MIC2131 datasheet(14/20 Pages) MICREL

14 / 20 page

Micrel, Inc.

MIC2130/1

April 2008

M9999-042108-C

Application Information

Passive Component Selection Guide

Transition losses in the power MOSFETs are not defined

by inductor value. However, the inductor value is

responsible for the ripple current which causes some of

the resistive losses. These losses are proportional to

IRIPPLE

2. Minimizing inductor ripple current therefore

reduces resistive losses and can be achieved by

choosing a larger value inductor. This will generally

improve efficiency by reducing the RMS current flowing

in all of the power components. The actual value of

inductance is really defined by space limitations, RMS

rating (IRMS) and saturation current (ISAT) of available

inductors. If we look at the newer flat wire inductors,

these have higher saturation current ratings than the

RMS current rating for lower values and as inductance

value increases, these figures get closer in value. This

mirrors what happens in the converter with ISAT

analogous to the maximum peak switch current and IRMS

analogous to output current. As inductance increases, so

ISWITCHpk tends towards IOUT. This is a characteristic that

makes these types of inductor optimal for use in high

power buck converters such as MIC2130/31.

To determine the ISAT and IRMS rating of the inductor, we

should start with a nominal value of ripple current. This

should typically be no more than IOUT(max)/2 to minimize

MOSFET losses due to ripple current mentioned earlier.

Therefore:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

−

⋅

Efficiency

SWITCH

MIN

ILRMS > 1.04 x IOUT(max)

ILSAT > 1.25 x IOUT(max)

Any value chosen above LMIN will ensure these ratings

are not exceeded.

In considering the actual value to choose, we need to

look at the effect of ripple on the other components in

the circuit. The chosen inductor value will have a ripple

current of:

()

OUT

SWITCH

RIPPLE

⋅

−

This value should ideally be kept to a minimum, within

the cost and size constraints of the design, to reduce

unnecessary heat dissipation.

Output Capacitor Selection

The output capacitor (COUT) will have the full inductor

ripple current ILRMS flowing through it. This creates the

output switching noise which consists of two main

components:

Noise

Capacitor

OUT

RIPPLE

Noise

ESR

RIPPLE

OUTPK

ESR

⋅

≈

−

If therefore, the need is for low output voltage noise

(e.g., in low output voltage converters), VOUT ripple can

be directly reduced by increasing inductor value, Output

capacitor value or reducing ESR.

For tantalum capacitors, ESR is typically >40m

Ω which

usually makes loop stabilization easier by utilizing a

pole-zero (type II) compensator.

Due to the many advantages of multi-layer ceramic

capacitors, among them, cost, size, ripple rating and

ESR, it can be useful to use these in many cases.

However, one disadvantage is the CV product. This is

lower than tantalum. A mixture of one tantalum and one

ceramic can be a good compromise which can still utilize

the simple type II compensator.

With ceramic output capacitors only, a double-pole,

double-zero (type III) compensator is required to ensure

system stability. Loop compensation is described in

more detail later in the data sheet.

Ensure the RMS ripple current rating of the capacitor is

above IRIPPLE ⋅ 0.6 to improve reliability.

Input Capacitor Selection

The input filter needs to supply the load current when the

high-FET is on and should to limit the ripple to the desire

value. The CIN ripple rating for a converter is typically

IOUT/2 under worst case duty cycle conditions of 50%.

IRMSCIN =

(

)

OUT

−

Where D = VOUT/(VINxeff)

It is however, also important to closely decouple the

Power MOSFETs with 2 x 10µF Ceramic capacitors to

reduce ringing and prevent noise related issues from

causing problems in the layout of the regulator. The

ripple rating of CIN may therefore be satisfied by these

decoupling capacitors as they allow the use of perhaps

one more ceramic or tantalum input capacitor at the

input voltage node to decouple input noise and localize

high di/dt signals to the regulator input.

Power MOSFET Selection

The MIC2130/31 drives N-channel MOSFETs in both the

high-side and low-side positions. This is because the

switching speed for a given RDSON in the N-Channel

device is superior to the P-Channel device.

There are different criteria for choosing the high- and

low-side MOSFETs and these differences are more

significant at lower duty cycles such as 12V to 1.8V

conversion. In such an application, the high-side

MOSFET is required to switch as quickly as possible to

minimize transition losses (power dissipated during rise

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