Re: 5mW boost converter

Hi Joerg,

Joerg wrote:
Hello Adam,


I'd wager the problem isn't shoot-through on the paralleled stages,
but cross-conduction in the oscillator stage. I've measured
disappointingly high Idd in 74hc14s I'd hoped to use as micro-power
oscillators. They aren't suitable (Idd close to a mA for slow
oscillators ISTR).

Thats agrees precisely with my findings too. For the 74HC14 oscillator
the VDD pin current drain is essentially independent of frequency until
you get >> 500kHz. My conclusion is cross conduction on the FET input
stage of the oscillator inverter. So your right, the 74HC14 cannot be
use as a micro power oscillator. My previous attempts were with three
gate RC oscillators using a 74HC04, but IDD was around 1.0mA. Even worse
was 3~4mA from the 74AC04. I was scratching my head for a while until I
realized I should be using a single gate Schmitt 74HC14 oscillator, but
that turned out less than ideal.


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.

But they do not like stuff heavily bumping into their substrate diodes.

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.

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?

Best,
James Arthur

.

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