LTC3549 datasheet(14/16 Pages) LINER

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LTC3549

3549f

Figure 4b. Burst Mode Efﬁciency, VOUT = 1.5V

Figure 4c. Load Step Response

Design Example

As a design example, assume the LTC3549 is used in a

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

operating from a maximum of 3.1V down to about 1.8V.

The load current requirement is a maximum of 250mA

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 3:

OUT

∆

⎛

⎝⎜

⎞

⎠⎟

•

–

(3)

Substituting VOUT = 1.5V, VIN = 3.1V, ΔIL = 100mA and

f = 2.25MHz in Equation 3 gives:

MHz

⎛

⎝⎜

⎞

⎠⎟

≅µ

2 25

100

15 1

.•

.–

For best efﬁciency choose a 350mA or greater inductor

with less than 0.3Ω series resistance. CIN will require an

RMS current rating of at least 0.125A ≅ ILOAD(MAX)/2 at

temperature.

For the feedback resistors, choose R2 = 137k. Then, from

Equation 3, R1 is 200k. Figure 4 shows the complete circuit

along with its efﬁciency curve.

APPLICATIO S I FOR ATIO

Figure 4a. High Efﬁciency Step-Down Regulator

RUN

CIN

4.7

µF

CERAMIC

VIN

1.8V TO

3.1V

LTC3549

VIN

MODE

3.3

µH*

22pF

200k

137k

3549 F04a

VFB

GND

COUT

4.7

µF

CERAMIC

VOUT

1.5V

*TDK VLF3012AT-3R3MR87

LOAD CURRENT (mA)

100

0.1

100

1000

3549 F04b

VIN = 1.8

VIN = 3.1

VIN = 2.5

µs/DIV

VOUT

100mV/DIV

AC COUPLED

ILOAD

200mA/DIV

3549 F04c

VIN = 2.5V

VOUT = 1.5V

ILOAD = 100mA to 250mA

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