Re: 5mW boost converter


Winfield Hill wrote:
Joerg wrote...

Strange, I have used the CD40106 as well as the 74HC14 in low
power PWM converters without any problems. They are both Schmitts,
meaning they won't dwell in the crossover phase any longer than
their switching speed lets them.
[ snip ]
Look at the Philips spec for the 74HC14, page 9, 5th line item:
http://www.semiconductors.philips.com/acrobat_download/datasheets/74HC_HCT14_3.pdf
30uA typical, and an oscillator doesn't spend a whole long time
at that very spot.

Not quite. Yes, once the threshold has been reached the switching
is fast and the extra current drain stops, but as the analog input
voltage slowly approaches the threshold the class-A current is high
and going higher, reaching a maximum just at the moment of switching.
See the graphs on page 14. So one gets a much higher supply-current
hit than expected if this hasn't been considered.

Yes, Figs. 10 and 11 illustrate the problem -- in operation, the
typical one-gate R-C Schmitt oscillator's input voltage range is
exactly the most disadvantageous vis-à-vis current consumption.

The 700uA-drawing circuit 74hc132 I mentioned before used a Motorola
74hc132 quad NAND-gate with R-C feedback to one input, and a
logic-level 'enable' signal applied to the other. All other
gate-sections were disabled. Dragging out the proto, I've re-measured
it.

It draws 700uA at Vdd=3.0v, not 4.0v as previously stated. At
Vdd=4.5v, the circuit draws 2.1mA.

.. 74hc132d
.. +------. +--+ Output:
.. enable+ >--------| --- \ | | 15mS pulse, 67Hz
.. | // |O----+-----> --+ +--
.. +---|--- / |
.. | +------' |
.. | |
.. | R1 |
.. | 2.2M |
.. o----/\/\/\--------o
.. | |
.. | R2 D1 |
.. | 68k In4148 |
.. +--/\/\/\---|<|----+
.. |
.. |
.. C1 -----
.. 22nF -----
.. |
.. |
.. ===
.. GND


Idd waveform @ Vdd=4.0v

.. _ ...__ 2.8mA
.. /| /|
.. / | / | rise/fall time = 11.4/1.6mS.
.. / | / |
.. / |/ |/_ ..__ 1.4mA
..


BTW, it's nice to see that parameter spec'd in the data***. But
one observes that a 30uA typical spec at Vcc -2.1V is not properly
at the tippy-top current peak! The stated 30uA pales in comparison
to the huge currents in figure 10 page 14, which peak at 330uA going
up and 420uA coming down. That's what we should be looking at IMHO.
The Philips graphs show that if you want 30 to 40uA current maximums
you'll have to run their 74hc14 at 2.0 volts supply! Hmm, too bad
they don't have any graphs between 2 and 4.5 volts, like 3.3 volts.


At any rate, I often end up avoiding the Schmitts for linear stuff.

Me too.

Regards,
James Arthur

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