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

I looked at some TPS511XX devices, and all need a 5 Volt Input. This will be a bootstrap problem for Rickman...

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.

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.

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

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.

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

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.

On 8 Oct 2006 06:08:00 -0700, "rickman" wrote: [snip]

Yes but some other poor bastard does.

;^)

Good one!

We also have emissions standards that are a lot tougher than FCC part

1. 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".

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.

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

Yes, efficiency and low noise require some real estate.

And also has a maximum theoretical efficiency well below the

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.

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.

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.

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.

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.

(snip)

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

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.

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

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.

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

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.

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.

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

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

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

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

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.

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.

A link to an article about using a capacitor as an intermediate power source:

"Supercapacitor boosts current from small battery"

- Tim