LM3661 datasheet(14/17 Pages) NSC

14 / 17 page

Application Information (Continued)

INDUCTOR SELECTION

There are a few things that one must consider when select-

ing an inductor for an application. They are the inductor DC

current rating, inductor ripple current, DC-resistance of the

inductor and value of the inductor. The DC current rating of

the inductor denotes the maximum current before the induc-

tor core enters saturation. Before selecting the DC current

rating of the inductor, an inductor ripple current must be

determined using Equation (1). The DC current of the induc-

tor should be the maximum output current of the circuit plus

half of the peak to peak current ripple of the inductor using

Equation (2).

(1)

(2)

A good estimate for the inductor ripple current would be

using a operation condition or assume the inductor ripple

current to be about 30% of the maximum output current of

the device. Consider the following example for LM3661

(when I

SEL = L); a 10 µH, 450 mA load current with 1.4V

output operates at 4.5V input and 600kHz in an application,

solving for

∆I

L using Equation (2) yields

∆I

L = 160 mA.

Therefore the maximum peak current (Equation (1)) in the

application will be 530 mA (I

O + 1/2

∆I

L). Thus, an inductor

with DC current rating of 600 mA or higher should suffice for

the application when I

SEL = L. For a more conservative

approach, it is best to select an inductor with a current rating

of the maximum switch peak current of the device. Note that

If smaller inductor is used in the application, the larger the

inductor ripple current (Equation (1)). Care must be taken to

select the inductor such that the peak current rating of the

inductor accounts for minimum inductance and maximum

current for the operating condition. Equation (3) can be used

to calculate the inductor value if the application conditions

are known:

(3)

Where f is the operating frequency,

∆I

L is the inductor current

ripple and Vo is the desired output.

Finally, the DC resistance (DCR) of the inductor also affect

the overall efficiency of the solution. Lower DCR is recom-

mended for better efficiency in handheld and battery oper-

ated applications. Consult inductor manufacture for this

specification. Table 2 lists suggested inductors and suppli-

ers.

Table2. Suggested Inductor and Suppliers

Part Number

Vendor

Web

D01608C-103

Coilcraft

www.coilcraft.com

P1174.103T

Pulse

www.pulseeng.com

P0770.103T

Ell6GM100M

Panasonic

www.panasonic.com

INPUT AND OUTPUT CAPACITOR

The LM3661 is designed for ceramic capacitor for its input

and output filters. Ceramic capacitors such as X5R and X7R

are recommended to use for input and output filters. These

provide an optimal balance between small size, cost, reliabil-

ity and performance. Do not use Y5V ceramic capacitors as

they have poor dielectrics performance over temperature

and voltage characteristics for a given value. Table 2 lists

suggested capacitors and suppliers.

A 10 µF input and 22 µF output ceramic capacitors are

suggested in figure 2 (Typical application circuit) for optimal

performance.

The input filter capacitor supplies current to the PFET switch

of the LM3661 in the first part of each cycle and reduces

voltage ripple imposed on the input power source. The out-

put filter capacitor smooths out current flow from the inductor

and reduce output voltage ripple. These capacitors must be

selected with sufficient capacitance and sufficiently low ESR

20098833

FIGURE 3.

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