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LTC3405

3405fa

APPLICATIO S I FOR ATIO

The basic LTC3405 application circuit is shown in Figure 1.

External component selection is driven by the load require-

ment and begins with the selection of L followed by CIN and

COUT.

Inductor Selection

For most applications, the value of the inductor will fall in

the range of 3.3µH to 10µH. Its value is chosen based on

the desired ripple current. Large value inductors lower

ripple current and small value inductors result in higher

ripple currents. Higher VIN or VOUT also increases the ripple

current as shown in equation 1. A reasonable starting point

for setting ripple current is ∆IL = 120mA (40% of 300mA).

∆ =

()( )

−

⎛

⎝

⎜

⎞

⎠

⎟

OUT

(1)

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 360mA rated

inductor should be enough for most applications (300mA

+ 60mA). For better efficiency, choose a low DC-resistance

inductor.

The inductor value also has an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor current peaks fall to approximately

100mA. Lower inductor values (higher ∆IL) will cause this

to occur at lower load currents, which can cause a dip in

efficiency in the upper range of low current operation. In

Burst Mode operation, lower inductance values will cause

the burst frequency to increase.

Inductor Core Selection

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Tor-

oid or shielded pot cores in ferrite or permalloy materials

are small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

electrical characteristics. The choice of which style induc-

tor to use often depends more on the price vs size require-

ments and any radiated field/EMI requirements than on

what the LTC3405 requires to operate. Table 1 shows some

typical surface mount inductors that work well in LTC3405

applications.

Table 1. Representative Surface Mount Inductors

MAX DC

MANUFACTURER PART NUMBER

VALUE CURRENT DCR HEIGHT

Taiyo Yuden

LB2016T3R3M

3.3µH

280mA

0.2Ω 1.6mm

Panasonic

ELT5KT4R7M

4.7µH

950mA

0.2Ω 1.2mm

Murata

LQH3C4R7M34

4.7µH

450mA

0.2Ω

2mm

Taiyo Yuden

LB2016T4R7M

4.7µH

210mA

0.25Ω 1.6mm

Panasonic

ELT5KT6R8M

6.8µH

760mA

0.3Ω 1.2mm

Panasonic

ELT5KT100M

10µH

680mA

0.36Ω 1.2mm

Sumida

CMD4D116R8MC 6.8µH

620mA

0.23Ω 1.2mm

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:

OMAX

OUT

required IRMS ≅

−

()

[]

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

IRMS = IOUT/2. This simple worst-case condition is com-

monly 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 manufac-

turer if there is any question.

The selection of COUT is driven by the required effective

series resistance (ESR). An ESR in the range of 100mΩ to

200mΩ is necessary to provide a stable loop. For the

LTC3405, the general rule for proper operation is:

0.1Ω ≤ COUT required ESR ≤ 0.6Ω

ESR is a direct function of the volume of the capacitor; that

is, physically larger capacitors have lower ESR. Once the

ESR requirement 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

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