LTC3613 datasheet(23/44 Pages) AD

23 / 44 page

LTC7131-1

Rev. 0

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APPLICATIONS INFORMATION

and any radiated field/EMI requirements. New designs for

surface mount inductors are available from Toko, Vishay,

NEC/Tokin, Cooper, TDK, Wurth Elektronik and Coilcraft.

Refer to Table 3 for more details.

Table 3. Representative Surface Mount Inductors

INDUCTANCE

(µH)

DCR

(mΩ)

MAX CURRENT

(A)

DIMENSIONS

(mm)

HEIGHT

(mm)

EATON FP1007 R3 Series

0.15

0.29

10.2 x 7.8

7.3

0.17

0.29

10.2 x 7.8

7.3

0.22

0.29

10.2 x 7.8

7.3

Wurth 744308 Series

0.15

0.18

10.2x8.10

0.17

0.18

10.2x8.10

0.18

10.2x8.10

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the trapezoi-

dal wave current at the drain of the top power MOSFET. To

prevent large voltage transients from occurring, a low ESR

input capacitor sized for the maximum RMS current should

be used. The maximum RMS current is given by Equation 8.

IRMS ≅IOUT(MAX)

VOUT

VIN

VOUT

−1

(8)

Equation 8 has a maximum at VIN = 2VOUT, where

IRMS ≅IOUT/2

This simple worst-case condition is commonly used for

design because even significant deviations do not offer

much relief. Note that ripple current ratings from capaci-

tor manufacturers are often based on only 2000 hours of

life which makes it advisable to further derate the capac-

itor, or choose a capacitor rated at a higher temperature

than required.

Several capacitors may also be paralleled to meet size or

height requirements in the design. For low input voltage

applications, sufficient bulk input capacitance is needed

to minimize transient effects during output load changes.

The selection of COUT is determined by the effective series

resistance (ESR) that is required to minimize voltage rip-

ple and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response. The output ripple, ΔVOUT, is

determined by Equation 9.

ΔVOUT < ΔIL

8 • f • COUT

+ESR

⎛

⎝

⎜

⎞

⎠

⎟

(9)

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

RMS current handling requirements. Dry tantalum, spe-

cial polymer, aluminum electrolytic, and ceramic capaci-

tors are all available in surface mount packages. Special

polymer capacitors are very low ESR but have lower

capacitance density than other types. Tantalum capacitors

have the highest capacitance density but it is important

to only use types that have been surge tested for use in

switching power supplies. Aluminum electrolytic capac-

itors have significantly higher ESR, but can be used in

cost-sensitive applications provided that consideration

is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteristics

and small footprints. Their relatively low value of bulk

capacitance may require multiples in parallel.

Using Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high rip-

ple current, high voltage rating and low ESR make them

ideal for switching regulator applications. However, care

must be taken when these capacitors are used at the input

and output. When a ceramic capacitor is used at the input

and the power is supplied by a wall adapter through long

wires, a load step at the output can induce ringing at the

VIN input. At best, this ringing can couple to the output and

be mistaken as loop instability. At worst, a sudden inrush

of current through the long wires can potentially cause

a voltage spike at VIN large enough to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R and X7R dielectric formulations. These

dielectrics have the best temperature and voltage char-

acteristics of all the ceramics for a given value and size.

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