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

[rickman]

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.

 

[PeteS]

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

 

[Uwe Bonnes]

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...

 

[rickman]

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.

 

[rickman]

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]

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

 

[rickman]

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.

 

[Wes Stewart]

I believe they just don't use the frequencies that
are impacted by the EMI.

Yes but some other poor bastard does.

 

[rickman]

Yes but some other poor bastard does.

;^)

Good one!

We also have emissions standards that are a lot tougher than FCC part
15. But we are in a metal enclosure and filter everything going in and
out so we don't have the problem of messing with the "other guy".

 

[joseph2k]

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.

 

[John Popelish]

Thanks for the tips, but I have looked at all of these devices as well
as many more. This is the third time I have done this same power
supply search for these parameters. If there was an inductive switcher
out there that did a good job at 10 mA of current, I would have found
it. But the combination of low current and high Vin seems to be
deadly.

Are you saying these efficiency specs are deadly, or wrong?

In addition, all but a few of the high Vin regulators are
non-synchrnous and many of those also require a boost diode. By the
time you add the sizes of all these components you are using a fair
hunk of real estate and not getting much in the way of efficiency.

Yes, efficiency and low noise require some real estate.

Designing an inductive switcher may well be complex. But a multimode
switched capacitor circuit is mostly a digital design along with
careful control of the detailed timing.

And also has a maximum theoretical efficiency well below the
100% theoretical efficiency of a buck regulator. Switched
capacitor voltage changes (with no inductors) are
essentially RC processes.

The only timing issue I am
concerned about is that the P channel FETs must be driven through N
channel FETs. These devices add much more delay to the path than the
logic circuits will. To make sure there is no shoot through I will
have to compensate these delays compared to the N channel FETs used on
the low side of the caps. I may have to add N channel FETs to drive
the N channel FETs just to equalize the delays.

But what I asked for help on was selecting an LDO that meets my
requirements.

I understand your request, I just don't yet understand why
you are making it. If you are severely limited in the EMI
department, then you will have trouble with a switched
capacitor step down circuit, as much as you will with an
inductive buck regulator. They both make noise.

I am suggesting that starting with something like the above
buck regulator and spending your effort on noise
containment, you will reach your goals and get higher
efficiency and lower noise than is possible for the same
real estate using your switched capacitor step down (without
additional inductive noise filter components) and an LDO
linear post regulator.

 

[rickman]

Are you saying these efficiency specs are deadly, or wrong?

They are not correct for the mode I will be using the part. On
inductive switchers a lot of power is used to keep the circuit
operating. So at low power levels the efficiency is poor and can even
be beat by an LDO. They get around this by essentially turning off the
switcher until the voltage drops enough to need the switcher again. So
it runs in a burst mode with a higher ripple and a variable frequency.
I can't work with the variable frequency so I am stuck using the parts
in the PWM mode which has too low an efficiency.

And also has a maximum theoretical efficiency well below the
100% theoretical efficiency of a buck regulator. Switched
capacitor voltage changes (with no inductors) are
essentially RC processes.

Why is theoretical efficiency even an issue? I have a design that over
a range of current will provide efficiencies between 70% and 95%
including the required drop out of the LDO. Of course this is not
built or tested so it may end up having some higher losses than I
expect due to quiescent current.

I understand your request, I just don't yet understand why
you are making it. If you are severely limited in the EMI
department, then you will have trouble with a switched
capacitor step down circuit, as much as you will with an
inductive buck regulator. They both make noise.

This circuit is not to deal with EMI. Besides, this should have a lot
better EMI performance just because there is no inductor. The reason
for this design is efficiency.

I am suggesting that starting with something like the above
buck regulator and spending your effort on noise
containment, you will reach your goals and get higher
efficiency and lower noise than is possible for the same
real estate using your switched capacitor step down (without
additional inductive noise filter components) and an LDO
linear post regulator.

Please explain how I can improve the efficiency of the inductive
switcher at 10 to 30 mA of output current. The simple inductive
switcher is not large. But it is not efficient at low currents either.
I don't understand how adding circuitry can improve that.

 

[John Popelish]

John Popelish wrote: (snip)
I can't work with the variable frequency so I am stuck using the parts
in the PWM mode which has too low an efficiency.

I don't understand why you can't work with variable
frequency, if you keep the noise under control.

Why is theoretical efficiency even an issue? I have a design that over
a range of current will provide efficiencies between 70% and 95%
including the required drop out of the LDO. Of course this is not
built or tested so it may end up having some higher losses than I
expect due to quiescent current.

Any time you connect two capacitors together that do not
match in voltage, as much energy is lost as is transferred.
The switch is essentially a resistor in series with the
charge transfer.

This circuit is not to deal with EMI. Besides, this should have a lot
better EMI performance just because there is no inductor.

Inductors are not inherently noisy. A shielded inductor can
be part of a very useful noise filter.

The reason for this design is efficiency.

You may teach me something new, if you can achieve high
efficiency with a switched capacitor voltage changer.

And when charge plows between two of those capacitors
through the low impedance of your switched, large current
pulses (many times the average load current) will occur, and
those can radiate a lot of noise, if you aren't careful.

Please explain how I can improve the efficiency of the inductive
switcher at 10 to 30 mA of output current. The simple inductive
switcher is not large. But it is not efficient at low currents either.
I don't understand how adding circuitry can improve that.

The LT3470 claims an efficiency of 64% with 24 volts in, 3.3
volts out and a 1 mA load (better with a 16 volt to 7 volt
input). If you add a small inductor to the input side and
maybe a ferrite bead to the output, the noise level can be
quite low. The frequency is, however variable, since this
part is a hysteretic controller. But that means that it
handles step load changes with guaranteed stability. The
main filter inductor could be less than 7mm square, like the
100 uHy Sumida CDRH6D28NP-101ND @ $1 each:
http://www.sumida.com/en/products/pdf/CDRH6D28.pdf

 

[Don Lancaster]

Any time you connect two capacitors together that do not match in
voltage, as much energy is lost as is transferred. The switch is
essentially a resistor in series with the charge transfer.

Not quite true.

If the voltage difference is very small, the charging efficiency can be
acceptable. Otherwise switched capacitor power sources would be totally
useless. Rather than typically offering 95 percent efficiency.

On resistively charging a capacitor from zero, most of the energy loss
happens early in the first time constant. By doing most of your charging
four or five time constants out, the losses can be considerably lower.

This requires that the charge consumed per cycle be much less than the
charge stored.

A fancier switchmode circuit that does an intermediate transfer to an
inductance can also eliminate this problem.


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[John Popelish]

Not quite true.

If the voltage difference is very small, the charging efficiency can be
acceptable. Otherwise switched capacitor power sources would be totally
useless. Rather than typically offering 95 percent efficiency.

On resistively charging a capacitor from zero, most of the energy loss
happens early in the first time constant. By doing most of your charging
four or five time constants out, the losses can be considerably lower.

This requires that the charge consumed per cycle be much less than the
charge stored.

I stand corrected on the efficiency possible. I just
simulated a 2 to 1 voltage switched capacitive voltage
reducer, and if the switch on resistance was low enough,
good efficiency was possible. But the large current spikes
I mentioned were also present. If I use a two phase, high
frequency 2 to 1 step down, everything quiets down pretty
well.

 

[Mark Borgerson]

They are not correct for the mode I will be using the part. On
inductive switchers a lot of power is used to keep the circuit
operating. So at low power levels the efficiency is poor and can even
be beat by an LDO. They get around this by essentially turning off the
switcher until the voltage drops enough to need the switcher again. So
it runs in a burst mode with a higher ripple and a variable frequency.
I can't work with the variable frequency so I am stuck using the parts
in the PWM mode which has too low an efficiency.

With a LOT of output capacitance, could you not end up with a
SMPS that runs at your 600KHz for 1msec and turns off for
100 msec. With that kind of duty cycle, you won't see much
EMI at anything other than 600KHz.

This would work if the power requirements are discontinuous---
part of the time at 100mA and part of the time at 10mA. But
it would be a problem if the power required could be anywhere
in the range between 10 and 100mA.


Mark Borgerson

 

[Tim]

If I have understood you correctly, your system has two operating modes
a) low power and b) high power. Could you thus use two power supplies
that could be enabled and disabled using FET switches according to your
system requirements by this CPLD? In low power mode you would use LDO
and in high current mode you would use switched mode power supply+LDO.

I have not designed switched mode power supplies, but I guess if you
reduce the switching frequency, you will improve efficienfy with the
expence of ripple which could be "filtered" by this LDO. Could you get
away with for example 1 kHz switching frequency? Or could an adjustable
switching frequency be more feasable?

Please note, this was all pure speculation and I haven't tried this at
home

- Tim

 

[Tim]

This one came into my mind: How about using some small rechargeable
batteries or a very high capacitance condensator as an intermediate
power source which is charged by a 100mA step-down-switcher as needed.
When the charger (switching regulator) is operating in high current
mode, its efficiency remains quite good, maybe > 90%. You may want to
add an LDO to filter out the ripple created by charger and
battery/condensator.

- Tim

 

[rickman]

With a LOT of output capacitance, could you not end up with a
SMPS that runs at your 600KHz for 1msec and turns off for
100 msec. With that kind of duty cycle, you won't see much
EMI at anything other than 600KHz.

This would work if the power requirements are discontinuous---
part of the time at 100mA and part of the time at 10mA. But
it would be a problem if the power required could be anywhere
in the range between 10 and 100mA.

Any number of things may be possible, but I have not yet found a
converter chip which will allow synchronization in PFM. In fact, they
typically use the same pin for selecting PWM/PFM and clock sync input.
The pin can not be held low and receive a clock at the same time.

 

[rickman]

If I have understood you correctly, your system has two operating modes
a) low power and b) high power. Could you thus use two power supplies
that could be enabled and disabled using FET switches according to your
system requirements by this CPLD? In low power mode you would use LDO
and in high current mode you would use switched mode power supply+LDO.

I have not designed switched mode power supplies, but I guess if you
reduce the switching frequency, you will improve efficienfy with the
expence of ripple which could be "filtered" by this LDO. Could you get
away with for example 1 kHz switching frequency? Or could an adjustable
switching frequency be more feasable?

Please note, this was all pure speculation and I haven't tried this at
home

Thanks for the ideas.

Typically our devices have several power consumption levels. But I
can't use multiple PS circuits for better efficiency because they don't
make any that I have found that are efficient at low currents. The
problem is not the dual mode, the problem is not the noise, the problem
is that there are *NO* switching converter chips that meet all the
requirements of input voltage, clock sync and good efficiency at low
current. The high Vin chip market starts with parts that work at 1.5
amps and goes up from there.