TOREX LDO User Manual
Technical Information Paper
A
No.0030(Ver.01)
LDO Current Foldback/Current Limiter Circuits
INTRODUCTION
This TIP is aimed at improving the knowledge of engineers and sales people, as it is not really possible to delv e into that level of
details on LDO datasheets. On the pages that follow, you will learn that current limiter/current foldback circuits of LDO’s come in many
different forms. Moreover, we will show that, by using the V
the output voltage and output current of the LDO will vary at start-up. This concept, briefly explained on the “XC6219 Current Foldback
Circuit” TIP for one type of LDO, will be further detailed here. Finally, since output current waveforms at start-up include a significant
amount of inrush current, this document will highlight as well the benefits of Torex LDO’s with inrush current protection.
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vs. I
characteristics, you can often anticipate –at least partially- how
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TYPES OF CURRENT LIMITING/CURRENT FOLDBACK CIRCUITS
The current limiting/current foldback circuits used by Torex LDO’s can be divided into 4 main types:
A) Current limiter, then current foldback;
B) Current foldback only;
C) Current limiter, then current limiter;
D) Current foldback, then current limiter.
Please note that the order selected is based on when the LDO is enabled and an exces sive amount of output curre nt is requested on
the output side of the LDO. For a “type A” LDO, this means that the current limiter is activated first, followed by the current foldback,
hence the above statement: “A) Current limiter, then current foldback”.
The below figure shows the typical V
Moreover, the below table highlights which type of current limiting/current foldback circuitry is used by the main Torex LDO’s.
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waveforms for these 4 circuits.
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In this table, most of the older Torex LDO's (XC6204, XC6219, etc) are of type A, where the current limiter circuit was added due to the
lack of accuracy of the current foldback circuit.
LDO series
XC6204 X
XC6209 X
XC6210 X
XC6214 X
XC6215 X
XC6216 X
XC6217 X
XC6218 X
XC6219 X
XC6220 X
XC6221 X
XC6222 X
XC6223 X
XC6224 X
XC6227 X
XC6501 X
XC6503 X
XC6504 X
XC6505 X
XC6602 X
XC6701 X
Type A Type B Type C Type D
Limiter TYPE
Then, around the year 2005, it became possible to use the foldback circuit as a curre nt limiter too. So Torex introduced LDO's of type
B, with a current foldback circuit that can detect accurately when an over-current condition occurs. T he protection circuits required for
types A and B LDO’s are smaller than the protection circuits for types C and D LDO’s, making it possible to integrate types A and B
LDO’s inside smaller packages. For example, ultra small-size LDO’s such as the XC6224 and XC6504 are of type A.
However, using current foldback can sometimes prevent the LDO from starting up when the initial output voltage on customers'
applications is slightly negative (down to -0.2V). Using an LDO whose first limiting circuit (at start-up) is a current limiter instead of a
current foldback can circumvent this issue. This is why type C LDO's (XC6223, XC6503, etc) were introduced. Despite it may appea r
obvious to many, we shouldn’t forget that the main reason for including a current limiter in types A and C LDO’s is the high accuracy of
the set current limit. But, although type C LDO's fix a problem at start-up, they create a new problem when an over- current condition
occurs because the decrease in V
current until V
Whereas both type D and type C LDO's can solve the problem faced by types A and B LDO's during start-up, type D LDO’s can also
avoid the aforementioned issue faced by type C LDO's during a n over-current condition. So Torex recently started the development of
such LDO's with the XC6505 series. It is worth noting as well that, thanks to the addition of a therma l shutdown circuit, some type C
LDO’s don’t face the over-heating problem during an over-current condition.
drops below 1V, leading to more over-heating than types A and B.
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activated by their over-current protection circuit is not accompanied by a decrease in output
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Because of all the above explanations, most of the new Torex LDO’s will be of type C or type D. Moreover, Torex keeps on improving
and reducing the size of these limiter circuits, so that they can be integrated in ultra small-size packages.
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FURTHER COMMENTS BEFORE DELVING INTO DETAILS
We are now going to introduce the behavior at start-up for various LDO’s: 1 of type A, 1 of type B and 2 of type C. Since the XC6505 is
currently the only Torex LDO of type D, it was considered unnecessary to dedicate a full chapter to it. Please note that series differ from
one another. As a consequence, since the below example for “type A” LDO focuses only on the XC6219 series, you shouldn’t use it to
draw any conclusion regarding the behavior of other series of “type A” LDO’s at start-up, as it is possible that they will behave differently.
The same remark applies for types B, C and D LDO’s. In any case, this document is a good starting point and can be kept as a reference
whenever you need to investigate the start-up behavior of a Torex LDO.
Since the waveforms that will follow often include the output current I
with the following circuit diagram. As can be seen, the current probe should be inserted between the output pin V
output capacitor CL. If the current probe is positioned after CL, critical information is lost as all the start-up generated inrush current will
be absorbed by CL.
Unless stated otherwise, C
For example, when the regulated output voltage of the LDO is 3.0V, VIN is set to 4.0V.
= CL = 1µF and the input voltage VIN is always set 1.0V above the regulated output voltage of the LDO.
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of the LDO, let’s clarify where this output current is measured
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of the LDO and the
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CASE STUDIES
1) LDOofTypeA
Our first example is going to be the XC6219B302MR. For that matter, the below 3 graphs are showing how the output current varies
when the XC6219B is enabled via its CE pin and needs to supply 3 different values of continuous current (20mA, 80mA and 150mA) to
an output load. All I
Please note that for the below graphs –and for all graphs that follow-, the output current was set with a variable resistor.
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waveforms look similar, apart from the fact that they peak at slightly different values.
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We are now going to study the start-up waveforms in further details, by using the case when the LDO is enabled and needs to supply
20mA. For that matter, the below waveforms are simply zoomed waveforms of the above-left graph. In addition to that, a second plot
exhibits the V
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characteristics of the XC6219B302MR.
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Initially, both V
the remain on the current foldback curve during start-up, so the values of V
foldback curve, as we are now going to see. First of all, the output current starts ramping up to about 20mA whereas V
(see V1, I1). Then, as V
coordinates of a point of the current foldback curve (see for example V2, I2). Following that, the output current reac hes the end of the
current foldback curve, which is as well the beginning of the current limiter curve (see V3, I3). The output current hardly increases any
further, while the output voltage ramps up to its set value. Finally, when V
stripped of its inrush current component and decreases down to the continuous current value req uested b y the output load (see V5, I5).
In the case of the XC6219, the latter step between (V4, I4) and (V5, I5) is a bit noisy for the output current.
T here are a few points worth mentioning regar ding the above behavi or: in a way, at start-up, the current foldback circuit acts like
some kind of inrush protection circuit by forcing the output current I
characteristics. Take for example the above case when V
possible, but it can’t because it is forced to remain on the foldback curve. So I
But the current foldback circuit is not a perfect inrush protection circuit, as you will notice by looking at the below graph, where the
LDO is enabled but the actual output current demand from the output load is 0mA.
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and I
are at 0V. Then, as was explained on the “XC6219 Current Foldback Circuit” TIP, V
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starts increasing, I
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and I
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increases as well, but in such a way that V
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reaches its set value (see V4, I4), the output current is
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to always remain on the foldback curve of the V
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= V2 = 0.7V. Due to inrush current, I
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= I2 = 100mA.
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always correspond to a point of the current
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and I
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and I
OUT
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always correspond to the
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have got
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remains at 0V
vs. I
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wants to increase as high as
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