Re: 5mW boost converter

Yes, I would say the measured 600uA (@3.3V) IDD I got from the 74HC14
running at 500kHz is a 'low power' circuit, but not 'micropower'. An
LMC555 under same conditions drained about 200uA.

Since you have found the 555 to be the lower power one, why don't
you make the next step - run it at 50 rather that 500 kHz. Over a 1 uF
capacitor this will yield something like 20 mV ripple p-p @ 1mA - is
this not good enough? The consumption should also drop dramatically
- may be not 10 times, but it will go low enough for >90% efficiency.
Well, if too much of the 200 uA go into the comparators you won't
gain much... but I would measure that before giving up.

Dimiter

------------------------------------------------------
Dimiter Popoff Transgalactic Instruments

http://www.tgi-sci.com
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Adam S wrote:
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.

Regards, Joerg

Yes, I would say the measured 600uA (@3.3V) IDD I got from the 74HC14
running at 500kHz is a 'low power' circuit, but not 'micropower'. An
LMC555 under same conditions drained about 200uA.
The gate output FETs are not the problem, its the input stage MOSFETs of
the Schmitt that suffer cross conduction. This is easily proved by
noting how current drain is a constant 600uA from 1Hz to several hundred
kilohertz.

.

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