Re: 5mW boost converter

Hello James,


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. They only consumed more when I went above a MHz or so but
that is to be expected from older CMOS processes.

That _is_ odd. Referring to my notes, the most recent time this bit
me was with a 74hc132 (quad 2-input NAND schmitt) 67Hz R-C oscillator.
Unloaded, it drew 700uA from a +4 volt supply.


Ouch, never had one that high.


That's been my experience right down the line with the HC-series
schmitts. I was possibly one of the first users of the 'HC14, or at
least would have been, except the early Nat. Semi and Motorola parts
dumped a HUGE, hysteresis-dependent bias current back at you through
the input! TI's parts did not do this but they came too late, so I had
to make my own (from 74hc244s).

Are you sure it was the 74hc14, as opposed to the 74c14?


Nope, never designed much with the C series. It wasn't mainstream enough in terms of pricing and availability. 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. Also, Adam mentioned 3.3V in which case it should be even lower. It's usually good policy to run just one input on the RC node so this stuff doesn't add up. Of course, if power mattered really badly one could add a little MOSFET up front.

Regards, Joerg

http://www.analogconsultants.com
.

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