Re: Simple problem: 0.20 V drop
- From: Winfield Hill <Winfield_member@xxxxxxxxxxx>
- Date: 18 Jan 2007 07:00:30 -0800
Colin Howarth wrote...
This is a well-known problem (for those using old cameras...):
Mercury oxide cells have a pretty flat 1.35 V open circuit voltage.
They have however been banned in many countries. The best replacement
would seem to be silver oxide which has a voltage of 1.55 V.
The current drawn (by the exposure meter) is low.
So, I'd like to drop 0.20 V along the way somewhere.
a) Zener diodes: don't go down to 0.2 V
b) Schottky diodes: supposed to have a forward drop of 0.15 to 0.45V
at 1 mA (according to wikipedia) but the drop is current dependent
and varies between samples.
c) Germanium diode?
d) voltage divider with a FET follower? a JFET ?
Is there a standard / best way to do this (with few, smd components)?
Today's instant ASCII design:
.. Micropower low-voltage LDO regulator:
.. drop 13% = 200mV at 1.55V (if k=0.87)
..
.. Vi___________________pnp Q3
.. | | | E C ----+-- OUT
.. R1 R3 | B | Vo
.. | | | | |
.. | +---- | -----? |
.. | Q1 | | Q2 npn |
.. | C C |
.. +---B B -----------?
.. | E--+--E k = Vo/Vi = R2/(R1+R2)
.. | | R4 = (kVi-0.55)/Iq
.. R2 R4 R3 = 2*0.60/Iq
.. | | choose Iq >= Iout/25
.. gnd gnd choose R1+R2 = 10 Vi/Iq
--
Thanks,
- Win
.
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