As asked in the header, does anybody have an idea for simply generating a low negative voltage for indicator leds on low output voltages e.g. 0.9V on FPGA core supplies ?Non-red leds (green, yellow, white) need at least 2V or more so a direct connection to low voltage regulators for indicating presence doesn't make sense.Of course a driving NPN could be used however on many voltage outputs a negative supply would be an alternative.Using some kind of phase output of buck regulators is one of the possibilities, is there any other simpler idea ?Thanks for all suggestions,Mike

I would look into switch capacitor circuits but I'm not sure I like the idea at all. You want to tie the cathode of the LED through a resistor to -1.5v. When the signal at 0.9v, you have 2.4 volts across the LED and it turns on. When the signal is at 0V, the LED has less than it's ON voltage, so it's off and you're hoping that the leakage current with 1.5v across the LED isn't high enough to pull the pin below ground enough to damage some ultra-low power 0.9v device. I would rethink that plan.

On Thu, 3 Nov 2022 16:41:02 -0400, Phil Hobbs snipped-for-privacy@electroWhere is the second one?

On Thu, 3 Nov 2022 20:57:08 -0400, Phil Hobbs snipped-for-privacy@electroI thought this is what he wanted to do. not.

fredag den 4. november 2022 kl. 01.57.17 UTC+1 skrev Phil Hobbs:as I understood it the idea was to turn on an LED when a 0.9V power rail is present

On Fri, 4 Nov 2022 09:59:35 -0400, Phil Hobbs snipped-for-privacy@electroWe leave OPs in the dust.

Idea for a simple -1.5V 20mA power supply?

Nov 02, 2022 Last reply: 3 years ago 28 Replies

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John Larkin 3 years ago

+0.9 to base, 0.65 Vbe drop, 0.25 across a 250 ohm emitter resistor to ground, 1 mA collector current through the LED, microamps of base current.

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Phil Hobbs 3 years ago

Sure, but that takes two resistors.

Cheers

Phil Hobbs

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John Larkin 3 years ago

Where is the second one?

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Phil Hobbs 3 years ago

IIRC the idea was to drive a LED from logic with a BJT and one resistor.

You FPGA-savvy folks may think of +0.9V as a logic high, but those of us still in the 3.3- or 5-V trailing edge of technology aren't there yet. ;)

Cheers

Phil Hobbs

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John Larkin 3 years ago

I thought this is what he wanted to do.

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Maybe not.

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Lasse Langwadt Christensen 3 years ago

as I understood it the idea was to turn on an LED when a 0.9V power rail is present

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Phil Hobbs 3 years ago

Right, the OP. There's that. ;)

Cheers

Phil Hobbs

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John Larkin 3 years ago

We leave OPs in the dust.

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Mike Randelzhofer 3 years ago

The original thought was exactly that, want a simple supply supervisor, a supervisor MCU can measure exactly but a quick view whats happening is always desirable - but if the cost of the led circuitry is a transistor, its IMHO a doubtful solution.

But the current source idea is nice because it shows small changes in voltage on the led. It also could be used to show IO pin states, another interesting application.

The original intent was to have a small negative supply which also could be useful in some analog circuitry.

Thanks for all ideas, Mike

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