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LTC3561

1A, 4MHz, Synchronous Step-Down DC/DC Converter

Linear Technology 

LTC3561

APPLICATIO S I FOR ATIO

Table 1. Representative Surface Mount Inductors

MANU-

FACTURER PART NUMBER

MAX DC

VALUE CURRENT DCR HEIGHT

Toko

A914BYW-2R2M-D52LC 2.2µH 2.05A 49mΩ 2mm

Coilcraft D01608C-222

2.2µH 2.3A 70mΩ 3mm

Coilcraft LP01704-222M

2.2µH 2.4A 120mΩ 1mm

Sumida CDRH2D18/HP-2R2 2.2µH 1.6A 48mΩ 2mm

Taiyo Yuden N05DB2R2M

2.2µH 2.9A 32mΩ 2.8mm

Murata LQN6C2R2M04

2.2µH 3.2A 24mΩ 5mm

Cooper SD3112-2R2

2.2µH 1.1A 140mΩ 1.2mm

TDK

VLF3010AT-2R2M1R0 2.2µH 1.0A 100mΩ 1.0mm

EPCO

B82470A1222M

2.2µH 1.6A 90mΩ 1.2mm

Input Capacitor (CIN) Selection

In continuous mode, the input current of the converter is

a square wave with a duty cycle of approximately VOUT/

VIN. To prevent large voltage transients, a low equivalent

series resistance (ESR) input capacitor sized for the maxi-

mum RMS current must be used. The maximum RMS

capacitor current is given by:

IRMS ≈ IMAX

VOUT (VIN − VOUT )

VIN

where the maximum average output current IMAX equals

the peak current minus half the peak-to-peak ripple cur-

rent, IMAX = ILIM – ∆IL/2.

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst case is commonly used

to design because even significant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours life-

time. This makes it advisable to further derate the capaci-

tor, or choose a capacitor rated at a higher temperature

than required. Several capacitors may also be paralleled to

meet the size or height requirements of the design. An

additional 0.1µF to 1µF ceramic capacitor is also recom-

mended on VIN for high frequency decoupling, when not

using an all ceramic capacitor solution.

Output Capacitor (COUT) Selection

The selection of COUT is driven by the required ESR to

minimize voltage ripple and load step transients. Typically,

once the ESR requirement is satisfied, the capacitance is

adequate for filtering. The output ripple (∆VOUT) is deter-

mined by:

∆VOUT

≈

⎛

∆IL ⎝⎜ESR +

1⎞

8fO COUT ⎠⎟

where f = operating frequency, COUT = output capacitance

and ∆IL = ripple current in the inductor. The output ripple

is highest at maximum input voltage since ∆IL increases

with input voltage. With ∆IL = 0.3 • ILIM the output ripple

will be less than 100mV at maximum VIN and fO = 1MHz

with:

ESRCOUT < 150mΩ

Once the ESR requirements for COUT have been met, the

RMS current rating generally far exceeds the IRIPPLE(P-P)

requirement, except for an all ceramic solution.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Alumi-

num electrolytic, special polymer, ceramic and dry tanta-

lum capacitors are all available in surface mount pack-

ages. The OS-CON semiconductor dielectric capacitor

available from Sanyo has the lowest ESR(size) product of

any aluminum electrolytic at a somewhat higher price.

Special polymer capacitors, such as Sanyo POSCAP, offer

very low ESR, but have a lower capacitance density than

other types. Tantalum capacitors have the highest capaci-

tance density, but it has a larger ESR and it is critical that

the capacitors are surge tested for use in switching power

supplies. An excellent choice is the AVX TPS series of

surface mount tantalums, available in case heights rang-

ing from 2mm to 4mm. Aluminum electrolytic capacitors

have a significantly larger ESR, and is often used in

extremely cost-sensitive applications provided that con-

sideration is given to ripple current ratings and long term

reliability. Ceramic capacitors have the lowest ESR and

cost but also have the lowest capacitance density, a high

voltage and temperature coefficient and exhibit audible

3561f

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