What's the hi-side current sense chip du jour?

I did that a lot as well, but with MIC4421 and its brethren. They pack more punch and there is a 2nd source from Microchip:

How did you manage negative pull? The LM5112 can't have more than 14V supply so capacitive drive wouldn't allow 10V devices for the FET.

That would result in a loud *KABLOUIE* :-)

And east of us we've already got black clouds forming. Severe thunderstorm and flash flood warning in effect. So it things go quite here you know why. Not good ...

Yes, it definitely is. Might even have to go to flyback because of it. Meaning more EMI, difficult to source magnetics. OTOH if life was too easy the world would not need analog engineers.

No, it's SEPIC. I am designing something with several converters in there and the bucks have the same issue. This thread started with a different topic, like most threads on s.e.d. do but at least it hasn't arrived on the hill yet :-)

Why can't all PWM chips have a MIC4422 or maybe even a 12A MOSFET driver in there? That would be so cool.

I like Larkin's approach... use a good hefty gate driver, and roll your own loop around it.

Most of the switchers I designed for GenRad Portable Products Division (nee Omnicomp) used LM339 + TL431 for the controller architecture ;-)

...Jim Thompson

If this doesn't work with good efficiency I might just do that.

My first ones consisted of 1/6th CD40106, pnp/npn follower, another bigger follower if needed, then the big FET. Plus lots of cheap discrete stuff around it. It's ok for mass production in Asia but small qty domestic fabrication is not so cool.

Anyhow, I have just sent LTC my "Dear Santa" list of things that could be improved in their switcher designs.

Its logic and power ground pins are separate, and the mosfet drive Vee can be below logic ground.

You can have all sorts of fun with this part.

ftp://jjlarkin.lmi.net/Inverter.jpg

John

Not ye most terrific load regulation there, unless I'm missing something ultra-clever--the loop gain looks pretty low.

Cheers

It's actually pretty stiff, plenty good enough to power a bunch of opamps. It runs continuous mode, so it doesn't need a lot of duty cycle change to adjust for a load change. I seem to recall regulation ballpark a tenth of a volt from 100 to maybe 800 mA, numbers roughly like that.

There aren't a lot of negative or inverter circuits around. I recall ever seeing only one true negative buck IC, some ancient Fairchild part maybe.

John

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While it is possible the Maxim part in question is defective, I assure you every design is benched and flogged by applications as well as design. This is true at any semi I worked at. Nobody wants a hardware recall.

More likely than not, the customer PCB is not designed well. [AKA pilot error.] Most companies sell (OK, give away if you look serious) demo boards with functioning circuits.

It is probably no surprise that most switchers are not tested in circuit on the handler. The inductance of the contactor makes it hard to run a switcher. [Charge pumps don't have such problems, so they are tested in the actual circuit.] There are usually test modes that allow the test engineer to measure the power fet on resistance, comparator trip points, etc.

I have tested a few customer boards over the years. In every situation but one, it was a component error, component defect, or layout problem. [OK, one was a bug on my part, but the part was being used in a funny manner nobody anticipated.]

Interesting--I wouldn't have thought so by looking at it, but then again I haven't designed a switcher in aeons--not since the 78S40 was the chip du jour!

Cheers

Phil Hobbs

Ah, IN_REF to VEE up to 5V. Interesting, thanks!

That looks almost like my first switcher, early 90's. Except that I used a CD40106 plus cheap pnp/npn follower to drive the FET. The device with it in there is still in production.

So far I can't remember a Maxim part ever being flawed (can't say the same about some other mfgs). I believe the engineers at Maxim are actually quite good. The problem seems to be more in the upper management. 100% of my design-out cases happened because Maxim was unable to deliver quantities in a timely and predictable manner. If that issue hasn't been recognized by upper management in decades I am not sure it ever will be :-(

It's quite similar with some (large) European companies. Excellent engineering combined with serioulsy flawed S&M departments. Consequently I avoid their parts if possible.

My first switcher was a hysteretic buck, using an uncompensated uA709, a 2N2905, and a 2N3055, with a schottky free-wheeling diode, a novelty at the time. It ran at 25 KHz, because I could hear 22K in those days.

John

The line regulation sucks, of course, but that didn't matter much in my application.

Lately we're using these...

and moving up the abstraction stack. Some of our products have 9 power rails!

John

MAX9690. And, in my opinion, its so-called replacement, the MAX9691.

My knowledge of flawed Maxim parts has sort of dropped off, since we don't design around Maxim parts any more.

John

Ok, that kind of gave away your age now :-)

My 3rd or 4th regulator was based on a salvaged PL504 pentode, on account of HV transistors being out of league for a kid back then. Still got it, still works.

So what's next? Automatic transmission in your VW?

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What really happens is the small companies don't get serviced well. You buy a few hundred K of parts, you get serviced. You buy a few hundred parts, you get samples but not follow on. I haven't been there in a decade, so I don't know how its run now, but many of the same fools are still in management

My point really was regarding parts out the chute not working, ans the other poster claimed. Now every semi buys and benches the competitions parts. Internally, they have reports of their warts, so nobody gets blindsided. Every part has some issue, but more or less meets the function and certainly passes the electricals in the datasheet.

DC/DC chips all have one critical node, namely the pin connected to the external inductor. Make the trace to long, it rings, radiates, and things get ugly. Some chips (OK, my designs) use a one shot designed to blank out any "decisions" while the LX pin moves. Who knows if the reference is wiggling, current is coupled capacitively into inputs, etc. Blanking decisions for a short period of time won't effect the control if you set the timing right. This is one of those things that doesn't show up in the datasheet block diagram.

I know you like to roll your own, but a good DC/DC is full of protection circuitry you never see. I've gone as far as adding watchdog timers to chips so if the idiot customer shorts out pins that effect control, the part can reset itself.

Some companies have S&M departments that will service anything that moves.