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LTC3532

3532fc

APPLICATIO S I FOR ATIO

SHDN/SS

VIN

VOUT

BURST

SW1

SW2

GND

3532 F06

LTC3532

Figure 6. Recommended Component Placement. Traces Carrying

High Current are Direct. Trace area at FB and VC Pins are Kept

Low. Lead Length to Battery Should be Kept Short

Inductor Selection

The high frequency operation of the LTC3532 allows the

use of small surface mount inductors. The inductor ripple

current is typically set to 20% to 40% of the maximum

inductor current. For a given ripple the inductance terms

are given as follows:

LBOOST >

VIN(MIN) •(VOUT –VIN(MIN))

f• ΔIL •VOUT

LBUCK >

VOUT •(VIN(MAX) –VOUT )

f• ΔIL •VIN(MAX)

where f = Operating frequency, Hz

ΔIL = Maximum allowable inductor ripple current, A

VIN(MIN) = Minimum input voltage

VIN(MAX) = Maximum input voltage

VOUT = Output voltage

IOUT(MAX) = Maximum output load current

For high efﬁciency, choose a ferrite inductor with a high

frequency core material to reduce core losses. The induc-

tor should have low ESR (equivalent series resistance) to

reduce the I2R losses, and must be able to handle the peak

inductor current without saturating. Molded chokes or chip

inductors usually do not have enough core to support the

peak inductor currents in the 1A to 2A region. To minimize

radiated noise, use a shielded inductor. See Table 1 for a

suggested list of inductor suppliers.

Table 1. Inductor Vendor Information

SUPPLIER

WEB SITE

Coilcraft

www.coilcraft.com

Murata

www.murata.com

Sumida

www.sumida.com

TDK

www.component.tdk.com

TOKO

www.tokoam.com

Output Capacitor Selection

The bulk value of the output ﬁlter capacitor is set to reduce

the ripple due to charge into the capacitor each cycle. The

steady state ripple due to charge is given by:

% RIPPLE_BOOST =

IOUT(MAX) •(VOUT –VIN(MIN))•100

COUT •VOUT

2 •f

% RIPPLE_BUCK =

8LCf2

•

(VIN(MAX) –VOUT ) • 100%

VIN(MAX)

where COUT = output ﬁlter capacitor in Farads and

f = switching frequency in Hz.

The output capacitance is usually many times larger than

the minimum value in order to handle the transient response

requirements of the converter. As a rule of thumb, the ratio

of the operating frequency to the unity-gain bandwidth of

the converter is the amount the output capacitance will

have to increase from the above calculations in order to

maintain the desired transient response.

The other component of ripple is due to the ESR (equiva-

lent series resistance) of the output capacitor. Low ESR

capacitors should be used to minimize output voltage

ripple. For surface mount applications, Taiyo Yuden or

TDK ceramic capacitors, AVX TPS series tantalum capaci-

tors or Sanyo POSCAP are recommended. See Table 2 for

contact information.

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