Switching power supply question

Probably because some clever person designed the loop so that it becomes more stable as you add capacitive loads.

John

If the dominate pole for the control loop is made up by the impedance of the supply feeding the output capacitor, then adding more capacitance just shifts the 3db corner frequency down. Then there are hysteresis controls that just bounce the output voltage between two slightly different voltages. These are unstable, by design, but the oscillation amplitude is controlled. Adding capacitance just slows the oscillation cycle.

But there are lots of other loop designs that are destabilized by adding enough pure capacitance across the output. Many need a little resistance in series with a significant part of the total capacitance to add a stabilizing zero to the total response.

Does that mean there can't be many clever persons in groups tasked with LDO design? SCNR ...

Actually, it will do so, unless you intend otherwise.

This is one of the basic tenets of electronic design.

RL

Apparently not. A couple of people make opamps that tolerate any capacitive load. The concept is pretty simple: the output pin is one end of the compensation cap; adding load capacitance doesn't add a pole to the loop, it just shifts the dominant pole down. The guys who do LDO's ought to be told about this.

John

Not quite as simple as that. There's some feed-around techniques used to isolate the load.

They can't, without an extra pin, and an external R/C... LDO's can't afford to waste power with implementing the R on-chip... a small R forces a large un-integrable capacitor.

I recently did one on-chip, but the current ability was only 15mA, charging an external capacitor, which was then dumped with 2.2A.

...Jim Thompson

This is what I had in mind, per Fig 1.

The load isn't isolated, it becomes part of the compensation. The transistors operate in "LDO" mode, namely with the output coming off the collectors, not the emitters.

The 8261 is a very handy gadget to have around.

John

It's a "simplified" schematic. Add up the poles if it were implemented purely as shown, and you'll see why a zero is required somewhere... for capacitive loads it needs to be associated with the output configuration.

BTDT (almost daily :-)

...Jim Thompson

It depends on the feedback function.

You get the same with motor speed control. Thats why someone had to use PID to control a motor.

"Jim Thompson" wrote in message

So LDO Designers aren't as stupid as might be assumed?

Robert H.

Cruise control is a feedback control system, yet I think I could load up my wife's station wagon with boxes and boxes of capacitors and have the cruise control work fine.

Just because many (but not all!) op-amp circuits go unstable when they're capacitively loaded doesn't mean that all electronic feedback circuits will go unstable when capacitively loaded.

But it's a Dollar a pop. Yikes.

That the point. We discrete designers do not mind at all to have to add a cap or even a more elaborate external loop. We are used to that from PWM controllers. It can't be too expensive to go from a SOT23 to a SOT23-5.

Let me be the first to say "use a PIC" in this thread. ADC->MICRO-

chips have enough bits, people may not notice the chatter.

First post must have been picked up by a wind gust (it's ugly out here right now).

We discrete designers would not mind an extra pin one bit. We are used to external parts from PWM chips. What's the big deal?

Yes but with a motor the type of load is normally well defined.(a rotating mass of some sort) and is designed for a specific application in mind.

Not so with power supplies where you can be connecting up to almost anything. I have seen a power supply osicllating once.

Wab

I never said it would - you can read my earlier comments. You obviously understand little aboyt control engineering. (If it's in a book it's either elementary,irrelevant or wrong!)

Wab

Some of them do oscillate if you put a large, low ESR capactive load on the output for exactly the reason you give.

DC/DC converter modules suffer from this because they have some fixed, compromise compensation that's usually designed for a small capacitive load. They ring like a bell when you put lots of low ESR C on the output. If you're designing the DC/DC converter you can compensate it properly for any load, but this doesn't normally apply to the prebuilt modules. I first noticed this effect when I started using OS-CON and POSCAP capacitors in my designs. These caps have lots of capacitance with low ESR.

A few years ago TI / POLA released a series of DC/DC converter modules which have programmable compensation (called TurboTrans), which allows you to tune the thing to have good performance for a particular (large) C load. I haven't used one yet, but they seem promising.

It's described in this PDF:

Regards, Allan

I've been waiting for someone to do some research on this topic. It has been solved for a long time.

National Semiconductor has an article on LDO ESR:

"Capacitors are key to voltage regulator design"

The LP2985 specs the minimum ESR at 5 milliohms. This is compatible with ceramic output caps:

The copyright on the datasheet is May, 2003. About 5 years ago.

Analog Devices has solved the ESR problem completely:

"Solving Stability Problems of Low Dropout Regulators"

The ADP3331 has no minimum ESR spec:

The copyright on the datasheet is also 2003.

For reference, here's a bunch of articles from various sources on LDO ESR. The url's are not shown since they degrade with time. But google finds them quickly.

AN1148 PDF 205,195 Compensating Low-Dropout Regulators AN682 PDF 879,026 New Generation of Low Dropout Regulators AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO SLVA068 PDF 68,595 LDO Fundamental Theory SLVA072 PDF 291,538 LDO Technical Review SLVA079 PDF 202,342 LDO Terms and Definitions SLVA115 PDF 85,754 Regulator ESR Stability SLYT151 PDF 468,245 Compensation Transient Response SLYT187 PDF 254,973 Understanding the Stable ESR Range SLYT194 PDF 246,355 LDO Linear Stability Analysis SR003AN PDF 73,945 Compensation for Linear Regulators SR004AN PDF 41,598 Linear Regulator Output Structures