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

Started by rickman October 7, 2006
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?

On 7 Oct 2006 11:49:48 -0700, "rickman" <gnuarm@gmail.com> 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?
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. John
In comp.arch.embedded rickman <gnuarm@gmail.com> 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.
... Did you look at the Texas Instrument TPS 6211X family? -- Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
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. The high Vin parts are all designed for 1.5 amps or higher so the efficiency goes south at lower currents. They typically get around this by using pulse skip mode or something similar, but we can't use that because we have to synchronize the freq to a common clock to help deal with the EMI. Thanks for the thought though. Yes, I only wish someone would make a decent low current, high Vin switcher.
Uwe Bonnes wrote:
> In comp.arch.embedded rickman <gnuarm@gmail.com> 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. > ... > > Did you look at the Texas Instrument TPS 6211X family?
These are not LDOs. They are inductive switchers. The problem is efficiency. Since we have to use PWM mode only, the best it will do is reaching 80% at 100 mA. But the efficiency gets quickly worse at lower power levels. Below 6 mA an LDO is more efficient even at 17 volts and is still better at low Vin up to about 25 mA. Since this circuit will be powered 100% of the time the efficiency matters even at the lower power levels. The switched cap circuit can stay above 80% over all voltages and down to 1 mA I estimate.
In comp.arch.embedded rickman <gnuarm@gmail.com> wrote:
> Uwe Bonnes wrote: > > In comp.arch.embedded rickman <gnuarm@gmail.com> 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. > > ... > > > > Did you look at the Texas Instrument TPS 6211X family?
> These are not LDOs. They are inductive switchers. The problem is > efficiency. Since we have to use PWM mode only, the best it will do is > reaching 80% at 100 mA. But the efficiency gets quickly worse at lower > power levels. Below 6 mA an LDO is more efficient even at 17 volts and > is still better at low Vin up to about 25 mA. Since this circuit will > be powered 100% of the time the efficiency matters even at the lower > power levels. The switched cap circuit can stay above 80% over all > voltages and down to 1 mA I estimate.
Why do you you have to stay with PWM mode only? -- Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
In sci.electronics.design rickman <gnuarm@gmail.com> wrote:
...
> 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?
For the High Vin LDO did you look at the Zetex ZLDO Series? The Torex XC6202 Series might also fit if you only need 100mA and is cheaper and smaller. -- Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
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. The high Vin parts are all designed for 1.5 amps >or higher so the efficiency goes south at lower currents. They >typically get around this by using pulse skip mode or something >similar, but we can't use that because we have to synchronize the freq >to a common clock to help deal with the EMI. > >Thanks for the thought though. Yes, I only wish someone would make a >decent low current, high Vin switcher.
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? John
Uwe Bonnes wrote:
> In sci.electronics.design rickman <gnuarm@gmail.com> wrote: > ... > > 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? > > For the High Vin LDO did you look at the Zetex ZLDO Series? The Torex > XC6202 Series might also fit if you only need 100mA and is cheaper and > smaller.
PWM mode has to be used because the switching has to be synchronized and you can't do that in PFM. Thanks for the suggestions. I don't know all the LDO makers and I figure I would get a few tips. The Torex part is not so low a drop out voltage at 670 mV, even at 30 mA it is 200 mV. The Zetex part looks like it will be a good choice although it comes in a larger package than I would prefer. It also is a fixed output voltage and the range is wide at about 3% tolerance. But this may well work. I don't know for sure what my range will be at the switcher output. We'll see when I prototype the circuit.
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.