Running uC off 1v5 to 15v range

AMACK · **Posted:** Mon Nov 21, 2011 8:30 am

Rob, Pete never sleeps :lol:

cravenhaven · **Posted:** Mon Nov 21, 2011 2:34 pm

graynomad wrote:

Ok, transistor guys, will this work? Does the emitter follow the collector until the zener starts conducting and then the emitter is .6 below the zener voltage?

Trouble with that cct is that with an input voltage of say 1.9V, your input to the REG711 is approx 1.2-1.3 which is below its minimum of 1.8V.

graynomad · **Posted:** Thu Nov 24, 2011 8:22 pm

Don't you get a loss of the VCEsat voltage which is often .2 or .3v?

cravenhaven · **Posted:** Sat Nov 26, 2011 11:16 am

graynomad wrote:

Don't you get a loss of the VCEsat voltage which is often .2 or .3v?

Well as I understand it.
The REG711 draws approx 100mA at its input and to put the transistor into saturation would therefore require approx 5-10mA of base current.

Assuming 5mA Base current and input voltage=1.9V min, then:
Rb = 1.9-1.8/.005 = 20 ohms
When 14 volts is applied to the input then you would be drawing 0.6A through the resistor = 7Watts dissipation plus the Zener dissipation = 0.6W

In any case the Vce Saturation voltage is going to be around 0.3V which makes your input to the REG711 at 1.5V which is below its guaranteed working voltage.

Allan

graynomad · **Posted:** Sat Nov 26, 2011 4:46 pm

Yep I get it, I can tweak the numbers a bit (I don't need 100mA for example) but at the end of the day it's not a very efficient way to do it.

It's the bloody range that's a problem, low V is easy, high V is easy, but not both.

If I decide to go ahead I may have to use a proper buck/boost chip.

Running uC off 1v5 to 15v range

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