LTC3521 datasheet(15/20 Pages) LINER

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LTC3521

3521f

from the following formulas, where f is the frequency in

MHz and L is the inductance in μH:

L P

OUT IN

OUT

L P

•

–

-P,BUCK

-P,B

(

)

OOOST =

(

)

V V

IN OUT

OUT

•

–

In addition to affecting output current ripple, the size of

the inductor can also affect the stability of the feedback

loop. In boost mode, the converter transfer function has

a right half plane zero at a frequency that is inversely

proportional to the value of the inductor. As a result, a

large inductor can move this zero to a frequency that is

low enough to degrade the phase margin of the feedback

loop. It is recommended that the chosen inductor value be

less than 10μH if the buck-boost converter is to be used

in the boost region.

Buck-Boost Output Capacitor Selection

A low ESR output capacitor should be utilized at the buck-

boost converter output in order to minimize output volt-

age ripple. Multilayer ceramic capacitors are an excellent

choice as they have low ESR and are available in small

footprints. The capacitor should be chosen large enough

to reduce the output voltage ripple to acceptable levels.

Neglecting the capacitor ESR and ESL, the peak-to-peak

output voltage ripple can be calculated by the following

formulas, where f is the frequency in MHz, COUT is the

capacitance in μF, L is the inductance in μH and ILOAD is

the output current in amps:

LOAD OUT

OUT

P-P,BOOST

P-P

(

)

–

•

,,BUCK =

(

)

• •

•

–

L C

OUT

OUT OUT

Since the output current is discontinuous in boost mode,

the ripple in this mode will generally be much larger than

the magnitude of the ripple in buck mode. In addition to

controlling the ripple magnitude, the value of the output

capacitoralsoaffectsthelocationoftheresonantfrequency

in the open loop converter transfer function. If the output

capacitor is too small, the bandwidth of the converter

will extend high enough to degrade the phase margin.

To prevent this from happening, it is recommended that

a minimum value of 10μF be used for the buck-boost

output capacitor.

Buck-Boost Input Capacitor Selection

Thesupplycurrenttothebuck-boostconverterisprovided

bythePVIN1pin.ItisrecommendedthatalowESRceramic

capacitor with a value of at least 4.7μF be located as close

to this pin as possible.

Inductor Style and Core Material

Different inductor core materials and styles have an

impact on the size and price of an inductor at any given

peak current rating. Toroid or shielded pot cores in ferrite

or permalloy materials are small and reduce emissions,

but generally cost more than powdered iron core induc-

tors with similar electrical characteristics. The choice of

inductor style depends upon the price, sizing, and EMI

requirements of a particular application. Table 4 provides a

samplingofinductorsthatarewellsuitedtomanyLTC3521

application circuits.

Table 4. Representative Surface Mount Inductors

MANU-

FACTURER

PART NUMBER

VALUE

MAX

CURRENT DCR HEIGHT

Taiyo Yuden NP03SB4R7M

4.7μH

1.2A

0.047

Ω 1.8mm

NP03SB6R8M

6.8μH

0.084

Ω 1.8mm

Coilcraft

MSS7341-502NL

5μH

2.3A

0.024

Ω 4.1mm

DT1608C-472ML

4.7µH

1.2A

0.085

Ω 2.92mm

Cooper-

Bussmann

SD7030-5R0-R

5µH

2.4A

0.026

Ω 3mm

SD20-6R2-R

6.2µH

1.12A

0.072

Ω 2mm

Sumida

CDR6D23MNNP-4R2 4.2µH

2.6A

0.052

Ω 2.5mm

CDRH4D16FB/ND-

6R8N

6.8µH

0.081

Ω 1.8mm

applicaTions inForMaTion

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