in my design a ispMACH 4032ZE CPLD (32Macrocells) operates as SPI slave driven by a moderate 8MHz master-clock. The part requires a 1.8V core supply drawing (estimated) tiny 100uA over the full temperature range according to the data sheet.

A well regulated 3.3V supply which is derived from the 6V...8.4V board supply via an LDO is available on the board. Since the CPLD is the only 1.8V part on the board a full-blown LDO regulator seems overkill and I wonder whether there are more cost- and/or space efficient solutions.

The board supply range of 6V...8.4V seemingly rules out most affordable dual output LDO regulators, even 1.8V zener diodes are similarly priced. (0.3€)

The most cost effective solution seems to be a resistor divider (1.8k/1.5) plus capacitor... however this seems somewhat wrong for a power rail.

Any comments?


I recognize that there are cheap LDOs that would easily satisfy my requirements and which I probably will use for the sake of predictability and robustness. Still, my question aimed at the slightly uncommon solutions such as using LEDs for voltage dropping.

Many thanks!

  • \$\begingroup\$ A 0.3€ zener diode as a regulator is too expensive ? A voltage divider may not fit the CPLD power requirements. Have you take a look at Linear LT3008/LT3009 (tiny LDOs but maybe a bit expensive for you) ? \$\endgroup\$
    – zeqL
    Dec 5, 2013 at 16:20
  • \$\begingroup\$ LDO's are available with budgetary pricing (qty 1000) down to $0.12 at least (so figure maybe $0.50 or $0.60 in small lots). A zener might be slightly cheaper, but also fussier to design with. \$\endgroup\$
    – The Photon
    Dec 5, 2013 at 17:28

2 Answers 2


If you don't need much current, you can maybe use an LED? A red LED has a voltage drop about 1.5V, which will drop your 3.3 down to 1.8V. It seems weird to use a diode or LED to regulate power, but this FPGA development board which I've used, the XSA-50: http://www.xess.com/shop/product/xsa-50/ (click on manual to see the schematic) uses a 1N4148 to do exactly that (derive 2.5V from 3.3V). Also, free power indicator LED for the 1.8V rail.

The correct thing to do usually would be to use a Zener, like this one from digikey. But I kind of like the LED idea better, and you might have one lying around.


Having taught students for quite a while, and designed my own production boards, I think any saving in money will be lost with time and annoyance.

Zener's are a very variable component, think +-20% out of the factory. LEDs are not really designed for stable Vf so also can give you grief when someone changes their color for cosmetics or a cheaper part (people see LEDs as non-critical components).

A SOT linear like the MIC5377YC5-TR is dirt cheap, at $0.26 for singles from Mouser, 150mA and will have incredibly better stability with current draw, temperature, part variability, etc. vs. the LED or Zener, while being the same cost or cheaper.


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