5mW boost converter

"Adam S" schreef in bericht news:44501718$0$25525$ snipped-for-privacy@news.optusnet.com.au...

The big weakness of CMOS in this sort of application is that both the P and N-channel MOSFETs are on during switchover, so you get some "shoot-through" current. If you used discrete logic-level P- and N-channel MOSFETs to drive the inductor, and organised the logic to give you non-over-lapping drive waveforms, you might do better - the circuit might not have to be much more complicated.

Or you can replace your inductor with a transformer wound on an RM4 cores, or one of the little planar cores, and use something really simple. like a Royer inverter, which gets up to around 90% efficiency in some situations.

Are you willing to consider other approaches?

I was thinking about the shoot through issue, and wondered whether changing to 40106 CMOS hex Schmitt trigger will make a difference. Then I discovered that the bias current for the LCD character module was essentially due to an onboard voltage divider comprised of 5 x 1 kohm resistors. Changing these to 10kohm will 1/10th the current consumption. At 100uA, a simple capacitor switcher such as the LM6642 that John Popelish suggested looks attractive.

Adam

The CMOS 555 may perform somewhat better in this application. Most of your current power waste is probably being lost due to cross conduction of the input buffer stage of the U1A inverter. Unfortunately this type of oscillator requires the input voltage to be roughly 1/2 the chip voltage, which maximizes cross conduction waste.

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).

The solution is to use something other than the 74hc14 for the oscillator, something with sharp edges to feed into the CMOS part.

A much lower frequency, low-duty cycle pulse for a driving waveform would save a bunch on switching losses too (but kills your synchronous rectifier scheme, and you'd need feedback for voltage regulation).

Outputting 1mA @ 5v and 60% efficiency should require 2.5mA @ 3.3v input into the 74hc14, so for the Idd(74hc14)=0.8mA you report the converter isn't loaded, yes? Still, 0.6mA of losses @ 3.3v with L1 disconnected leaves 0.4mA of losses unaccounted for. That might be the paralleled driver stages -- you could disable them, test, and thus be sure.

James Arthur

Yes, just saw this. If you can live with its -3v output (or can afford to cascade two), a capacitive switcher sounds simple, and is spunky enough for this load.

ISTR some regulating gadgets rather like this one, but which have multiple stages and can output double the input voltage. Not super-efficient, but simple and solid. Made by LTC perhaps? Yes:

James Arthur

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.

I measured 200uA supply current of a 555 CMOS configures as 50% duty cycle oscillator running at 500kHz. To get self starting synchronous rectification, paralleled 74HC04 hex inverters will need to be added on the output.

And maybe some provision to make sure -VOUT doesn't charge up to

As seductive as synchronous rectification is, in this instance the inductor has ~8 volts flyback across it, so a schottky rectifier's 0.4v drop will only contribute ~ 0.4v/8v =3D 5% overall efficiency loss. It's pretty easy to lose that and more in switching losses in the sync. rectifier, which already has an ohmic loss as well.

But, all these are moot: you've already got the thing beat just by upping the LCD's divider resistances 10x. Touch=E9.

James Arthur

I recently did a driver circuit for LCD shutters using a LTC6906 for an oscillator and a 4060 to provide drive for a capacitor voltage doubler (very efficient), LCD shutter drive signal, and other timing. A couple other parts are used to do level translation and drive the LCD shutters. The LTC6906 draws around 15 uA at an output freq of 13 kHz. LTC6906 is one of Linear Technology's wonderful resistor-set oscillators. Total current consumption of the circuit drew; while driving a 10nF shutter capacitance; 25 uA in-phase, 320 uA out-of-phase.

Making R-C oscillators out of various standard logic family inverters consumes 100s of uA at 10kHz, partly due to the slow rise time on the input.

snip

At this low power drain, most of the energy in the circuit should be just circulating. Peak to peak magnetizing current in the choke will be in the order of 2mA ppk, with the values shown, but this should mostly be returned to the rails. The 100ohm output driver resistance is the source of loss in the actual energy conversion.

If you dropped the oscillator frequency, the parasitic cross-conduction in the fets of the paralleled output structure should reduce dramatically. This will be much more than is present in independantly conneced gates.

Freewheeling magnetizing current might be more efficiently returned to the rails using shottky signal diodes, if the inductor value isn't raised accordingly, to keep magnetization constant.

By the way, I think you'll find it works just as well without biasing the duty cycle away from 50% ( ie without R2).

RL

Hello Adam,

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.

The HD44780 based LCD character modules are actually capable of running for a few months from just 2 AA cells.

As I said, modifying all the bias divider voltage resistors from standard 1k to say 22k and by driving logic inputs of DB0-DB7, RS, R/W to high state when idling so that HD44780 internal pull ups of these pins do not add extra current, then these LCD character modules are capable of consuming as little as 150uA@3V supply for the logic and another 50uA@5V for the bias. Thats a big shift from 1mA@5V specified in most applications.

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

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

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Adam S wrote:

Hello Adam,

Are you sure you haven't received some bad apples there? The Philips spec says the typical added current if an input is held in the linear region is 30uA. I guess that would be just one in your case. Even if you held all six there that shouldn't result in 600uA. And this spec is for

Regards, Joerg

Hello James,

Ouch, never had one that high.

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

Regards, Joerg

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.

BTW, it's nice to see that parameter spec'd in the datasheet. 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.

CT14_3.pdf

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-=E0-vis current consumption.

The 700uA-drawing circuit 74hc132 I mentioned before used a Motorola

It draws 700uA at Vdd=3D3.0v, not 4.0v as previously stated. At Vdd=3D4.5v, the circuit draws 2.1mA.

. 74hc132d . +------. +--+ Output: . enable+ >--------| --- \\ | | 15mS pulse, 67Hz . | // |O----+-----> --+ +-- . +---|--- / | . | +------' | . | | . | R1 | . | 2.2M | . o----/\\/\\/\\--------o . | | . | R2 D1 | . | 68k In4148 | . +--/\\/\\/\\---| BTW, it's nice to see that parameter spec'd in the datasheet. But

Me too.

Regards, James Arthur