LTC3608 datasheet(13/24 Pages) LINER

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LTC3608

3608fa

APPLICATIONS INFORMATION

Inductor Selection

Given the desired input and output voltages, the induc-

tor value and operating frequency determine the ripple

current:

IL =

VOUT

f L

VOUT

VIN

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efﬁciency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a tradeoff between

component size, efﬁciency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). The largest ripple current

occurs at the highest VIN. To guarantee that ripple current

does not exceed a speciﬁed maximum, the inductance

should be chosen according to:

VOUT

f IL(MAX)

VOUT

VIN(MAX)

Once the value for L is known, the type of inductor must

be selected. High efﬁciency converters generally cannot

afford the core loss found in low cost powdered iron cores.

A variety of inductors designed for high current, low volt-

age applications are available from manufacturers such as

Sumida, Panasonic, Coiltronics, Coilcraft and Toko.

CIN and COUT Selection

The input capacitance CIN is required to ﬁlter the square

wave current at the drain of the top MOSFET. Use a low ESR

capacitor sized to handle the maximum RMS current.

IRMS ≅IOUT(MAX)

VOUT

VIN

VOUT

–1

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

IRMS = IOUT(MAX)/2. This simple worst-case condition is

commonly used for design because even signiﬁcant de-

viations do not offer much relief. Note that ripple current

ratings from capacitor manufacturers are often based on

only 2000 hours of life which makes it advisable to derate

the capacitor.

The selection of COUT is primarily determined by the

ESR required to minimize voltage ripple and load step

transients. The output ripple ΔVOUT is approximately

bounded by:

VOUT

IL ESR +

8fCOUT

Figure 3. Maximum Switching Frequency vs Duty Cycle

2.0

1.5

1.0

0.5

0.25

0.50

0.75

3608 F03

1.0

DROPOUT

REGION

DUTY CYCLE (VOUT/VIN)

Figure 2. Correcting Frequency Shift with Load Current Changes

CVON

0.01μF

RVON2

100k

RVON1

30k

VOUT

(2a)

(2b)

VON

ITH

LTC3608

CVON

0.01μF

RVON2

10k

2N5087

RVON1

10k

3608 F02

VOUT

INTVCC

VON

ITH

LTC3608

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