LTC3403 datasheet(10/16 Pages) LINER

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LTC3403

3403f

Table 1. Representative Surface Mount Inductors

Part

Value

DCR

MAX DC

Size

Number

(

µH)

(

ΩMAX)

Current (A)

WxLxH (mm3)

Sumida

1.5

0.068

0.90

3.2 x 3.2 x 1.2

CDRH2D11

2.2

0.098

0.78

3.3

0.123

0.60

Sumida

2.2

0.041

0.85

3.2 x 3.2 x 2.0

CDRH2D18/LD

3.3

0.054

0.75

4.7

0.078

0.63

Sumida

2.2

0.116

0.95

3.5 x 4.1 x 0.8

CMD4D06

3.3

0.174

0.77

4.7

0.216

0.75

Murata

1.0

0.060

1.00

2.5 x 3.2 x 2.0

LQH32C

2.2

0.097

0.79

4.7

0.150

0.65

Taiyo Yuden

1.0

0.080

0.78

1.8 x 2.5 x 1.8

LQLBC2518

1.5

0.110

0.66

2.2

0.130

0.60

Toko

2.2

0.14

1.14

4.6 x 4.6 x 1.2

D412F

3.3

0.20

0.90

4.7

0.22

0.80

CIN and COUT Selection

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle VOUT/VIN. To prevent large

voltage transients, a low ESR input capacitor sized for the

maximum RMS current must be used. The maximum

RMS capacitor current is given by:

required I

RMS

OMAX

OUT

≅

[(

–

)] /

This formula 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 the capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of life.

This makes it advisable to further derate the capacitor, or

choose a capacitor rated at a higher temperature than

required. Always consult the manufacturer if there is any

question.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically, once the ESR require-

ment for COUT has been met, the RMS current rating

generally far exceeds the IRIPPLE(P-P) requirement. The

output ripple VOUT is determined by:

∆≅ ∆









I ESR

OUT

where f = operating frequency, COUT = output capacitance

and IL = ripple current in the inductor. For a fixed output

voltage, the output ripple is highest at maximum input

voltage since IL increases with input voltage.

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount configurations. In the case

of tantalum, 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 tantalum. These are

specially constructed and tested for low ESR so they give

the lowest ESR for a given volume. Other capacitor types

include Sanyo POSCAP, Kemet T510 and T495 series, and

Sprague 593D and 595D series. Consult the manufacturer

for other specific recommendations.

The bulk capacitance values in Figure 1(a) (CIN = 10µF,

COUT = 4.7µF) are tailored to mobile phone applications, in

which the output voltage is expected to slew quickly

according to the needs of the power amplifier. Holding the

output capacitor to 4.7

µF facilitates rapid charging and

discharging. When the output voltage descends quickly in

forced continuous mode, the LTC3403 will actually pull

current from the output until the command from VREF is

satisfied. On alternate half cyles, this current actually exits

the VIN terminal, potentially causing a rise in VIN and

forcing current into the battery. To prevent deterioration

of the battery, use sufficient bulk capacitance with low

ESR; at least 10

µF is recommended.

APPLICATIO S I FOR ATIO

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