TLE727x - Output Capacitor

Robust Dimensioning of the Output Capacitor
TLE727x-2 – Ultra-Low Quiescent Current LDOs
Z8F52274290
Application Note
Rev. 1.01, 2014-09-26
Automotive Power
Robust Dimensioning of the Output Capacitor
Z8F52274290
Table of Contents
1
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
3.1
3.2
3.3
3.4
3.5
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts . . . . . . . . . 5
Influence of Control Loop’s Reaction Time on Output Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Influences on Control Loop’s Reaction Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Consequences for the Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Influence of the Output Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4
4.1
4.2
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage Regulator 15
How to Avoid Big Current Transients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
How to Dimension the Output Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5
Additional Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6
Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Application Note
2
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Abstract
1
Abstract
Note: The following information is given as a hint for the implementation of our devices only and shall not
be regarded as a description or warranty of a certain functionality, condition or quality of the device.
This Application Note is intended to provide a description on how to dimension the output capacitor of a
linear voltage regulator with ultra-low quiescent current to obtain a reliable supply circuit.
Application Note
3
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Introduction
2
Introduction
In nearly all of today’s electronic systems, low energy consumption is one of the major challenges. As the
systems’ performance is increasing in all kinds of applications, intelligent solutions need to be found to
compensate at the same time the increasing energy demand. These then help either to save energy costs or
to efficiently use limited energy resources.
One example for limited energy resources can be found within the automotive area: The car’s battery provides
a limited amount of energy. Therefore, any electronic system being permanently connected to the battery
faces hard requirements regarding its energy consumption.
For supply ICs, saving energy means keeping the internal current consumption as low as possible. The
challenge is here to ensure a proper operation at very low bias currents of all functionalities that are usually
present within the IC.
To meet these requirements for linear voltage regulators, Infineon has set a benchmark on the market with its
ultra-low quiescent current voltage regulators. These products combine, besides the usual functionalities and
an outstanding quality, sophisticated feature sets like Window-Watchdog and Reset with an ultra-low current
consumption of only 28µA (TLE7273-2). Without additional features, the ultra-low quiescent current voltage
regulators reach a current consumption of even 20µA (TLE7274-2). In addition, to reduce the overall system
cost, Infineon implemented for these regulators an advanced control loop concept that requires only a very
small output capacitor for control loop stability.
This document is split in two parts: In the first one (Chapter 3), the challenge for circuit design for linear
voltage regulators of combining ultra-low quiescent current with very small output capacitors is
demonstrated. The second part (Chapter 4) shows a procedure for dimensioning the output capacitor to set
up a reliable and robust supply circuit with ultra-low quiescent current. Therefore, if you’re just interested in
quickly setting up your application using an ultra-low quiescent current linear voltage regulator, you can skip
the theoretical part and directly jump to Chapter 4.
Application Note
4
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
3
Theoretical Background on Ultra-Low Quiescent Current Control
Loop Concepts
In this chapter, the challenge for circuit design of combining in a linear voltage regulator ultra-low quiescent
current with very small output capacitors is demonstrated.
For this, we’ll look inside a linear voltage regulator and have a closer look on the influence of the output
capacitor.
3.1
Influence of Control Loop’s Reaction Time on Output Voltage
The main part of every linear voltage regulator is the integrated control loop, that is controlling the voltage
regulator’s output voltage. Depending on the application requirements, different control loop concepts can
be implemented. Each one of these has different advantages and disadvantages regarding their performance.
One important criterion for performance of a control loop is its so-called “reaction time”. Let’s see what it is
and why it is important:
Figure 1 shows a simple application circuit with a microcontroller supplied by a linear voltage regulator. Let’s
assume that the application has a standby mode and a normal operating mode, but is never completely shut
down. This means the microcontroller is always supplied, but in standby mode with few mA or even less (let’s
take 1mA as simple example), in normal operating mode with several 10mA up to few 100mA (let’s assume
70mA). Now, at the moment the microcontroller is triggered to go from standby mode to normal operating
mode, the result is typically a fast current transient with rise times below 500ns at the voltage regulator’s
output.
IQ
rise time < 500ns
70mA
1mA
I
VBatt
Figure 1
Linear VReg
e.g.
TLE42754
Q
GND
t
IQ
µC
CQ
Simple Application Circuit
Figure 2 shows the typical behaviour of a standard (not ultra-low quiescent current) linear voltage regulator
at this current transient. The output voltage is set to 5V. It can be seen that at the moment of the current
transient, the voltage is dropping down by ∆V to a minimum value (“Min. Voltage”). During this phase, the
voltage regulator’s control loop is not reacting yet. Only after this short time, called “Reaction Time”, the
control loop is acting and sets the output voltage back to the nominal value by increasing the output current.
It is now obvious that the reaction time is a performance criterion for a control loop as: The faster the control
loop’s reaction time, the smaller the voltage drop at current transients.
Application Note
5
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
IQ
Standard
70mA
1mA
t / µs
VQ
5V
ΔV
Min. Voltage
t / µs
Reaction Time
Figure 2
Output Voltage at Current Transient at Standard Linear Voltage Regulator
In case the microcontroller is triggered to go from normal operating mode to stand-by mode, a negative
current transient is resulting and the behaviour is the other way round. This is shown in Figure 3. The
correlation between ∆V and reaction time is the same, the faster the reaction time, the smaller the voltage
peak.
IQ
Standard
70mA
1mA
t / µs
VQ
Max. Voltage
5V
ΔV
t / µs
Reaction Time
Figure 3
Output Voltage at Negative Current Transient at Standard Linear Voltage Regulator
Application Note
6
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
Let’s now focus on the reaction time of a control loop of an ultra-low quiescent current linear voltage
regulator.
Figure 4 shows now in addition the output voltage of an ultra-low quiescent current linear voltage regulator
(right picture). It can be easily seen that its reaction time is longer, hence its voltage drop higher.
a) Standard
IQ
IQ b) Ultra Low-Quiescent Current
70mA
70mA
1mA
t / µs
1mA
VQ
5V
t / µs
VQ
Min. Voltage
ΔV
5V
Min. Voltage
t / µs
t / µs
Reaction Time
Figure 4
ΔV
Reaction Time
Output Voltage at Current Transient: a) Standard versus b) Ultra-low Quiescent Current
Linear Voltage Regulator
The same conclusion can be found at a negative current transient shown in Figure 5: The control loop’s
reaction time at an ultra-low quiescent current voltage regulator is longer, therefore the voltage peak is
higher.
Application Note
7
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
a) Standard
IQ
IQ b) Ultra Low-Quiescent Current
70mA
70mA
1mA
t / µs
1mA
VQ
t / µs
VQ
Max. Voltage
5V
ΔV
Max. Voltage
t / µs
t / µs
Reaction Time
Figure 5
ΔV
5V
Reaction Time
Output Voltage at Negative Current Transient: a) Standard versus b) Ultra-low Quiescent
Current Linear Voltage Regulator
Thus the question is: Why is the reaction time of an ultra-low quiescent current linear voltage regulator higher
than that for a standard one?
3.2
Influences on Control Loop’s Reaction Time
Therefore, let’s discuss the influences on the control loop’s reaction time.
For this, we’ll look inside a linear voltage regulator. Figure 6 delivers a simplified insight, one can see that all
basic functions like the voltage reference, the error amplifier and the protection functions need to be biased.
I
Q
[Input]
[Output]
Ibias
Driver
Protect
+
–
Linear Voltage Regulator, e.g. TLE7274-2
Figure 6
Biasing inside a Linear Voltage Regulator
Application Note
8
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
But what has biasing to do with reaction time, where do the delays come from?
In silicon technology, delays are generally caused by capacitances. Let’s now see, why these capacitances are
present. Some of these are intentionally integrated capacitors, added by the IC designer, some of them are
unwanted, but inevitable due to the used silicon technology. These are then the so-called parasitic
capacitances.
Both types cause delays within the silicon according to Equation (3.1)
t
delay
C ⋅ ΔV
= ---------------I
bias
(3.1)
Note: This simplified formula provides a sufficient indication as bias currents and capacitance values are
nearly constant.
Let’s now apply one theoretical example: Assume that the bias current for a device is Ibias = 30 µA, an
integrated capacitor has C = 30 pF, and there’s a voltage delta of ∆V = 1 V. The calculated delay according to
Equation (3.1) would be then tdelay = 1 µs.
Parasitic Capacitances
This type of capacitance is unwanted, but present in every silicon technology. It is typically not one specific
capacitor, but a capacitance value that is resulting from several parasitic effects. Hence it is difficult to
quantize it to concrete values, but we’ll at least give an indication here: Depending on the silicon technology,
it is normally in the range of several pF and has at linear voltage regulators a minor, but not neglectable
influence on delays. By using the above formula and a voltage delta of ∆V = 0.5 V, one would get delays in the
range of 0.5 µs .. 1 µs per capacitance for an ultra-low quiescent current concept, for a non-ultra-low
quiescent current concept considerably less (tdelay < 0.05 µs per capacitor). Again, these values are intended
as theoretical indication to understand the influence of parasitics within the technology on the delays, in real
products different values can occur.
Power Stage
Depending on the application’s requirements, different kinds of power stages are used for linear voltage
regulators. Basically two types of power stage families can be found, bipolar types and MOSFET types.
To get an idea on the delays occurring in these different types, let’s simplify their circuitry like shown in
Figure 7. Also here, all values need to be considered as indication to understand the power stage’s influence
on delays, as well as the difference between concepts and power stage types. Real products might show
different values.
Application Note
9
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
Bipolar
Ibias
Icomp
Ccomp
Ibase
tdelay =
Cpar
Vc
MOSFET
Ibias
C * ΔV
Vc
Icomp
tdelay =
Cgate
standard 1.
Icomp = 50µA
=> t = 0.8µs
2. Icomp = 5µA
=> t = 8µs
standard 1.
Figure 7
Ibias
Examples:
VC = 6V, Cgate = 30pF
Examples:
VC = 0.8V, Ccomp = 50pF
ultra-low
quiescent current
C * ΔV
ultra-low
quiescent current
Ibias = 50µA
=> t = 3.6µs
2. Ibias = 5µA
=> t = 36µs
Delays in Power Stages
In a bipolar power stage, the main delay results from the compensation capacitor and the parasitic base
capacitance. In a MOSFET power stage, the delay is caused by the gate capacitance, worst case occurs during
start-up, when the gate capacitance needs to be fully charged (∆V = VC = 6 V).
Important to notice is that the delay caused by MOSFET power stages is typically higher, and the delay caused
by ultra-low quiescent current concepts is even higher.
To sum up, the longest delay is caused by the combination of an ultra-low quiescent current concept with a
MOSFET power stage. Therefore, linear voltage regulators implementing this combination show a slower
reaction time of the control loop than standard linear voltage regulators.
Output Capacitor
At the output of a linear voltage regulator a capacitor (“Output Capacitor”) needs to be connected. Depending
on the implemented control loop concept, it acts either as simple buffer, or is in addition even part of the
control loop and plays therefore an important role in terms of stability.
In terms of control loop’s reaction time, as a simple qualitative approach, it can be summarized that - similar
to the above mentioned capacitances - higher capacitance values of the output capacitor cause slower
reaction times of the control loop.
3.3
Consequences for the Application
We have seen so far that the reaction time of a linear voltage regulator’s control loop is caused by delays
resulting from different capacitances. It was also shown that this reaction time causes variations on the
voltage regulator’s output voltage at the moment of current transients. Now, let’s concentrate on the
consequences that these voltage variations have within an application.
Potential Malfunction of Load
Every load connected to a linear voltage regulator’s output has a specified operating range. Let’s take as
example Infineon’s XC2000 microcontrollers: They specify proper operation for the “Upper Voltage Range”
within a supply voltage range between 4.5 V .. 5.5 V. In case the supply voltage, that is the linear voltage
regulator’s output voltage, is outside this range even for short time, proper operation is not ensured any more.
A reliable supply behaviour is shown in Figure 8.
Application Note
10
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
Standard
70mA
IQ
1mA
t / µs
VQ
Max. Operating
Voltage Load
Max. Voltage
5V
Min. Voltage
Min. Operating
Voltage Load
t / µs
Figure 8
Reliable Supply, Voltage Variation at Current Transients within Load’s Operating Range
Potential Damage of Load
Another painful, but least probable consequence might happen when the voltage variations of the load’s
supply are exceeding its absolute maximum ratings. For example, many microcontrollers have 6 V as absolute
maximum voltage specified; in this case, any voltage peak higher than 6 V on the supply voltage must be
avoided. However, as the absolute maximum ratings are always at least as high as the maximum value of the
operating range, the occurrence of this effect is less probable. Figure 9 shows, how the supply voltage should
be.
Standard
70mA
IQ
1mA
t / µs
VQ
! Absolute Maximum Rating Load !
Max. Operating
Voltage Load
Max. Voltage
5V
Min. Voltage
Min. Operating
Voltage Load
t / µs
Figure 9
Reliable Supply, Voltage Variation at Current Transients within Load’s Operating Range and
below its Absolute Maximum Rating
Most Probable when Voltage Monitoring Present: Unwanted Reset Triggered
The two previously mentioned cases are not necessarily detected at the moment of their occurrence, except
if the load is completely damaged. Compared to this, the effect of high voltage variations on the voltage
regulator’s output voltage is most probably seen, when a circuit monitoring the voltage is integrated and the
voltage variations lead to voltage drops below the monitoring threshold.
Application Note
11
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
This can be e.g. a Reset feature integrated in the voltage regulator. In this case, an additional pin on the
voltage regulator is connected to a supplied microcontroller’s RST pin. When the monitored voltage (= voltage
regulator’s output voltage = microcontroller’s supply voltage) falls below the Reset Threshold, the additional
pin resets the microcontroller. This can also happen for short voltage drops at current transients: When the
voltage drop at a current transient is too high and falls below the Reset Threshold, an (unwanted) reset is
triggered. Figure 10 illustrates an acceptable voltage drop at a current transient.
IQ
Standard
70mA
1mA
t / µs
VQ
5V
Min. Voltage
ΔV
Reset Threshold
Min. Operating
Voltage Load
t / µs
Figure 10
Reliable Supply, Voltage Drop at Current Transient above Reset Threshold
3.4
Influence of the Output Capacitor
So far, we have seen why voltage variations at a voltage regulator’s output voltage can occur at current
transients and what are potential consequences then. But we still don’t know yet what can be done to prevent
these consequences. Therefore, let’s now focus on the influence of the voltage regulator’s output capacitor.
As mentioned earlier, at the output of a linear voltage regulator a capacitor needs to be connected. We’ve
heard that - depending on the implemented control loop concept - this output capacitor acts either as simple
buffer, or is in addition even a part of the control loop and plays therefore an important role regarding the
loop’s stability. Infineon’s voltage regulators that implement these concepts, specify a min. value for the
output capacitor that is at least required, and a range for its parasitic series resistor (“Equivalent Series
Resistance” = “ESR”) that has to be respected to maintain stable regulation. Figure 11 shows this
specification for the TLE7273-2.
Figure 11
Specification of Requirements for TLE7273-2’s Output Capacitor
These specified requirements for the output capacitor need to be respected at all times.
Application Note
12
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
Apart from this, as the capacitor acts always as a buffer, it has always a direct influence on the voltage
variation at load steps. Let’s now see why.
IQ Ultra Low-Quiescent Current
70mA
t / µs
(e.g.) VBatt
Q 1 mA
I
VQ
5V
CQ
VESR
ΔVC (discharge capacitor)
Snapshot
Vreg, e.g.
TLE7273-2
theoretical capacitor
discharge curve
ESR
1mA
C
IQ
VQ
VESR
VC
t / µs
time of snapshot
Figure 12
Equivalent Circuit for Snapshot of Linear Voltage Regulator at Current Transient
Figure 12 shows at the left side again the voltage variation at a current transient for an ultra-low quiescent
current linear voltage regulator. The equivalent circuit at the right side shows a snapshot of the voltage
regulator at the moment the current transient occurs. At this point, the voltage regulator doesn’t recognize
the transient yet and can be modelled by a current source adjusted to the previous current, for our example
to 1 mA. Nearly all current (70 mA - 1 mA) needs therefore to be sourced by the output capacitor CQ. As every
capacitor contains an ESR, at the output voltage a small immediate drop can be recognized at this point.
Then, as long as the voltage regulator’s control loop doesn’t react yet, CQ still needs to source nearly all
current (70 mA - 1 mA), the output voltage has therefore the shape of a typical capacitor discharge curve
during this period and is dropping by ∆VC.
It is now obvious that increasing the capacitance value of CQ leads to a smaller voltage variation because of a
smaller ∆VC. This ∆VC is smaller as a bigger capacitor buffers more energy and is discharged more slowly. This
is shown in Figure 13.
Just to mention, it can be also seen in this figure, what we heard in Chapter 3.2: Increasing the output
capacitance increases the control loop’s reaction time. This means, even if increasing the capacitance value
leads to smaller voltage variations, the relation between capacitance and voltage variation is not
proportional, e.g. the double of the capacitor’s size does not lead to half of the voltage variation, its still more
than a half.
Application Note
13
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Theoretical Background on Ultra-Low Quiescent Current Control Loop Concepts
IQ Ultra Low-Quiescent Current
IQ Ultra Low-Quiescent Current
70mA
70mA
1mA
t / µs 1mA
VQIncreased Output Capacitance
VQ
5V
VESR
5V
ΔVC (discharge capacitor)
theoretical capacitor
discharge curve
Figure 13
t / µs
VESR
ΔVC (discharge capacitor)
theoretical capacitor
discharge curve
t / µs
Reaction Time
Reaction Time
Comparison Variation of Output Voltage at Current Transient for Different Output
Capacitance Values
t / µs
Generally speaking, higher capacitance values will always lower the voltage variations. This is especially
important to keep in mind for ultra-low quiescent current linear voltage regulators, as their control loop is
optimized for ultra-low quiescent current and shows a slower reaction time due to very small bias currents.
Hence, for an application requiring ultra-low quiescent current, the output capacitor of the linear voltage
regulator has to be sized accordingly, not only to fulfill the voltage regulator’s requirements in terms of
stability, but also to buffer sufficiently the worst-case current transients within the application.
3.5
Conclusion
Let’s summarize what we have seen so far:
•
Control loop’s reaction time at current transients influences the linear voltage regulator’s output voltage:
The faster the reaction time, the smaller variations of the output voltage.
•
Control loop’s reaction time is caused by delays within the silicon. It depends therefore on the capacitance
values of integrated capacitors respectively parasitic effects and the bias currents. The higher the
capacitances and the lower the bias currents, the longer the delays and the reaction time.
•
Potential risks of too high voltage variations are: 1. triggering unwanted Reset, 2. malfunction of supplied
microcontroller by exceeding its specified operating range, 3. damage of load by exceeding its max.
ratings.
•
Output capacitor of linear voltage regulator buffers output voltage at current transients: Increasing the
capacitance lowers the voltage variations at current transients and avoids the mentioned risks when
dimensioned correctly.
From this information, we can conclude:
Keeping the quiescent current at a linear voltage regulator ultra-low and at the same time the output
capacitor very small, is physically contradicting, as these regulators must have very low bias currents to keep
the current consumption ultra-low. Therefore, at current transients, the control loop’s reaction time is always
slower and the voltage variations are always higher than at a standard linear voltage regulator. Thus, one
needs to pay attention, when these voltage regulators are used in an application where big current transients
can occur: Even if a control loop concept is implemented that requires only small capacitors to maintain loop
stability, higher capacitance values might be needed to buffer big current transients and avoid risks like
unwanted resets or malfunction of supplied microcontrollers.
Application Note
14
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
4
Dimensioning the Output Capacitor of an Ultra-Low Quiescent
Current Voltage Regulator
Infineon’s ultra-low quiescent current voltage regulators combine the ultra-low quiescent current concept
with a MOSFET power stage. This offers the lowest possible current consumption even at sophisticated
feature sets and requires only very small output capacitance values for the control loop’s stability (TLE72732: CQ ≥ 470 nF). Another advantage of the MOSFET power stage is that the current consumption remains ultralow even at high output currents.
Due to this ultra-low quiescent current concept, the control loop’s reaction on current transients is slower
than for standard voltage regulators. This correlation is demonstrated in Chapter 3. When these voltage
regulators are used in applications where big current transients can occur, e.g. at the transition from standby to normal operation, high voltage variations at the voltage regulators’ output voltage with consequences
on the application (see Chapter 3.3) might result.
To avoid these high voltage variations, basically two solutions are possible:
1. Avoid big current transients wherever possible.
2. Increase the output capacitor to buffer the voltage regulator’s output voltage.
In the following we will first provide hints on 1., how to avoid big current transients, then focus on 2. and
provide a method to dimension the output capacitor correctly.
4.1
How to Avoid Big Current Transients
To keep the output capacitor’s size as small as possible, one should first of all try to avoid big current
transients within the application. As the most critical transients appear at the start up or at the transition from
standby to normal operation, here several recommendations:
•
Many microcontrollers provide solutions for a so-called “soft-start”, the different blocks are then started
step by step; if possible for your application, apply this kind of start up.
•
If your microcontroller doesn’t implement a “soft-start”, try to manually implement delays during the
microcontroller’s start up and avoid a start of all blocks at the same time.
•
Switch off I/Os by default when starting the microcontroller and switch them on step by step after a delay.
•
If several loads are present in the application, switch them on step by step after a delay.
4.2
How to Dimension the Output Capacitor
Step 1: Check for Worst-Case Current Transients within Application
As mentioned above, first make sure to avoid big current transients as much as possible. When this is ensured,
the worst-case current transients within the application need to be evaluated. A “worst-case” current
transient is a high transient starting at very low currents < 5 mA.
It can be estimated, when a detailed documentation of all loads is available. It would be then mostly at the
transition from stand-by (very low current) to normal operation (high current), when several loads are
switched on at the same time.
The more realistic (and mostly easier) way is to measure it at the bench. For this, run the application and
watch either the overall supply current or - if possible - the voltage regulator’s output current as well as its
output voltage at the scope (see Figure 14). At this time, connect as output capacitor a type that just respects
the minimum values required for stability, no need to consider its buffer effect yet. Try to run all possible
Application Note
15
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
application conditions and note the current transients caused by their transitions. As mentioned, note
especially high current transients starting at very small currents < 5 mA, these are generally the most critical.
I
Linear VReg
e.g.
TLE7274-2
VBatt
Figure 14
IQ
Q
CQ
GND
VQ
Load
e.g.
µC
Measurement Setup for Evaluation of Worst Case Current Transients
Step 2: Define Maximum Allowed Voltage Variation ∆Vmax at Current Transient
Now, you need to define, which maximum voltage variation at a worst case current transient is allowed within
your application. As this definition strongly depends on the application, we concentrate here on providing
generic hints:
In case you have an undervoltage monitor, e.g. Reset, implemented, its threshold voltage might be a target as
minimum voltage to maintain. Example: Let’s assume an implemented Reset with a threshold at max. 4.8 V
for a 5 V supply voltage, a target for the maximum allowed voltage variation would be ∆Vmax = 5 V 4.8 V = 200 mV.
Another parameter that should be considered is the operating range of connected loads. As a malfunction of
the load could be a consequence, its operating range should not be exceeded. Example: The min. operating
voltage of a connected microcontroller is at 4.5 V. When supplying it with 5 V and no undervoltage monitor
present, a target for the maximum allowed voltage variation would be ∆Vmax = 5 V - 4.5 V = 500 mV.
In Figure 15 both scenarios are shown.
In any case, we recommend to consider within your maximum voltage variation a reasonable safety margin.
Application Note
16
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
IQ
70mA
1mA
VQ
example for worst case
current transient
t / µs
5V
Min. Allowed
Voltage
Reset Threshold
Min. Operating
Voltage Load
ΔVmax
t / µs
Figure 15
Example for Definition of the Maximum Allowed Voltage Variation ∆Vmax
Step 3: Select Output Capacitor
Now we know the “worst-case” current transient as well as the max. allowed voltage variation. With this
information, the graphs in “Typ. Perf. Characteristics Output Voltage Variation ∆VQ vs. Output Current
Transient ∆IQ” on Page 19 can be used as guideline for selecting the right capacitor. These graphs show for
different output capacitors the relation between voltage variation ∆VQ and current transient ∆IQ, using
different start currents as a parameter. They were generated for all of Infineon’s ultra-low quiescent current
voltage regulators that are available today and are therefore valid for:
•
TLE7270-2
•
TLE7272-2
•
TLE7273-2
•
TLE7274-2
•
TLE7276-2
•
TLE7278-2
•
TLE7279-2
Note: As the worst-case for the voltage regulator is at high temperature and as current transients won’t
happen at Tj = -40 °C, graphs are provided for two temperatures each, Tj = 25 °C and Tj = 150 °C.
Let’s take an example to see how to work with these graphs:
We assume as worst-case current transient 0.5 mA to 60 mA and a max. allowed voltage variation of 200 mV.
Furthermore, let’s say that the voltage regulator can reach at the worst-case transient high temperatures.
With this information, we concentrate only on the graphs where Tj = 150 °C. At all these graphs, we need to
consider only the curve with IQ1 = 0.5 mA as a parameter, as our worst-case transient starts at 0.5 mA. Start at
the curve for a 470 nF capacitor. At the considered curve, check at 60 mA whether the voltage variation is less
than 200mV. If not, go on with the curve for the next capacitor and do the check again, otherwise you have
found a potentially suitable capacitor for your application and can proceed with Step 4. This example is
illustrated in Figure 16.
Application Note
17
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
Step 4: Verify Selected Output Capacitor within Application
After having selected a potential output capacitor, you should solder it in the application and run it again at
all possible conditions, like done at Step 1. Watch again the supply current / voltage regulator’s output current
and its output voltage with the scope like shown in Figure 14. In case the voltage variation stays within your
defined maximum value, the selected capacitor is the right one. Otherwise, go for the next higher capacitor
value and do the verification test again.
Figure 16
Example for Selection of a Suitable Capacitor Using the Below Graphs
Application Note
18
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
Typ. Perf. Characteristics Output Voltage Variation ∆VQ vs. Output Current Transient ∆IQ1)
Conditions: CQ = 470 nF ceramic type; Tj = 25 °C
Conditions: CQ = 470 nF ceramic type; Tj = 150 °C
0,90
0,90
I Q1 = 0.1 mA
I Q1 = 0.5mA
0,80
0,80
I Q1 = 0.1 mA
I Q1 = 1 mA
0,70
0,70
I Q1 = 0,5 mA
I Q1 = 1 mA
I Q1 = 5 mA
0,60
0,60
I Q1 = 10 mA
0,50
Δ V Q [V]
Δ V Q [V]
I Q1 = 5 mA
I Q1 = 10mA
0,40
0,50
0,40
0,30
0,30
0,20
0,20
0,10
0,10
0,00
0,00
0
30
60
90
120 150 180
210 240
0
270 300
30
60
90
120 150 180
210 240
270 300
Δ I Q [mA]
Δ I Q [mA]
Conditions: CQ = 1 µF ceramic type; Tj = 25 °C
Conditions: CQ = 1 µF ceramic type; Tj = 150 °C
0,90
0,90
0,80
0,80
I Q1 = 0.1 mA
I Q1 = 0.1 mA
I Q1 = 0.5mA
0,70
0,70
I Q1 = 0,5 mA
I Q1 = 1 mA
I Q1 = 1 mA
0,60
0,60
I Q1 = 5 mA
0,50
Δ V Q [V]
Δ V Q [V]
I Q1 = 5 mA
I Q1 = 10mA
0,40
I Q1 = 10 mA
0,50
0,40
0,30
0,30
0,20
0,20
0,10
0,10
0,00
0,00
0
30
60
90
120 150 180
210 240
0
270 300
Application Note
30
60
90
120 150 180
210 240
270 300
Δ I Q [mA]
Δ I Q [mA]
19
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
Typ. Perf. Characteristics Output Voltage Variation ∆VQ vs. Output Current Transient ∆IQ1)
Conditions: CQ = 2.2 µF ceramic type; Tj = 25 °C
Conditions: CQ = 2.2 µF ceramic type; Tj = 150 °C
0,80
0,80
I Q1 = 0.1 mA
0,70
I Q1 = 0.1 mA
0,70
0,60
I Q1 = 0,5 mA
0,60
I Q1 = 1 mA
0,50
I Q1 = 5 mA
I Q1 = 0.5mA
I Q1 = 1 mA
I Q1 = 5 mA
Δ V Q [V]
Δ V Q [V]
0,50
I Q1 = 10mA
0,40
0,40
0,30
0,30
0,20
0,20
0,10
0,10
I Q1 = 10 mA
0,00
0,00
0
30
60
90
120 150 180
210 240
0
270 300
30
60
90
120
150 180
210 240
270 300
Δ I Q [mA]
Δ I Q [mA]
Conditions: CQ = 4.7 µF ceramic type; Tj = 25 °C
Conditions: CQ = 4.7 µF ceramic type; Tj = 150 °C
j
0,70
0,70
0,60
0,60
I Q1 = 0.1 mA
I Q1 = 0.5mA
I Q1 = 0.1 mA
I Q1 = 0,5 mA
0,50
I Q1 = 1 mA
0,50
I Q1 = 1 mA
I Q1 = 5 mA
0,40
Δ V Q [V]
Δ V Q [V]
I Q1 = 5 mA
I Q1 = 10mA
0,30
I Q1 = 10 mA
0,40
0,30
0,20
0,20
0,10
0,10
0,00
0,00
0
30
60
90
120 150 180
210 240
0
270 300
Application Note
30
60
90
120
150 180 210 240
270 300
Δ I Q [mA]
Δ I Q [mA]
20
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
Typ. Perf. Characteristics Output Voltage Variation ∆VQ vs. Output Current Transient ∆IQ1)
Conditions: CQ = 6.8 µF tantalum type, ESR = 0.3 Ω; Conditions: CQ = 6.8 µF tantalum type,
Tj = 25 °C
ESR = 0.3 Ω; Tj = 150 °C
0,60
0,60
0,50
0,50
I Q1 = 0.1 mA
I Q1 = 0.5mA
I Q1 = 0.1 mA
I Q1 = 1 mA
I Q1 = 0,5 mA
0,40
I Q1 = 1 mA
0,30
I Q1 = 5 mA
I Q1 = 10 mA
I Q1 = 5 mA
Δ V Q [V]
Δ V Q [V]
0,40
I Q1 = 10mA
0,30
0,20
0,20
0,10
0,10
0,00
0,00
0
30
60
90
120 150 180
210 240
270 300
0
30
60
90
Δ I Q [mA]
120
150 180
210 240
270 300
Δ I Q [mA]
Conditions: CQ = 10 µF tantalum type, ESR = 0.26 Ω; Conditions: CQ = 10 µF tantalum type,
Tj = 25 °C
ESR = 0.26 Ω; Tj = 150 °C
0,50
0,50
0,45
0,45
I Q1 = 0.1 mA
I Q1 = 0.5mA
I Q1 = 0,5 mA
0,40
I Q1 = 0.1 mA
0,40
I Q1 = 1 mA
I Q1 = 1 mA
0,35
I Q1 = 5 mA
0,35
I Q1 = 5 mA
I Q1 = 10 mA
0,30
I Q1 = 10mA
Δ V Q [V]
Δ V Q [V]
0,30
0,25
0,25
0,20
0,20
0,15
0,15
0,10
0,10
0,05
0,05
0,00
0,00
0
30
60
90
120 150 180
210 240
270 300
0
Δ I Q [mA]
Application Note
30
60
90
120
150
180 210
240 270
300
Δ I Q [mA]
21
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Dimensioning the Output Capacitor of an Ultra-Low Quiescent Current Voltage
Typ. Perf. Characteristics Output Voltage Variation ∆VQ vs. Output Current Transient ∆IQ1)
Conditions: CQ = 22 µF tantalum type, ESR = 0.26 Ω; Conditions: CQ = 22 µF tantalum type,
Tj = 25 °C
ESR = 0.26 Ω; Tj = 150 °C
0,50
0,50
I Q1 = 0.1 mA
0,45
0,45
0,40
0,40
0,35
0,35
I Q1 = 0.1 mA
I Q1 = 0,5 mA
0,30
I Q1 = 1 mA
Δ V Q [V]
Δ V Q [V]
0,30
0,25
I Q1 = 5 mA
I Q1 = 10mA
0,20
I Q1 = 0.5mA
I Q1 = 1 mA
0,25
I Q1 = 5 mA
I Q1 = 10 mA
0,20
0,15
0,15
0,10
0,10
0,05
0,05
0,00
0
30
60
90
120 150 180
210 240
0,00
270 300
0
Δ I Q [mA]
30
60
90
120 150 180 210
240 270 300
Δ I Q [mA]
Conditions: CQ = 47 µF tantalum type, ESR = 0.3 Ω; Conditions: CQ = 47 µF tantalum type, ESR = 0.3 Ω;
Tj = 25 °C
Tj = 150 °C
0,50
0,30
0,45
0,25
I Q1 = 0.1 mA
0,40
I Q1 = 0.1 mA
0,35
I Q1 = 0.5mA
0,20
I Q1 = 1 mA
I Q1 = 5 mA
Δ V Q [V]
Δ V Q [V]
0,30
0,25
I Q1 = 0,5 mA
I Q1 = 10 mA
0,15
I Q1 = 1 mA
0,20
I Q1 = 5 mA
0,15
0,10
I Q1 = 10mA
0,10
0,05
0,05
0,00
0
30
60
90
120 150 180
210 240
0,00
270 300
0
Δ I Q [mA]
30
60
90
120 150 180 210
240 270 300
Δ I Q [mA]
1) valid for: TLE7270-2, TLE7272-2, TLE7273-2, TLE7274-2, TLE7276-2, TLE7278-2, TLE7279-2
Application Note
22
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Additional Information
5
Additional Information
Information regarding the product portfolio of Linear Voltage Regulators with ultra-low quiescent current as
well as the related data sheets containing the product information and specification can be found on our
webpage: http://www.infineon.com.
Application Note
23
Rev. 1.01, 2014-09-26
Robust Dimensioning of the Output Capacitor
Z8F52274290
Revision History
6
Revision History
Revision
Date
Changes
1.01
2014-09-26
Infineon Style Guide update; Editorial changes
1.0
2009-09-30
Initial document
Application Note
24
Rev. 1.01, 2014-09-26
Trademarks of Infineon Technologies AG
AURIX™, C166™, CanPAK™, CIPOS™, CIPURSE™, CoolGaN™, CoolMOS™, CoolSET™, CoolSiC™, CORECONTROL™, CROSSAVE™, DAVE™, DI-POL™, DrBLADE™,
EasyPIM™, EconoBRIDGE™, EconoDUAL™, EconoPACK™, EconoPIM™, EiceDRIVER™, eupec™, FCOS™, HITFET™, HybridPACK™, ISOFACE™, IsoPACK™, iWafer™, MIPAQ™, ModSTACK™, my-d™, NovalithIC™, OmniTune™, OPTIGA™, OptiMOS™, ORIGA™, POWERCODE™, PRIMARION™, PrimePACK™,
PrimeSTACK™, PROFET™, PRO-SIL™, RASIC™, REAL3™, ReverSave™, SatRIC™, SIEGET™, SIPMOS™, SmartLEWIS™, SOLID FLASH™, SPOC™, TEMPFET™,
thinQ!™, TRENCHSTOP™, TriCore™.
Other Trademarks
Advance Design System™ (ADS) of Agilent Technologies, AMBA™, ARM™, MULTI-ICE™, KEIL™, PRIMECELL™, REALVIEW™, THUMB™, µVision™ of ARM Limited,
UK. ANSI™ of American National Standards Institute. AUTOSAR™ of AUTOSAR development partnership. Bluetooth™ of Bluetooth SIG Inc. CAT-iq™ of DECT
Forum. COLOSSUS™, FirstGPS™ of Trimble Navigation Ltd. EMV™ of EMVCo, LLC (Visa Holdings Inc.). EPCOS™ of Epcos AG. FLEXGO™ of Microsoft
Corporation. HYPERTERMINAL™ of Hilgraeve Incorporated. MCS™ of Intel Corp. IEC™ of Commission Electrotechnique Internationale. IrDA™ of Infrared Data
Association Corporation. ISO™ of INTERNATIONAL ORGANIZATION FOR STANDARDIZATION. MATLAB™ of MathWorks, Inc. MAXIM™ of Maxim Integrated
Products, Inc. MICROTEC™, NUCLEUS™ of Mentor Graphics Corporation. MIPI™ of MIPI Alliance, Inc. MIPS™ of MIPS Technologies, Inc., USA. muRata™ of
MURATA MANUFACTURING CO., MICROWAVE OFFICE™ (MWO) of Applied Wave Research Inc., OmniVision™ of OmniVision Technologies, Inc. Openwave™ of
Openwave Systems Inc. RED HAT™ of Red Hat, Inc. RFMD™ of RF Micro Devices, Inc. SIRIUS™ of Sirius Satellite Radio Inc. SOLARIS™ of Sun Microsystems,
Inc. SPANSION™ of Spansion LLC Ltd. Symbian™ of Symbian Software Limited. TAIYO YUDEN™ of Taiyo Yuden Co. TEAKLITE™ of CEVA, Inc. TEKTRONIX™ of
Tektronix Inc. TOKO™ of TOKO KABUSHIKI KAISHA TA. UNIX™ of X/Open Company Limited. VERILOG™, PALLADIUM™ of Cadence Design Systems, Inc.
VLYNQ™ of Texas Instruments Incorporated. VXWORKS™, WIND RIVER™ of WIND RIVER SYSTEMS, INC. ZETEX™ of Diodes Zetex Limited.
Trademarks Update 2014-07-17
www.infineon.com
Edition 2014-09-26
Published by
Infineon Technologies AG
81726 Munich, Germany
© 2014 Infineon Technologies AG.
All Rights Reserved.
Do you have a question about any
aspect of this document?
Email: [email protected]
Document reference
Doc_Number
Legal Disclaimer
THE INFORMATION GIVEN IN THIS APPLICATION NOTE
(INCLUDING BUT NOT LIMITED TO CONTENTS OF
REFERENCED WEBSITES) IS GIVEN AS A HINT FOR THE
IMPLEMENTATION OF THE INFINEON TECHNOLOGIES
COMPONENT ONLY AND SHALL NOT BE REGARDED AS
ANY DESCRIPTION OR WARRANTY OF A CERTAIN
FUNCTIONALITY, CONDITION OR QUALITY OF THE
INFINEON TECHNOLOGIES COMPONENT. THE
RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY
ANY FUNCTION DESCRIBED HEREIN IN THE REAL
APPLICATION. INFINEON TECHNOLOGIES HEREBY
DISCLAIMS ANY AND ALL WARRANTIES AND
LIABILITIES OF ANY KIND (INCLUDING WITHOUT
LIMITATION WARRANTIES OF NON-INFRINGEMENT OF
INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD
PARTY) WITH RESPECT TO ANY AND ALL INFORMATION
GIVEN IN THIS APPLICATION NOTE.
Information
For further information on technology, delivery terms
and conditions and prices, please contact the nearest
Infineon Technologies Office (www.infineon.com).
Warnings
Due to technical requirements, components may
contain dangerous substances. For information on the
types in question, please contact the nearest Infineon
Technologies
Office.
Infineon
Technologies
components may be used in life-support devices or
systems only with the express written approval of
Infineon Technologies, if a failure of such components
can reasonably be expected to cause the failure of that
life-support device or system or to affect the safety or
effectiveness of that device or system. Life support
devices or systems are intended to be implanted in the
human body or to support and/or maintain and
sustain and/or protect human life. If they fail, it is
reasonable to assume that the health of the user or
other persons may be endangered.