High Vin LDO with truely low dropout in small package (long post)

Rocky wrote:
> rickman wrote:
>
> >
> > I do have some concerns about the losses in the switcher.  But if they
> > are too high to run the switcher and LDO from 7 volts, I can always add
> > a 1:1 mode for the low end of the input voltage.
>
> Just out of curiosity - what frequency will the switcher run?

I believe I could run it at any freq and still meet the "requirements".
 There is a common signal to sync switchers at 600 kHz which I will
use.  The intent is to sync the inductive switchers since they radiate.
 I believe the switched cap circuit will be much quieter and so should
not need to be run at this freq, but I will sync it just to ward off
the kibitzing.

Rene Tschaggelar wrote:
> A switcher in burst mode has less transients than in
> PWM mode. Thus generates less EMI. If you want low
> EMI, then you have to choose a switcher with
> controlled/slower transients.

Reducing the EMI does not solve the problem.  In essence they have said
that the EMI problem can not be reduced enough to call it "solved".  So
instead they want to control the specific frequencies to known values.
Ideally they would even control the EMI frequencies so that they are
not in the way of the frequency in use at the time.  But I don't see
where they do this.  I believe they just don't use the frequencies that
are impacted by the EMI.

rickman wrote:
> We often need to power low current circuits (<100 mA) from a wide
> battery voltage of 7 to 16.5 volts.  It is hard to find switching
> regulators that will do this efficiently.  Sometimes we can just write
> off the wasted power and use an LDO if the current is low enough, but I
> am looking to find a way to reclaim this lost efficiency.
> 
> I am thinking of building a switched capacitor voltage converter that
> can lower the input voltage over this input voltage range to something
> that is suitable for an LDO.  The circuit would use a CPLD (powered by
> this circuit) to generate the control signals for a small army of P and
> N channel FETs (or possibly analog switches) to control three flying
> capacitors.  I have done a lot of searching and found a 2 x 3 mm
> complementary FET that has the required characteristics that should
> work well.  I have also found a 4 channel comparator with a built in
> reference.  Now I only have two remaining problems.
> 
> The first is figuring out how to start up the circuit.  I have come up
> with a couple of ideas that will bypass the switches until the output
> voltage is up to snuff and the CPLD starts running the switches.
> However this is hard to do without exposing the LDO following the
> switched cap converter to the full Vin of 16.5 volts.
> 
> This is the second problem.  If I try to find an LDO with Vin up to
> 16.5 volts, output current up to 100 mA and dropout voltage of 200 mV,
> I come up short.  Considering some losses in the switching circuit,
> even to work with a dropout of 200 mV will realistically require the
> output voltage to be lower than the 3.3 volts I would like to use.  But
> I can likely live with 3.2 or even 3.1 volts if the accuracy on the LDO
> is good enough to keep it above 3.0 volts worst case.
> 
> I can't raise the 7 volt Vin minimum requirement, so I am stuck with a
> 200 mV dropout.  I can get this in a low Vin device, but not a high Vin
> device in a small package.  So far I have tried to keep this as simple
> as possible and not used anything like a "pre-regulator" or Zener
> diode.  But I'm not happy with my choices.
> 
> Anyone have any suggestions on a better way to improve this circuit?
> 

If the object of the exercise is to get higher efficiency and pass 
emissions, then perhaps you might look at the latest batch of buck 
controllers from TI, LTC, Max et.al. as others have already mentioned.

I use these because I have handheld units (indeed, I am in the middle of 
a design for one now) and power efficiency is sorta important.

The fact that these units go into a burst mode (or PFM or even pseudo 
PFM) has not been an issue at compliance testing. In fact, I have lower 
point emissions in burst mode (because it gets spread across the 
spectrum, one might surmise).

I use a number of controllers in the latest design, and one series that 
would work (there are others of course) is the TPS511xx series from TI. 
External FETs, typical efficiency at 10mA load (3.3V output) > 80%, 
about 90% at 100mA. 1mA efficiency 45%, FWIW.

Just my $0.02

Cheers

PeteS

In comp.arch.embedded PeteS <peter.smith8380@ntlworld.com> wrote:

> I use a number of controllers in the latest design, and one series that 
> would work (there are others of course) is the TPS511xx series from TI. 
> External FETs, typical efficiency at 10mA load (3.3V output) > 80%, 
> about 90% at 100mA. 1mA efficiency 45%, FWIW.

I looked at some TPS511XX devices, and all need a 5 Volt Input.
This will be a bootstrap problem for Rickman...
-- 
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

Jim Granville wrote:
> The LP2951/LP2954 family get close. Micrel & Advanced Monolithic do
> 'better' versions of these, with -20/+60V ip.
>
> You could also look at placing the LDO _before_ the switch-cap, since
> you say that has low Rs. That halves the LDO current, and also makes the
> dropout a smaller % of higher voltage.
>
> Also, because you DO have a higher voltage, that opens up boosted gate
> drive schemes, most often found in higher current regulators.
>
> Have you looked at LED drive Switch modes ? - these are getting smarter
> all the time, and often have mode changes, and are designed to deliver
> low load currents, at high efficencies - because they target handheld apps.

I'm not sure why you are recommending the LP2951/LP2954 parts.  Their
drop out is 500 to 600 mV.

I looked very hard at *all* of the switchers I could find including the
LED drivers since we also have to drive LEDs with dimming control.

Uwe Bonnes wrote:
> In comp.arch.embedded PeteS <peter.smith8380@ntlworld.com> wrote:
> 
> 
>>I use a number of controllers in the latest design, and one series that 
>>would work (there are others of course) is the TPS511xx series from TI. 
>>External FETs, typical efficiency at 10mA load (3.3V output) > 80%, 
>>about 90% at 100mA. 1mA efficiency 45%, FWIW.
> 
> 
> I looked at some TPS511XX devices, and all need a 5 Volt Input.
> This will be a bootstrap problem for Rickman...

That may be true, yet he has 7V as a minimum Vin, which easily satisfies 
the requirements. (My minimum is 5.8V, incidentally, and it works fine).

If that's an issue, then try something like the LTC1735 (which can be 
set for low currents). The minimum current through the device (in a 
couple of designs I have it in) is about 4mA, so _very_ low current 
efficiency is not good at all.

There are numerous other solutions, of course.

My point is there _are_ SMPS (inductive buck) controllers that are 
efficient across the Vin and load range Rickman desires, provided you 
are willing to let them operate in burst mode.

Cheers

PeteS

PeteS wrote:
> If the object of the exercise is to get higher efficiency and pass
> emissions, then perhaps you might look at the latest batch of buck
> controllers from TI, LTC, Max et.al. as others have already mentioned.
>
> I use these because I have handheld units (indeed, I am in the middle of
> a design for one now) and power efficiency is sorta important.
>
> The fact that these units go into a burst mode (or PFM or even pseudo
> PFM) has not been an issue at compliance testing. In fact, I have lower
> point emissions in burst mode (because it gets spread across the
> spectrum, one might surmise).

The problem is not emissions compliance testing.  It is internal EMI.
We have much tougher goals to meet and we know ahead of time that we
can't actually get the EMI low enough to solve the problem.  So we put
the EMI at known frequencies and deal with it other ways.


> I use a number of controllers in the latest design, and one series that
> would work (there are others of course) is the TPS511xx series from TI.
> External FETs, typical efficiency at 10mA load (3.3V output) > 80%,
> about 90% at 100mA. 1mA efficiency 45%, FWIW.
>
> Just my $0.02

Thanks for your comments.  This part is actually very far from what I
need.  In PWM mode the graphs show less than 50% efficiency at 100 mA
and below 10% at 10 mA.  A straight linear regulator is between 20% and
47% efficient over the input voltage range.  This part is also not
synchronizable.

What I am doing may be a bit of overkill.  But I want to see how
practical it is.  If it works out well for currents up to 100 mA I will
see if I can find a vendor who would be willing to put the controller
and switches into a chip.  I know there are other switched cap
converters, but they almost universally fall into two camps, the simple
doubler/inverter parts and the low Vin parts.  It would be very useful
to us to have a small chip that could convert these higher voltages
efficiently at lower currents.

PeteS wrote:
> My point is there _are_ SMPS (inductive buck) controllers that are
> efficient across the Vin and load range Rickman desires, provided you
> are willing to let them operate in burst mode.

Yes, that is the problem.  Our constraints prevent us from doing that.
We have to operate at a fixed 600 kHz rate in this application.

Mike Harrison wrote:
> On 7 Oct 2006 12:27:36 -0700, "rickman" <gnuarm@gmail.com> wrote:
>
> >John Larkin wrote:
> >> Revisit the idea of using a buck switcher? That doesn't seem difficult
> >> over a mere 2:1 or so input range, and it will be a lot simpler than a
> >> switch+cap array, control logic, and an LDO.
> >
> >Yes, simple, but very inefficient.  At 100 mA it may not be so bad at
> >around 50-60% typically, but at 10 mA or lower the efficiency goes to
> >heck in a handbasket.
>
> If you're worried about 10mA why on earth are you considering using a CPLD for anything...?

I don't follow.  The CPLD is the only part on this board that needs
power all the time.  Since it was there I figured I could use the extra
logic to control a switched cap converter and get the power consumption
down from 50 mW to 10 mW or so.  I don't actually expect it to use 10
mA all the time.  It has to do two thing, run a multiplexer circuit at
13 MHz any time it is not "powered down".  It also has to receive
commands over an SPI interface to control a few relays.  The relays are
latching so they don't consume current when they are not being
switched.  The duty cycle is very, very low since they are only changed
on mode changes.  I figure they can live rich, full lives powered by an
LDO from Vbat.  But the controller has to be up 100% of the time to
receive commands and run the mux.

So my initial estimate is that the CPLD controller runs at about 3 mA
typ and up to 10 mA or so when there is activity on the SPI interface.
If this works well I may use the same design on another board where the
typ current is more like 20-40 mA up to 100 mA max.  Or I may use the
TI switcher to provide 5 volts and then use other parts to generate 3.3
and 1.8 volts.

rickman wrote:

> John Larkin wrote:
>> A simple homemade hysteretic switcher should be efficient down to
>> roughly zero current, if you allow a bit of ripple. As far as syncing
>> with a clock to reduce EMI, well, I don't understand that... isn't it
>> better to spread out the spectral lines instead of heaping them all on
>> top one another?
> 
> Two problems with that idea.  First spreading the spectrum may or may
> not reduce the problem.  For example, using a moving frequency for the
> clock may result in a test measurement that is lower, but does it
> really reduce the interference problem or does it just allow you to
> pass a test?   The interfering spur is still the same amplitude, it is
> just moving while you test and so is integrated over a wider frequency
> range giving an average lower reading.
> 
> Secondly, in the case of power supplies, you will be generating spurs
> either way, sync'd or not sync'd.  But if you sync all the supplies to
> the same clock, at least they are all creating the same harmonics.
> There are other ways to deal with the spurs since you can't get rid of
> them.
> 
> If it is easy to make a switcher with good efficiency at low currents,
> why aren't there chips available to do that?  We get a fair amount of
> attention from the vendors because we sell a lot of units.  They all
> try to sell me the same 1.5 Amp high Vin switchers with low efficiency
> at low currents.  The TI part is the best one I have seen so far and it
> is terrible below about 30 mA.

Oh fiddlesticks.

Ok. low power, low dropout, high efficiency regulator is simple; don't use a
"chip" go discrete.

Your volume is enough to attract sales critters, but not enough to interest
them in a targeted design, even using their own parts.

-- 
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller