Active Passives NATIONAL

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Active Passives- Demystifying Capacitors &

Inductors in Portable Power Systems

Mathew Jacob

Today’s topic is Active Passives - Demystifying Capacitors and Inductors in Portable Power Systems. Today’s seminar will be given by Applications Engineering Manager, Mathew Jacob.

Active Passives - Demystifying Capacitors and Inductors in Portable Power Systems. The title for this presentation is very interesting. Normally, we expect passives to just have the manufacturing tolerances. This can be either plus or minus 20% or plus or minus 10% or as per the manufacturer’s data sheet. But what we see in portable power systems as the component sizes become smaller and smaller, the passives are becoming more challenging and there’s quite a lot of variation associated with it, and this seminar looks at this in detail.

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© 2006 National Semiconductor Corporation© 2005 National Semiconductor Corporation

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Contents

•Capacitors•Inductors•Appendix – Tantalum capacitors

The contents for the seminar – capacitors, inductors, and in the end we have an appendix section, tantalum capacitors.

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Components do vary –in fact quite a lot

The old myth of component variation +/-20% is no longer applicable. Component manufacturers are becoming aggressive and perhaps releasing parts that are just good enough to keep up with the competitors in the size vs. value war.

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Components do vary, in fact, quite a lot. The old myth of component variation plus or minus 20% is no longer applicable. Component manufacturers are becoming aggressive and perhaps releasing parts that are just good enough to keep up with the competitors in the size versus value war.

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CAPACITORS

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Capacitors. This section of the seminar deals with ceramic capacitors.

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Ceramic capacitors: EIAJ-EIA Case Size Conversion

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In the ceramic capacitors, there are two naming conventions for the case sizes: one is a metric system and the second is an English system. So we have to be careful when we are comparing the capacitors from different manufacturers. Some use the metric system whereas some use the English system. For example, an 0603 case size capacitor in the English system is called a 1608 case size in the metric system.

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Ceramic capacitors: Impedance and Capacitance

Typical ESR-/Z/ Curve

Here in this slide, we can see the impedance and ESR versus frequency for a typical ceramic capacitor. And also, in the slide here, you see a formula for the ceramic capacitor value. As you can see here, it’s proportional to a constant, the thickness of the layer and the numbers of layers are all factors that determine the capacitance value.

What we see towards the right in the figure is the impedance starts dropping as we go higher in frequency because the impedance of a capacitor is 1 by 2 piFC and as the frequency goes higher, the impedance keeps dropping. At a certain point, the capacitance has a resistance frequency and the impedance, the capacitive portion of the capacitor and the inductive portion of the capacitor cancels out each other and you have just the resistor which is the ESR. And that’s what you see a dip in the impedance at 2 megahertz at that point. As we go again higher in frequency, the capacitor impedence becomes inductive.

At 2 megahertz, the ESR for a typical ceramic capacitor is below 10 milliohms and this makes a very good factor when you’re doing a load transit. And also, the stability of a converter has to be designed for this low ESR.

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Characteristics of Impedance

Impedance vs. Frequency

This is, again, a presentation of the impedance versus frequency curve. As we see on the left side, at lower frequencies it’s capacitive. Towards the mid region, 2 megahertz as we saw in the previous chart, the capacitive impedance is equal to the inductive impedance and the impedance of the capacitor is equal to the ESR. As you go again higher on frequency, the inductive portion takes over and you can see the impedance of the capacitor increases as we go higher in frequency.

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Equivalent Series Resistance

Factors Affecting ESR

• Inner electrode thickness• Inner electrode material• Number of layers• Inner electrode aspect ratio

(L/W)• Limitations due to component

thickness• Number of paralleled

terminations• Electrode coplanarity (surface

flatness)• Electrode metallization density• Application frequency (yes, ESR

changes)

Equivalent series resistance. What are the factors affecting ESR? We can see the couple of factors that affect ESR and some of them are: number of layers, the number of paralleled terminations, the electrode metallization density, and also it’s very important, the application frequency affects ESR. For example, if the converter is operating at 100 kilohertz compared to 2 megahertz. The capacitor ESR is going to be very different and as we can see on the right, a 10-microfarad capacitor esr chart is shown for different case sizes.

One thing we notice here is as we go towards a higher case size capacitor, 10 microfarad, the ESR is lower. Again, what happens if you keep the same case size and keep increasing the capacitance? For example, 10 microfarad in 2012 case size, has a higher esr compared to a 100-microfarad capacitor. The ESR is lower for the higher value capacitor in the same case size.

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Equivalent Series Inductance

Factors Affecting ESL

• Case size• Inner electrode aspect

ratio (L/W)• Number of layers• Number of paralleled

terminations• Cover layer thickness

Equivalent series inductance. Factors affecting ESL: the case size, the number of layers, and again, the number of paralleled terminations affect ESL. The figure on the right, what is seen here is the same 10-microfarad capacitor in different case sizes. What we can notice is the smaller case size has the smallest ESL compared to the bigger case size.

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ESL – Values by case size

This is a pretty interesting table. It gives you the ESL values by case size and as mentioned earlier, the smaller case size capacitors like 0603 have the smallest ESL compared to the bigger case sizes.

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10uF,6.3V cap

1206

0805

0603

0402 2009

1996

2000

2004

2.2uF available 2005

Ceramic capacitors-a look at timelines

3.2mm x 1.6mm x 1.6/0.85mm(H)

2mm x 1.25mm x 1.25/0.85mm(H)

1.6mm x 0.8mm x 0.8mm(H)

1mm x 0.5mm x 0.8mm(H)

Ceramic capacitors. Let’s look at the timelines. For this, I am considering a 10-microfarad, 6.3-volt cap. It’s interesting to look at the history of this capacitor value. It was available in 1996 as a 1206 case size capacitor. The dimensions of this capacitor also are given towards the right – 3.2 times 1.6 millimeter and the height was 1.6 millimeter and also available in 0.85mm. In 2000, this capacitor’s value changed its case size to an 0805 case size capacitor. The dimensions are smaller as we see on the right side. In 2004, it is available in an 0603 case size capacitor. The dimensions are 1.6 millimeter times 0.8 millimeter and the height also is 0.8 millimeter.

In 2009, it is predicted that this 10 microfarad, 6.3 volt will be available in an 0402 case size capacitor. The 2.2 microfarad is available right now in an 0402 case size.

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-50 0 50 100Temp [degC]

Cap

acita

nce

[uF] 10uF, 6.3V, A

10uF, 6.3V, B

4.7uF, 6.3V, A4.7uF, 6.3V, B

10uF, 6.3V, 0805, A

10uF, 6.3V, C

• All caps are 0603 unless mentioned• DC bias=1.8V

Capacitance values vs. dc bias and temperature

Let’s look at capacitance values versus DC bias and temperature. Typically, capacitor tolerances are plus or minus 20% or plus or minus 10% from the manufacturer. Apart from this, there is also a temperature variation of plus or minus 15% say for an X5R or an X7R capacitor. Not regarding all this, another factor that we have to consider, DC bias. Here we can see a 10-microfarad 0603 capacitor is compared with an 0805 capacitor. And also, the same capacitor, 0603 capacitor is compared among different manufacturers, A, B, and C. The DC bias applied is 1.8 volt.

Let’s look at manufacturer A. At minus 30ºC, the 10-microfarad, 6.3-volt cap at 1.8-volt DC bias is only 5.7 microfarad as we see here. For manufacturer B, the value is 4.25 microfarad. Manufacturer C, it is 3.75 microfarad. So we can see there’s a big difference with DC bias over temperatures from manufacturer to manufacturer. When this is compared to an 0805 case size capacitor that you see on the top, the value is 6.7 microfarad at minus 30ºC and the same variations can be seen over temperature.

So what this tells us is even though the capacitor is a 10-microfarad capacitor, in the actual application, the circuit is only seeing a 5.7-microfarad capacitance when using A or a 4.25-microfarad capacitance when using manufacturer B. The same is the story with the 4.7-microfarad 0603. A 4.7-microfarad 0603 at 1.8-volt DC bias is only 3.2 microfarad from manufacturer A and 2.5 microfarad from manufacturer B.

What are the reasons for this lowering in capacitance value with DC bias? Let’s have a look at that.

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DC Bias is a phenomena that occurs in ferroelectric dielectrics.These include X7R, X5R, Y5V, Z5U and others. It does not apply to Class I dielectrics like C0G. Basically, the electrical fieldacross the cap "polarizes" the inner molecular structure and this cases a temporary change in the K constant and unfortunately, only lowers it. So when you remove the electrical field, the inner structure re-aligns itself and the cap value comes back.

Factors affecting DC bias: • K constant• Layer thickness• % of rated voltage• Grain size of material

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What is the effect of DC Bias?

DC Bias is a phenomenon that occurs in ferroelectric dielectrics. These include X7R, X5R, Y5V, and others. It does not apply to Class I dielectrics like C0G. Basically, the electrical field across the cap polarizes the inner molecular structure and this cases a temporary change in the K constant, and unfortunately, only lowers it. So when you remove the electrical field, the inner structure realigns itself and the cap value comes back or the capacitance value comes back to the value without the DC bias.

So this is the main reason why the capacitance value is decreasing when you apply DC bias. The inner molecular structure is changing and that causes the change in the K constant. If you remember the equation for the capacitor, C is equal to K, a constant, times different factors. So once this K is lower, the capacitor’s value is lower.

What are the factors affecting DC bias? K constant, layer thickness, percentage of rated voltage, grain size of material. In the previous chart, we saw the capacitance values at 1.8-volt DC bias. Instead of 1.8, if I had applied 3.3-volt DC bias for a 6.3-volt capacitor, the percentage reduction in the capacitance values would have been worse. And also, the grain size of the material affects it. That also explains the reason why as we go towards a smaller case size which is a finer grain size of material, the DC bias phenomenon is much more predominant and it gets worse as we go to smaller case sizes.

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Because specs are valid only for specified test conditions:

Why do measured values even at no dc bias seem to be out of spec?

Sometimes we may ask, why do measured values even at no DC bias seem to be out of spec? For example, you get a 10-microfarad capacitor from a manufacturer. You measure on a LCR meter and the actual value is only 7 microfarad or 6.8 microfarad. But you still think the capacitors that have been shipped out to the manufacturing line have to meet the plus or minus 20% tolerances. But the reason why we see a low value when we do the measurements is because the capacitor values in manufacturing for the production has specific conditions associated with them. For example, if it’s a greater than 10-microfarad capacitor, you have certain AC voltage and a frequency at which the measurements are done. And that’s the condition where the specs are guaranteed.

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Ceramic capacitors –Class I and II

What do you mean the Class I and Class II and what are the difference between Class I and Class II capacitors? A Class I capacitor is made from ceramic materials and these are not sensitive to temperature changes so the capacitance variation over temperature is very small.

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Ceramic capacitors –Class I and II

A Class II capacitor like an X5R or the X7R or Y5V is made from ceramic materials that are derived from a barium titanate base which is temperature sensitive. This causes temperature variations for the capacitor value. For example, we know that an X7R capacitor has a temperature tolerance of plus or minus 15% over the temperature range of minus 55 to 125ºC. Similarly, an X5R has a temperature range of minus 55 to 85ºC.

Additionally, capacitors made from this barium titanate base are ferroelectric and, therefore, susceptible to aging in which a capacitor’s ability to hold capacitance will decrease over time if left in an unheated or uncharged state. Class I capacitors are not ferroelectric and, therefore, do not age.

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Aging of Ceramic capacitors

The next slide addresses the question of aging of ceramic capacitors. What do we mean by aging? Aging is generally referred to as a decrease in capacitance over time in Class II capacitors. It is a natural and unavoidable phenomenon that occurs in all ferroelectric formulations used as the dielectric material. If you remember, the Class II capacitors used barium titanate as a common in the dielectric material and that’s the reason why it has aging as one of the factors. Aging is reversible and occurs due to changes in the dielectric’s crystal structure with temperature and time.

On the right, you can see the capacitance change versus time for a C0G, X7R, and a Y5V. The Y5V is the worst of all compared to a C0G or an X7R.

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Steps in finding Min. COUT

2.2 µF, 25V, 1206 measured value at T=-30°C, 1.8V dc bias = 1.73 µF

So far, we have looked at ceramic capacitors and its affect on DC bias. With the DC bias, we know the capacitance value can be as low as 4 microfarad when using a 10-microfarad capacitor. So how do we make sure when using these type of capacitors that the converter is stable? That’s the next topic.

To make sure the converter is stable with the 10-microfarad 0603 capacitor, we saw from the DC bias curve this value from manufacturer A can go down as low as 5.7 microfarad at minus 30ºC. In order to prove that this switcher or the DC/DC converter is stable, what is done is a 2.2-microfarad, 25-volt, 1206 case size capacitor. With 1.8-volt DC bias at minus 30, the measured value is 1.73-microfarad capacitance. With this value we can see the load transient is unstable. Even the steady state waveform is unstable. So that’s one 2.2-microfarad 1206 case size.

The reason for choosing a big case size and a higher voltage rating is we want to make sure that we use parallel combinations of these capacitor so that we can check at what point does the converter become stable or at what point below which the converter becomes unstable.

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This value correspondsto lowest value of capacitance with a 0603 cap at -30°C

Proving that the device is stable with an 0603 capacitor

6.6 µF (3 of 2.2 µF, 25V, 1206 in parallel) measured value at T=-30°C, 1.8V dc bias = 5.61 µF

The next slide we see three of these 2.2-microfarad capacitors, 1206 in parallel. The measured value is 5.61 microfarad. Now, we see the load transient response looks good, the steady state waveforms look good. This value, 5.61, corresponds to the lowest value of capacitance with an 0603 cap at minus 30ºC from manufacturer A. So that just means that a 10-microfarad, 0603 6.3-volt cap from manufacturer A can be used in this particular switching converter or a DC/DC converter.

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Apart from Steady state waveforms and transient behavior over the temperature, range, bode plots also give a good indication of the stability of the part with the smaller case size capacitors like 0603.

DC bias characteristics of a capacitor can make a converter unstable. Make sure capacitor used satisfies the minimum capacitance value given in the datasheets. If this information is not available check with the actual capacitors planned to be used in the production under worst case conditions (usually cold temp). Be wary of the differences among manufacturers when specifying alternate suppliers.

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Min. COUT - Conclusion

So let’s look at the conclusion for the minimum COUT studies. Apart from steady state waveforms and transient behavior over temperature, range, bode plots also give a good indication of the stability of the part with the smaller case size capacitors like 0603.

DC bias characteristics of a capacitor can make a converter unstable. Make sure capacitor used satisfies the minimum capacitance value given in the datasheets. If this information is not available, check with the actual capacitors planned to be used in the production under worst case conditions, usually cold temperature. Be wary of the differences among manufacturers when specifying alternate suppliers. Remember the difference between manufacturers A, B, and C. So if a component has been evaluated with a manufacturer A, it does not mean it will be stable with B and C because the B and C had much lower values of capacitance at minus 30ºC.

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10µF,16V,1210

Temperature Characteristics of Y5V Capacitors

This slide shows the temperature characteristics of a Y5V capacitor. As we can see, the tolerance of the specs are plus 22 to minus 82%. And with DC bias, it’s even different. The whole profile changes with DC bias. So this is one of the reasons why a Y5V capacitor is not recommended for most of the DC/DC converter applications because of the wide variation in the capacitance. A 10-microfarad capacitor can be as low as 2 microfarad in the application.

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4.7µF,10V,1206

Temperature Characteristics of X5R Capacitors

The temperature characteristics of X5R capacitors. Here we can see the window of plus or minus 15% for an X5R over the temperature range minus 55 to 85ºC. X5R capacitors are recommended for applications like DC/DC converters.

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Ripple current rating

10uF,0603,X5R

This slide shows the ripple current rating for a 10-microfarad 0603 X5R capacitor. As we can see, the ripple current rating changes with frequency of operation and for a typical frequency at 2 megahertz, the ripple current rating is more than 1 amp for a temperature rise of 10ºC. So the ripple current rating also has to be considered when designing converters and selecting capacitors, but this is inherently not a problem for ceramic capacitors because of its high ripple current ratings compared to tantalum capacitors.

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INDUCTORS

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Inductors.

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Inductors- Size progression

LM3671 microsmd1.06mm x 1.39mm x 0.6mm

2 x 2 x1mm

4.5 x 6.6 x2.9mm

The next topic is inductors and the next slide shows the size progression of an inductor over a couple of years. A 2.2-microhenry inductor is shown on the left. It is available in a size 4.5 by 6.6 by 2.9 millimeter height a couple of years back. As we go towards the right, the inductor sizes get smaller and smaller and it is available in a 2 by 2 times 1 millimeter height now. On the top, we see the actual size of an LM3671 microsmd. It’s available in a 1.06 millimeter by 1.39 millimeter by 0.6-millimeter height package. So as we can see, the inductors are getting smaller and it’s approaching the size of the chip. And this is good news because the total solution size for a DC/DC converter will be much smaller than it was a couple of years back.

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Inductors

• Selecting Inductor Values• How current limit specs for a magnetic buck are specified

• Inductor selection parameters• Losses in Inductors• Coil vs. Chip Inductors• Shielded vs. Non shielded inductors

Let’s look at some of the topics that we will cover today. Selecting inductor values, how current limit specifications for a magnetic buck converter are specified, inductor selection parameters, losses in inductors, coil versus chip inductors, and shielded and non-shielded inductors are some of the topics we are going to cover.

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L such that: 2∆iL/I < 20 to 80%

iL(t)

t0 DTs Ts

IiL(0)

iL(DTs)ΔiLKnown

Vg=5.5VV=1.5Fs=3MHzD=V/VgI=800mA

Dfi2VV

LsL

g

Δ

−=

L=1µH

Select ILIM for switches based on worst case conditions

50% ripple current ratio

Practice Problem: Selecting the Inductor value for a Buck converter

Pk to pk ripple=2ΔiL

Here is a practice problem selecting the inductor value for a buck converter.On the top is a very familiar waveform. The inductor current to a buck converter is seen, ?iL is the average to peak ripple current. The peak-to-peak ripple current is 2 times ?iL. The known factors are given on the left. The maximum input voltage is 5.5; output voltage is 1.5 volt, switching frequency 3 megahertz, and the output current 800 milliamps. We can use this inductance selection equation for a buck converter given on the right, L is equal to Vg minus V divided by 2?iLfS times D where fS is the switching frequency.

We can select L such that the ripple current ratio, 2?iL by I is in the range from 20 to 80%. The higher ripple current ratio we select, the more the losses are in the converter. The smaller the ripple current ratio we select, the bigger the value of the inductance that we have to choose in the design. In this case, a 50% ripple current ratio is selected and a 1-microhenry inductor is selected for this application.

Now, we have to select ILIM or the current limit for the switches based on worst case conditions to guarantee 800-milliamps output current.

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Datasheet ParametersUnitsMaxTypMinConditionParameterSymbol

MHz3.332.7Internal oscillator frequency

FOSC

mATBDTBDTBDSwitch current limitILIM-PFET

Application ParametersOperating Ratings (Notes 1, 2)

0mA to 800mARecommended Load Current Vo= 1.5V

2.7V to 5.5VInput Voltage Range

How do we select ILIM-PFET min value once L=1.0uH is selected?

Practice Problem:Selecting Current Limit

The datasheet table that we see here looks very familiar. One is oscillator frequency and the current limit for the PFET. The oscillator frequency we know from design targets that the variation is plus or minus 10%. So we see here the minimum frequency is 2.7, typical 3, and max 3.3 megahertz. The current limit is the one that we have to define or select right now. The input voltage range and the recommended load current is again recorded from the previous slide, the max voltage is 5.5 and the maximum output current is 800 milliamps. We selected 1 microhenry, the previous slide. We’re going forward to select the ILIM-PFET minimum value. That is a TBD in this sheet.

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Worst case (∆iL)Vin,max = Vg=5.5V (application specific)Vo=1.5V (application specific)Fosc,min = 2.7MHz (datasheet specific)L= worst case value, 30% tolerances(manufacturer specific)

DLf

VVi

s

gL 2

−=Δ

ILIM-PFET min = I(max) + worst case(∆iL)

Worst case (∆iL) = 340mA

ILIM-PFET min = I(max) + worst case(∆iL) = 800mA + 340mA = 1140mA

ILIM-PFET min = 1140mA

Practice Problem:Selecting Current Limit

ILIM-PFET minimum is equal to the I(max) or the maximum output current, 800 milliamps in this case, plus the worst case (?iL). The worst case (?iL) can be computed from this formula, Vg minus V by 2LfS times D. The VIN max is application specific, in this case 5.5. VOUT is application specific, 1.5 volt. Oscillator frequency is 2.7 megahertz. That’s the worst case oscillator minimum frequency that’s datasheet specific. L is the worst case value, 30% tolerances can be considered. Again, this is manufacturer specific. Some manufacturers may have 10% tolerances.

Once we know the worst case (?iL), we can calculate the ILIM-PFET minimum value by adding I(max) plus the worst case (?iL). We get 1140 milliamps.

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• Once we know ILIM,min, then typ and max are obtained by design and production tolerances. • ILIM,typ = 10 to 15% higher than ILIM,min• ILIM,max = 10 to 15% higher than ILIM,typ

Note: Tighter tolerance in current limit specs offered by National partshelp to choose the smallest inductor for the application.

Current limit values defined

The next slide we see here, the 1150 milliamps is selected as the minimum value of current limit. Finding the typical and max values are easy. It’s 10 to 15% higher is the ILIM typical from the ILIM minimum. ILIM max is 10 to 15% higher than ILIM typical. Tighter tolerance in current limit specifications offered by National parts help to choose the smallest inductor for the application. But the point that is to be noted here is when you have a tight tolerance for the current limit, your ILIM-PFET max value can be kept to a low value and the smaller size inductance can be used in the application.

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Choosing the Inductor

We went through the process of Inductor Value selection for a given Application and determined Current limit required to guarantee a certain output current.

Once we know current limit we can proceed with Inductor selection (either among different types from one manufacturer or different Manufacturers)

Choosing the inductor. We went through the process of inductor value selection for a given application and determined current limit required to guarantee a certain output current. Once we know current limit, we can proceed with inductor selection either among different types from one manufacturer or different manufacturers.

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Typical Inductor Specifications –Coilcraft LPO series

That ends the practice problem. What we have seen, we selected 1 microhenry and we have selected the ILIM-PFET min, typical, and max. Now, let’s look at some general inductor specifications.

In this case, a Coilcraft LPO series inductor is taken. The point that I’ve highlighted is Isatvalue and the notes that go along with it. A 2.2-microhenry inductor is considered for this description. Isat 1.2 amp that we see here is the DC current at which the inductance value drops by 10% from its typical value without any current. Also, we see an Irms rating of 0.9 amps for a 2.2-microhenry inductor here.

What this means is – the note says that Irms current is average current for a 40ºC rise over 25ºC ambient temperature. So one thing to note here is the 0.9 Irms current is lower than the Isat rating of this particular inductor. We’ll go over this in more detail to understand the concepts behind this.

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LPO3010-2.2µHIsat=1.2A(from datasheet)

Inductance vs. Current

The inductance versus current curve is seen here for the 2.2-microhenry inductor that we described in the previous slide. The datasheet spec 1.2 amps as the saturation current reading. We can see here three curves. This is over temperature at ambient temperature 25ºC, 85ºC, and minus 40ºC. We can see the inductance versus current remains flat until a point and then it starts dropping off. The ambient temperature rating has a higher point at which inductance starts dropping off. At cold, it's lower and the minimum value for the current at which the inductance value drops and reaches a 10% point is at 85ºC.

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Isat

Isat1

This is an example of a curve that can be requested from manufacturers.

Saturation current deratingwith Temperature

This is an example of a curve that can be requested from manufacturers. As we have seen, the Isat rating is not a constant. It varies with temperature from the previous curve. So this curve shows the Isat rating at the ambient temperature and the percentage drop in the Isatrating at hot. For example, in this particular type of curve that we see here, at 85ºC, the Isatrating drops by 20% compared to the Isat rating at ambient temperature.

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10.133.3µH±30%ELLVGG3R3N

1.30.0922.2µH±30%ELLVGG2R2N

1.50.0731.6µH±30%ELLVGG1R6N

1.60A0.052W1.0µH±30%ELLVGG1R0N

CurrentTol.±20%Tol. +20%,-20%

RatedDCRInductancePart Number

Courtesy: Panasonic

Rated current is the smaller of the two:1) saturation current (where Inductance drops by 30%) 2) the rms current at which the Temp rises by 40ºC above 25ºC.

What does rated current mean in the specification?

What does rated current mean in the specification? In the previous example of the CoilcraftLPO series, we saw there was an Isat rating plus an Irms rating in the datasheet specification table. In this example from Panasonic, we see just one specification for the rated current. Again, a 2.2-microhenry inductor is chosen. The rated current is 1.3 amps. The rated current is the smaller of the two as per the notes: saturation current where inductance drops by 30% or the rms current at which the temperature rises by 40ºC above the ambient 25ºC. So it can be either of these two cases. Either the saturation inductance value has to drop by 30% or it has to have a 40 degrees temperature rise. Let's look at this in more detail.

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ELLVGG2R2N L-I DATA

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

IDC(A)

L(uH

)

25degC -30degC 85degC

Datasheet spec

~Isat

Inductance vs. Current Isat >> Datasheet spec

The inductance versus current for this particular Panasonic 2.2-microhenry inductor is seen here. As you can see here, the datasheet spec, 1.3 amps, but we see the inductance value drops by 30% only around 1.8 amps and then also at hot, 85ºC. So we have to figure out why is the datasheet spec 1.3 and not 1.8 amps?

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Temperature rise in Inductors

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40º temp rise current

Datasheet spec

Temp rise spec ~ Datasheet spec

The next slide explains exactly why. Here we can see the temperature rise versus DC current. At 1.3 amps, the 2.2 microhenry has a 40º temperature rise over the ambient temperature. So now we understand why this inductor has been specified as 1.3 amps in the datasheet. The temperature rise is the dominating factor for this inductor specification and not the Isat rating.

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Make sure Isat1 ≥ ILIM Min

Make sure Isat1 ≥ ILIM Max

Aggressive Method

Conservative Method

Where Isat1 = saturation current at 85ºC

Caution: Some types of application determine choice.

Note: Irms ratings need to be considered for determining worst case Component temperature. It does not help to select inductors that meet Isat requirements but have much lower Irms ratings which will cause temp rise to exceed manufacturers recommendations.

Approaches in selecting Isatrating of Inductor

Let's look at some approaches in selecting the Isat rating for an inductor. In the previous example, we selected the ILIM-PFET minimum value of 1150 milliamps and also we have a max value to account for the tolerances of the ILIM. One way we can select inductors is to make sure Isat1 is greater than ILIM minimum. This is an aggressive method. The other way is to make sure Isat1 is greater than or equal to ILIM maximum. This is a conservative method where Isat1 is equal to saturation current at 85ºC. If you remember, the saturation current at 25ºC Isat is higher than the saturation current at 85ºC Isat1. So it's always better to consider Isat1 when making choices like this.

Note – rms ratings need to be considered for determining worst case component temperature. It does not help to select inductors that meet Isat requirements but have much lower rms ratings which will cause temperature rise to exceed manufacturers' recommendations.

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Inductors –AC resistance vs. Frequency

This curve can be used to compare inductors from differentmanufacturers

The AC resistance versus frequency for inductors is also a curve that's sometimes missing from manufacturer datasheets. The normal specifications include Isat, Irms ratings, all the rated currents, DCR, DC resistance, and the important curve from the manufacturers is AC resistance versus frequency. This curve can be used to compare different inductors from different manufacturers. So as you can see, at low frequencies, the AC resistance versus frequency is really flat. It keeps going up as we increase the frequency. Let's look at how this impacts losses of the converter.

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vL(t) Vg – V

t– V

D'TsDTs

Switchposition: 1 2 1

iL(t)

t0 DTs Ts

IiL(0)

iL(DTs) ∆iI -> dc losses2∆i -> ac losses

Calculating losses in the inductor-Buck converter

Calculating losses in the buck converter. The top figure shows the voltage across the inductor in the different phases of a buck converter. The bottom curve or the bottom picture shows the inductor current flowing in a buck converter. In this case, it's a continuous conduction mode converter and the average current flowing to the inductor is equal to the output current. ?i is the average to peak ripple current. 2?i is the peak-to-peak ripple current. The average current flowing in this inductor, capital I, is responsible for all the DC losses in the converter. 2?i, or the peak-to-peak ripple current, is responsible for all the major AC losses in the converter.

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Losses in inductor include both dc and ac losses

Example:Fsw=2MhzL=2.2uH

Rdc at low freq = 0.2ohmsESR at 2Mhz = 1ohms

∆I=0.289A (calculated)

Vg=5.5VV=1.5V

Low freq loss = I2x Rdc

High freq loss = (∆I)2)/12 x ESR

Calculating Losses in Inductor

Let's look at how we can calculate losses in the inductors. The first thing we have to understand is the losses in the inductors include both DC and AC losses. In this particular case, the switching frequency is 2 megahertz, the inductance is 2.2 microhenry. We know the value of ?i from the buck converter equations. The 2.2-microhenry inductor, looking at the curve we can see at low frequencies, the DC resistance is 200 milliohms. That's what we have here, DC resistance at low frequencies, 200 milliohms. The ESR or the AC resistance at 2 megahertz is 1 ohm, so at 2 megahertz for 2.2 microhenry 1 ohm is the AC resistance value. The low frequency loss is I2x loss, I2 times the DC resistance. A high frequency loss is ?I2 divided by 12 times the ESR at that particular frequency.

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Io=50mA

• Low freq loss = I2x Rdc = 0.0005W

• High freq loss = (∆I)2)/12 x ESR=0.007W

• Total loss= 0.0005+0.007 = 0.0075W

• AC losses = 93% of total loss in inductor

Io=600mA

• Low freq loss = I2x Rdc = 0.072W

• High freq loss = (∆I)2)/12 x ESR=0.007W

• Total loss= 0.072+0.007 = 0.079W

• AC losses = 9% of total loss in inductor

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Note: importance should be given not only to DCR but also ESR at the frequency of operation of the converter.

AC and DC losses in perspective-Buck converter

Let's look at two cases: output current of 600 milliamps and output current of 50 milliamps. As we can predict, at the higher output currents the DC losses or the low frequency loss dominates. At the lighter loads, the DC losses are much smaller. The high frequency losses or the AC losses is only 9% of the total losses in the inductor at 600 milliamps. At 50 milliamps, the AC losses is equal to 93% of the total losses in the inductor. So we can see how great the impact is on the ESR or the AC resistance of the inductors at 2 megahertz on the AC losses of inductors.

Note – importance should be given not only to DCR but also ESR at the frequency of operation of the converter.

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0

5

10

15

20

25

30

35

40

0.01 0.1 1 10

Mhz

Q

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1 2 3 4 5

Mhz

ESR

(ohm

s)

Q Factor vs. Frequency

ESR vs. Frequency

A

B

A

B

Q Factor and ESR

⍵=2πFR= ESR

Q Factor and ESR. This slide shows an interesting phenomenon. The two charts that we see here: one, ESR versus frequency, and the second, Q Factor versus frequency. The inductor A has a low ESR versus frequency whereas inductor B the ESR versus frequency keeps climbing up at higher frequencies. The inductor A has a high Q Factor compared to inductor B at higher frequencies. The formula for Q Factor seen here, Q is equal to??L by R where R is equal to ESR at the particular frequency. So it is very understandable why the inductor A with a low ESR value has a high Q Factor because the ESR term is in the denominator. Let's look at how this places an impact on the efficiency of a converter.

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50.00

55.00

60.00

65.00

70.00

75.00

80.00

85.00

90.00

95.00

0.1 1 10 100 1000

Iout (mA)

effc

y (%

)

A B

LM3671-2MHz, Vo=1.8V, L=2.2uH

DCR_A=200mohms3.3 x 3.3 x 1.4DCR_B=100mohms3 x 3 x 1mm

A

B

A=High Q, B=Low Q

Effect of Q factor and ESR on Efficiency

The effect of Q Factor and ESR on efficiency is seen in this slide. This is a LM3671, 2-megahertz converter. The output volt is 1.8 volt. A 2.2-microhenry inductor is used, inductor A and inductor B. The number inductor A has a low ESR and a high Q in the previous slide. As we can see here, the inductor has a higher efficiency at light loads, up to 100 milliamps. The reason for this is the lower AC losses. We know that the inductor with low ESR and a high Q has lower AC losses and this is a dominating factor why the inductor A has a higher efficiency compared to inductor B at loads less than 100 milliamps, this apart from the fact that the A has a DC resistance of 200 milliohms compared to 100 milliohms for inductor B.

So this proves the point that at lower loads or light loads, the DCR is not a dominating factor. The ESR and the Q Factor is the key parameters to get high efficiency. At heavy loads, the DCR or the DC resistance plays the most important part and as we can see, the inductor A dips below the efficiency of inductor B because of its high DCR.

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Coil vs. Chip Inductors

Winding Type Chip

cross-sectional imageWire coil Ferrite core

Ag coil Ferrite

Typically, coil type of inductors has been used in wide variety over the past couple of years. A new trend or a new adoption also has been coming up is the chip type of inductors. It's called multilayer inductors. These are also been started to use widely among manufacturers for DC/DC converters.

The difference between the two can be seen here: the winding type of inductor versus the chip type of inductor. The winding type of inductor typically has a ferrite core and the coil is wound around the center leg of the ferrite core. Whereas for chip type of inductor, we have layers of the coil and the ferrite is encapsulated – the chip is encapsulated by the ferrite material.

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Coil vs. Chip Inductors - L vs. Idc

0

0.5

1

1.5

2

2.5

3

0 100 200 300 400 500 600 700 800 900 1000

Idc(mA)

L(uH

)

Coil A

Coil B

F=1MHz

Wire Wound Inductors vs. Multilayer Inductors

Chip XDCR=150mohms

Chip Y

L= 2.2uH

DCR=70mohms

Coil versus chip inductors. Let's look at the L versus DC current as we see here, coil A and coil B. As is representative of a coil type of inductors or wire wound type of inductors, the inductance value stays pretty flat up to the saturation current rating and then it starts dropping off. In this point, even up to 1 amp, the inductor has not started saturating. So you can see the coil A and coil B is flat. The coil A is slightly getting affected by the higher DC currents.

Compared to the chip type of inductors, chip X and chip Y. We can see that chip X has a much sharper roll off compared to the coil type of inductors. And even at 1 amp, the value is only 1.5 microhenry for a 2.2-microhenry inductor. Chip Y is even worse. The value is 0.5 microhenry at 1 amp current. So this is one of the major differences between the chip and coil type of inductors. The coil type of inductors provide a flat inductance versus current profile till the saturation rating, whereas the chip type of inductors have a gradual or sloping type of profile for the inductance versus current. In this case, the Isat ratings do not come into the picture and the major specification factor for the chip type of inductors is the rmscurrent rating or the current at which the temperature rises by 40ºC above ambient.

Also, one thing to note here is what is the difference between the two type of inductors that you see here, chip X and chip Y? The chip X inductor has a DCR of 150 milliohms and the chip Y has a DCR of 70 milliohms. Can this play an impact on why chip Y has a much sharper roll off compared to chip X? Yes. The chip X inductor manufacturer uses lower permeability material. This means to get 2.2-microhenry inductor, more number of turns are required. This means you have a higher DCR.

At the same time, the lower permeable material helps in the roll off. At higher currents, a roll off is much gradual. The chip Y inductor manufacturer uses a high permeability material. In this case, a lower number of turns is required to make 2.2 microhenry so you get a lower DCR. But at the same time, the inductance roll off is much more sharper compared to chip X versus current.

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3) New method of shielding

Courtesy: Panasonic

Shielded Inductors vs. Non shielded Inductors

Shielded inductors versus non-shielded inductors. There’s a couple of things to be careful about when using inductors. As we saw, one is the Isat rating. The second is the rms current rating of the inductor for temperature rise. The DCR is important. The ESR and the Q Factor is important. Now we get into the interference part of the inductors. Typically, an inductor with no shielding is basically just a drum core with a wire wound around it. A magnetic shielded inductor is the same drum core with a wire, but you also have a ring core surrounding it. This gives the better shielding effect to reduce the leakage flux lines coming out from the sides of the inductor.

A new method of shielding is seen here and it’s adhesive kind of glue which has a magnetic shielding effect surrounding the wire and the core. In this case, the ring core can be eliminated and still magnetic shielding properties are maintained.

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Measurement Setup for Leakage measurements

Let’s look at the measurement setup for leakage measurements or for measuring the flux lines for comparing a shielded inductor and a non-shielded inductor.

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Unshielded

Magnetic field lines –Unshielded Inductor

The magnetic field lines for an unshielded inductor are seen here. As we can see here, we see a quite of bit of magnetic field surrounding the inductor which could affect other components situated near it.

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Shielded

Magnetic field lines –Shielded Inductor

The magnetic field lines of a shielded inductor are seen here. The shielded inductor has a much more contained field, the magnetic fields. And it’s not quite wide as we saw for a non-shielded inductor.

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CIN

COUT L

3 worst case conditions happening

2 worst case conditions happening

External components in Perspective

Let’s look at external components in perspective. We know that any type of converter like a buck or a boost converter uses components like CIN input capacitors, COUT output capacitor, and inductors. So when we have all these three components, there are a couple of things that can happen. Either you can have two worst cases happening together or you can have three worst cases happening together.

So all these factors should be considered when designing a system and the worst case definitely is all the three worst case happening together. You have to be careful when using a chip type of inductor compared to a coil type of inductor. As we saw, a chip type of inductor has a much sharper roll off versus current. And all the formulas that we use for calculating ILIM of the PFET were assuming a 30% tolerance for the inductance. And when you use inductance, there is a much sharper roll off.

These factors have to be taken in account and the formula has to be because ILIM-PFET is manufacturer specific. It’s already a released part for which ILIM cannot be changed. So when a inductance, that has a sharper roll off is used, the only thing that can be changed is the output current rating of the part. So an 800-milliamps part may not be 800 milliamps when using a chip type of inductor. It may be much lower than that so appropriate caution should be taken to use the formulas and use the worst case inductance values to calculate what is the maximum output current that the application can use when using a chip type of inductor.

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• To the capacitor and inductor manufacturers like Coilcraft, TDK, Murata, FDK, Taiyo-Yuden, Panasonic etc. who supported and provided data and inputs when requested.

• To Vik Sangha who suggested a title for this seminar.

• To Chris Burket, TDK for some material content

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Acknowledgements

Last but not the least, acknowledgements. To the capacitor and inductor manufacturers like Coilcraft, TDK, Murata, FDK, Taiyo-Yuden, Panasonic who supported and provided data and inputs when requested for some of this material. To Vik Sangha who suggested a title for this seminar. To Chris Burket, TDK, for some material content.

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5353

APPENDIX

Tantalum Capacitors in Comparison to Ceramic

In this appendix section of this seminar, there’s a few slides that show a comparison within tantalum capacitors and ceramic capacitors.

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ESR

This compares ESR values of these capacitors.

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Filtering Performance

The filtering performance at 500 kilohertz

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100 kilohertz

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and 50 kilohertz is seen here. No capacitor, an electrolytic capacitor, a tantalum capacitor, and a multilayer ceramic capacitor is used to see the effects of the filtering. And as you can see, the spectrum is the lowest with the ceramic capacitor because of its inherent filtering properties.

The reason for the improvement in filtering we saw in the previous slide is because the impedance of the ceramic capacitor and the ESR is much lower at certain frequencies compared to the tantalum capacitors.

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Breakdown Voltage

The breakdown voltage comparison is seen here.

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Ripple current rating

Ripple current rating should be considered.Power dissipation capability varies by case size. Iripple(@Freq)= sqrt(Pmax/ESR)

Good apps note on Tantalum capacitors can be foundat: http://www.vishay.com/docs/40031/appripp.pdf

Ripple current rating. Ripple current rating should be considered carefully when using tantalum capacitors. The power dissipation capability can vary by the case size. So once we know the power dissipation rating of a case size, we can calculate the ripple current at that application frequency by the square root of the Pmax by the ESR at the application frequency. Apps note on tantalum capacitors can be found on the following Web site.

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Thank you !

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