Q on 12V 5A regulator

"Robert Baer"

** That is an awfully long cable run to drive a 12 volt load.

Wouldn't it be easier to move the supply closer ?

..... Phil

Did you put in the capacitors the LDO needs right near it? Are your resistors inductive?

You haven't indicated whether the non-uniform current drain has an effect on output regulation. If not, it could just be that you are seeing the 100mA variation of quiescent current that occurs as a single unit sees minimum headroom. You might consider ignoring it, if no functional problems result, until proper test equipment and loads allow a proper solution to become practical.

As output voltages are not identical (and even when they are), there is likely to be motor-boating around the threshold.

Placing series output resistors on an LDO seems rather ham-fisted. You've not mentioned your specified performance aims; it's not really possible to offer constructive alternatives until you do.

Obviously 220mV of load regulation is not considered to be a problem - this could qualify a lot of LDOs with nominally higher headroom requirements, as their spec will have assumed a much tighter regulation requirement in the spec formulation.

The LD29300 goes up to 3A @400mV, which could reduce your component count and potential modes of irregular parallel performance.

A discrete circuit is an obvious choice, if you're already going to the trouble of hooking up 5 linears, but I assume you're stuck out in the wilderness with only a soldering iron and a Digikey catalog delivery service.

RL

...ain't easy to move the mountain or the prophet... ..so ize gottza phaque it...

The caps are as close as the parts allow; the power resistors are wier wound but the spikes are in the tens to hundreds of microseconds wide with similar but larger seperation. Spike rate increases as input voltage increases.

** The variable load (due to thermal changes) of the lamp appears to not make a difference; the output looks reasonably regulated (with some of the spikes showing - undoubtedly due to the "bypass"/"shunt" resistance. ** Gets worse as input voltage increses. ** Just followed the same scheme recommended for the 1, 2, and 5A LDOs made by the same company - which is the same thing built into the IC of another LDO they make. ** Ideally, input should be able to change from12V to 15V and the load voltage remain at 12V (actualload current is stillunknown but presumed to be 5A). ** If i have too much trouble with the presen design, i may consider that part - especially if the drop decreases as load current decreases. ** Good call! ** Actually, a buck/boost DC/DC would be the perfect solution. But it seems they cannot be had off theshelf and i cannot design a workable one from absolute scratch.

What was the part number of this LDO so I can remember never to use it?

It still sounds like it is oscillating. Some LDOs oscillate if the ouput side capacitor is too good. You may need to put a resistance in series with the output side capacitor.

NOTES: The LT1083CP-12 data sheet, page 9 shows how to parellel them and it seems that this scheme of output resistors is good for the 1A to 7.5A units. In looking at datasheeets for the MIC29500, the LM2937ET-12 and the LF1200V type parts, that there *IS* mention of caps with too low ESR but one of the datasheets has a graph that shows stability from 0.001 up to some max - so there is no indication as to exactly (or evenapproximately) what the acceptable low end ESR might be: 20 micro-ohms??? There is also mention of instability if there is a thermal overload, but a 25 ohm resistive load shows one spike as the input goes above the threshold. And there is a statement that one could ignore oscillations (say what?). I am beginning to think that the "shunt"/"bypass" resistors have something to do with the problem - maybe a crazy idea of feed-forward gain (there is a lag from output change to the LDO output due to the series resistor and the 47uF capacitor). I am going to try removing some of those 2.7 ohm resistors to see if ther is any waveform difference.

** Worst case, i will have to toss what i have and try the MIC29752WWT. BTW,how does one find the ESR of a capacitor when the only spec is 100KHz impedance? I do not want to buy a bunch of differnt caps and then attempt to measure ESR without a decent meter (or metering circuit to build).

Spiking is exactly the same using four 2.7 ohm resistors in parallel for the "shunt"/"bypass" as using two in parallel. With zero resistors (ie: "normal" datasheet configuration), there is definite very large oscillations: step up and semi-RC ramp down on both input and output. This is obviously the case of thermal shutdown as mentioned in the datasheet - unacceptable. with only one 2.7 ohm resistor, the oscillations are faster with pulses more narrow. this might actually be better.

The paralleling method for LT1083 adjustable devices is not applicable to a fixed regulator, particularly one from another mfr with a differing construction.

2.7 ohm resistors? What...where...how.....why.... You were using ~200mOhm parts in series with the output of 1A regulators (LM2940) to assist in sharing.

The use of tantalum output caps above 22uF/per or low ESR AL electrolytics (at room temp) 100uF/per should suffice for ESR targeted by the LM2940 device. Note that the concern re esr is only really applicable below 200mA/per. The 220mOhm/per swamping resistor should cover low end limitations for ESR, under all conditions, so load the caps on to the common load point with a clear conscience.

I'm not sure what you are describing as 'spikes'. Is this a voltage or current irregularity (if current, how measured)? Rep rate, amplitude and duration, in each instance, please.

RL

The LT1083 is a fixed voltage regulator, which makes your statement slightly confusing.

The 2.7 ohm "shunt"/"bypass" resistors are connected from the input to the load and are to share current when the input goes above the regulated voltage - mainly to decrease regulator dissipation. The 0.2 ohm resistors in series with the regulators are to balance the load amoung them.

The (square-wave looking) spikes at the input are roughly 200uSec wide and spaced roughly 400uSec; depends on the input drive. Amplitude is 16V at peak and about 12.4V at bottom.

The LT1083 is a 7A5 three-terminal adjustable regulator from Linear Technology.

A waste of parts and pointless in applications where regulation is expected over the full load range.

Your source impedance is too high. Wack it with the largest electrolytics you've got lying around.

RL

There may be some valid reasons for confusion here.

There are two data sheets, one describing the LT1083 as fixed, the other as being adjustable.

fixed

adjustable

Fixed versions are 'dashed' voltage ie LT1083-5 LT1083-12

Pax?

RL

Check; i think i may have mentioned the LT1083-12; presently i am using the LM2840CT-122.

This is off the wall. But increase your shunt resistors, or put a diode in series, or both.

I do not think those ideas are off the wall. By removing 3 of the 4 shunt resistors (which increases the shunt resistance), the spikes are smaller and faster - which i think looks good. And the diode idea ensures the differential voltage must be at least

0.6V before they do their job. I had thought of that but now that someone else (you) mentioned that, i will give it a try. Also, i believe now that the "12V 50W' lamp actually draws more than the 5A design which may be a major factor. So i will try to find a lower wattage auto lamp.

Adding a capacitor to the *input* of each regulator fixed the oscillation problem.

Hi Robert,

so which regulator did you use in the end and what was the input cap value?

regards,

IanM

Am using ten LM2940CT-12 in "parallel" to handle up to 5 amps; outputs each have a 0.2 ohm resistor in series to load as well as the requisite 47uF "loading" cap. Ooe other regulator type specified a minimum of 1uF at the input, so since the inputs of these are busbar paralleled, i "overdid" it with

4.7uF flat (square) ceramic caps right at the IC leads; making for a total of 47uF at the input.