Which is better? Max1674 or Max1795/96/97

I have an LS/O that runs about 350 mA at 3.1 volts. I made it go with a Max1674 chip.

I've been looking at Max1795 / Max1796 / Max1797. It doesn't require a .1 uF capacitor like the max1674. So that's one less part.

The summary data sheet is here:
http://www.maxim-ic.com/quick_view2.cfm?qv_pk=2377

1. How would one of these (which one?) work instead of the max1674?

2. If I used a max1796 would it shut off the LS if the output current went above 500 mA.

Hello there Jeff,

Looks to me like the max output of the 1797 could
end up being 400ma. That sounds enough for a
standard 1w LS.
Max chips have to be tried though, as they dont
always put out the power they say they will
at the input voltage levels they say.

Looks like the shutdown feature is input voltage
related, not output current related.
There is a separate pin to force the device
into shutdown. This means the output doesnt
shut down because of high load current, but
rather the internal transistor goes into
current limit which in turn limits the output
current. To shut the device down, you would
apply the appropriate signal to the shut down
pin.

You will probably have to have a decent inductor
to get this to operate normally with low input
voltages. Note also the start up is much higher
then 0.8v for high load currents.

Good luck with your LED circuits,
Al

Originally posted by MrAl:

Looks to me like the max output of the 1797 could end up being 400ma. That sounds enough for a standard 1w LS.
Max chips have to be tried though, as they don't always put out the power they say they will at the input voltage levels they say.

Click to expand...

<font size="2" face="Verdana, Arial">On the max1674 data sheet it says:
Steady-state output current: min 300 mA, typical 420 mA at 3.3 volts output.

On the max1797 data sheet it says:
Steady-state output current: min 400 mA, typical 550 mA at 3.3 volts output with battery at 2 volts.

When I set up the max1674, it worked fine from 0 to 300 mA, but then at current levels even a little above that it started to get hot.

Can I say that chip is only good for 300 mA, but it's within spec. because the range that it'll provide is between 300 and 420 mA due to manufacturing variations?

So then if I were to use a max1797, I should be able to get at least 400 mA, and maybe as much as 550 mA at Vout=3.3 volts with battery voltage at least 2. volts?

Hello again Jeff,

That sounds about right.
The Max chips are nice, but they seem to overrate them a little
for some reason.

I find that some manufacturers specs are more reliable then others.
If they say "500ma typical" and you never get a chip that puts out
500ma, that's not typical to me
On the other hand, they do spec a min, and since it does that min
then it's still in spec according to them.
I also find that when they do spec a min you have to take that
as the final word. You cant rely on the typical spec anyway,
even though other specs will be typical in actual use.

One other thing that bothers me is the output voltage spec,
which doesnt state 3.3v output exactly, but rather "3.17v to 3.43v".
Have you had any problems with the output voltage level being too
low or too high?

Also, from the data sheet the efficiency should be around:
88% at 2.4v input
78% at 2.0v input

Did you get good efficiencies so far?

Also from the data sheet:
Startup voltage for 400ma load is 2.0v, but into an LS load it
might work a little lower then that too.
Have you tried to measure this yet on any of the chips you have tried?

Good luck with your LED circuits,
Al

Someplace on the maxim web site I saw a note about samples. They said the samples might not be as good as the ones they sell. I guess they give away all the stuff that's not quite 100%.

I still like my 4 led max757 light with the dimmer. The output voltage is nice and steady for whatever the setting is.

I had a max1674 going on a breadboard for awhile, but I took it apart before I got any efficiency measurements. It had a trim pot for variable voltage and it seemed to work very well with an LS up to 300 mA. But more than that, no good. It got hot fast. In retrospect it makes sense since 300 mA is the minimum specified max. current, and the chip was a free sample.

I never tried either of them at fixed output voltage, always variable.

I just ordered a couple of max1797 samples. I've got one more surfboard that I think it will fit.

That'll keep me busy for a couple of months. I'll post my impressions.

Hi again Jeff,

Have you tried the Max1703 chip yet?
At first glance, this chip looks like
it will take care of 5w LS's and breeze
over 1w LS's.
Takes 1 to 3 cells as input, but
for 3.3v output 2 cells will work.

Check it out if you get a chance,
it does look promising.
Also, do you know if Max sells their
chips directly too?

Take care for now,
Al

I know about the MAX1674/5 but not the others. I will say this about the 1674/5. The 1675 has a fixed 0.5A switch current limit. There is a MAXIOUT formula that predicts very accuractly how much current you can theoretically can put out given input voltage, output voltage, output load current, inductor etc... This I've plotted many places for reference on my web site if you are interested.

As for the 1674, it has a 1A internal switch current limit. At 2.5V input, 3.3V output, you can get 400+mA of output current. It can climb as high as 500-600mA of current if you raise the input to 3V and get a hot low Vf Luxeon.

I probably have looked at the 1703 and others, but, have not played with them.

Also, ordering samples from Maxim is not 2nd hand stock. It's from the standard line and I'm not aware of them ever giving out 2nd's or bad parts as part of the sample program. I've worked with almost since the company was founded some 10+ years ago.

WayneY

The samples are not as good as the ones you buy.
I ordered a MAX727 that should be 2A max but I can't get more than 1.2A without making the chip too hot to touch.

Star882,

I'm sure you will find that by adjusting inductor, capacitor or other factors will alter how much current you can get, so, I really doubt you have a bogus IC.

WayneY

The IC is a sample.

Max1703 does look like a good one. I don't know if you can buy directly from Maxim. I've been using samples.

Max1703 has 16 little pins but I think I could solder it to something. This is my techinque using a $12. soldering iron and a bobby pin:

http://edusite10.tripod.com/led3/smount/hold.htm

It worked really well for a max1674 with the 8 pin micro max package.

Originally posted by star882:
The IC is a sample.

Click to expand...

<font size="2" face="Verdana, Arial">which means they picked one out of a batch of a million and sent it to you for free.
It does not mean that they specifically manufacture inferior products and call them 'samples'.

Hello again Jeff,

Looks like some good ideas there

Yes, the 1703 looks like it will be able to
handle a 1w LS rather easily even if the chip
is a little overrated by Maxim

Good luck with it,
Al