LM3495 datasheet(17/26 Pages) NSC

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Design Considerations (Continued)

minimum inductance must be used based on the R

DSON of

the low-side FET and the desired switching frequency. As

with switching frequency, the inductance used is a tradeoff

between size and cost. Larger inductance means low current

ripple and hence low output voltage ripple. However, less

inductance results in smaller, less expensive devices. An

inductance that gives a ripple current of 30% to 40% of the

maximum load current is a good starting point (

∆i

O = 30% to

40%*I

O). Minimum inductance should be calculated from this

value, using the maximum input voltage, as:

By calculating in terms of amperes, volts, and megahertz,

the inductance value will come out in micro henries. The

second minimum inductance equation specific to the

LM3495 is:

By calculating in terms of milliohms and kilohertz the induc-

tance value will come out in micro henries.

For this design:

Whichever equation gives the higher value for inductance is

the one which should be followed.

The second criterion for selecting an inductor is the peak

current carrying capability. This is the level above which the

inductor will saturate. In saturation the inductance drops off

severely, often to 20% to 30% of the rated value. In a buck

converter, peak current, I

PK, is equal to the maximum load

current plus one half of the ripple current. For this example:

PK = 10A + 1.5A = 11.5A

Hence an inductor must be selected that has a peak current

rating greater than 11.5A and an average current rating

greater than 10A. To ensure a robust design, the inductor

selected should maintain approximately 50% of its rated

inductance during the worst-case peak current from an out-

put short circuit. For a low-side current limit the peak current

during an output short circuit can be estimated as I

CL plus

∆i

(O-MAX).

∆i

(O-MAX) is calculated by substituting zero for

output voltage in the expression for

∆i

O. Inductor core ma-

terials with soft saturation characteristics are preferred. One

inductor that meets the peak current guidelines is an off-the-

shelf 1.0 µH component that can handle a peak current of

18A and an average current of 14A. The inductor current

ripple and peak inductor current should be recalculated for

the selected inductance value, L

ACTUAL, by rearranging the

equation for minimum inductance:

OUTPUT CAPACITOR

The output capacitor in a switching regulator is selected on

the basis of capacitance, equivalent series resistance (ESR),

size, and cost. An important specification in switching con-

verters is the output ripple voltage,

∆v

O. At 500 kHz the

impedance of most capacitors is very small compared to

ESR, hence ESR becomes the main selection guide. In this

design the load requires a 1% ripple, which results in a

∆v

of 10 mV

P-P. Maximum ESR is then:

ESR

MAX is 10 m

Ω. Multi-layer ceramic, aluminum electro-

lytic, tantalum, solid aluminum, organic, and niobium capaci-

tors are all popular in switching converters. Generally, by the

time enough capacitors have been paralleled to obtain the

desired ESR, the bulk capacitance is more than enough to

supply the load current during a transient from no-load to full

load. In this example the load could transition quickly from

0A to 5A, (or from 5A to 0A), so moderate bulk capacitance

is needed. Two MLCC capacitors rated 100 µF, 6.3V each

with ESR of 3 m

Ω will work well.

VLIN5 DECOUPLING CAPACITOR

The VLIN5 pin should always be decoupled with a 2.2 µF,

10V-rated ceramic capacitor placed as close as possible to

the VLIN5 and PGND pins of the LM3495. The decoupling

capacitor should have a minimum X5R or X7R type dielectric

to ensure that the capacitance remains stable over the ex-

pected voltage and temperature range.

INPUT CAPACITOR

The input capacitors to a buck regulator are used to smooth

the large current pulses drawn by the inductor and load

when the high-side FET is on. Due to this large AC stress,

input capacitors are usually selected on the basis of their AC

rms current rating rather than bulk capacitance. Low ESR is

beneficial because it reduces the power dissipation in the

capacitors. Although any of the capacitor types mentioned in

the Output Capacitor section can be used, MLCCs are com-

mon because of their low ESR and because in general the

input to a buck converter does not require as much bulk

capacitance as the output. Input current, I

rms, can be calcu-

lated using the following equation:

A good estimate for the maximum AC rms current is one-half

of the maximum load current. For this example, the rms input

current can be estimated as 3.5A. Regardless of the type

and number of capacitors used, every design will benefit

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