturer.

It seems your assumption is right. Like Tim W. says its probably safe to power the board with 5V. Most low voltage switchers in that range accept up to 6V input but the OP must check first.

How much current? Do you expect big load transients, like from CPU idle to full-blast?

Sounds a little risky to me. I don't think 1N4007s are very well controlled, and regulation will be soft even on a single unit.

John

yeh, I've seen chips meant to be powered by a li-ion battery have abs. max voltage rating around 4V

-Lasse

The input supply range as per the datasheet is is 3.7V to 4.2V. The CPU has in-built cascaded regulators that generate 3.3V for IO, 2.5V for DDR and 1.2V for CPU core.

-M