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LTC3409

3409fc

Design Example

As a design example, assume the LTC3409 is used in a

2-alkaline cell battery-powered application. The VIN will be

operating from a maximum of 3.2V down to about 1.8V.

The load current requirement is a maximum of 600mA

but most of the time it will be in standby mode, requiring

only 2mA. Efﬁciency at both low and high load currents

is important. Output voltage is 1.5V. With this information

we can calculate L using Equation 2:

f• IL

VOUT 1–

VOUT

VIN

(2)

Substituting VOUT = 1.5V, VIN = 3.2V, ΔIL = 240mA and

f = 1.7MHz in Equation 2 gives:

1.7MHz • 240mA

1.5 1–

1.5

3.2

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟ 2.2μH

For best efﬁciency choose a 750mA or greater inductor

with less than 0.3Ω series resistance. CIN will require

an RMS current rating of at least 0.3A ≅ ILOAD(MAX)/2 at

temperature.

For the feedback resistors, choose R2 = 133k. R1 can then

be calculated from Equation 2 at 191K. Figure 4 shows the

complete circuit along with its efﬁciency curve.

Table 2 below gives 1% resistor values for selected output

voltages.

VFB

GND

VIN

SYNC

RUN

MODE

LTC3409

2.2μH

191k

10pF

CIN

4.7μF

VIN

1.6V TO 5.5V

133k

3409 F04

VOUT

1.5V

0.6A

COUT

10μF

CER

L1: SUMIDA CDRH2D18/LD

Figure 4

LOAD CURRENT (mA)

100

0.1

100

1000

3409 F04b

1.8VIN

3.1VIN

2.5VIN

Burst Mode Efﬁciency, 1.5VOUT

APPLICATIONS INFORMATION

VOUT

0.85V

51.1k

133k

1.2V

127k

133k

1.5V

191k

133k

1.8V

255k

133k

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