5

LTC3426

3426fa

OPERATIO

The LTC3426 is a monolithic 1.2MHz boost converter

housed in a 6-lead SOT-23 package. The device features

fixed frequency, current mode PWM control for excellent

enables the device to maintain high efficiency over a wide

range of load current. Operation of the feedback loop

which sets the peak inductor current to keep the output in

regulation can be best understood by referring to the Block

Diagram in Figure 1. At the start of each clock cycle a latch

in the PWM logic is set and the NMOS switch is turned on.

The sum of a voltage proportional to the switch current

and a slope compensating voltage ramp is fed to the

positive input to the PWM comparator. When this voltage

exceeds either a voltage proportional to the 2A current

limit or the PWM control voltage, the latch in the PWM

logic is reset and NMOS switch is turned off. The PWM

control voltage at the output of the error amplifier is the

amplified and compensated difference between the feed-

back voltage on the FB pin and the internal reference

voltage of 1.22V. If the control voltage increases, more

current is delivered to the output. When the control voltage

limited to a minimum of 2A. The current limit helps protect

the LTC3426 internal switch and external components

connected to it. If the control voltage decreases, less

current is delivered to the output. During load transients

control voltage may decrease to the point where no

switching occurs until the feedback voltage drops below

the reference. The LTC3426 has an integrated soft-start

feature which slowly ramps up the feedback control node

from 0V. The soft-start is initiated when SHDN is pulled

high.

APPLICATIO S I FOR ATIO

Setting the Output Voltage

is set by the formula:

V

R

R

⎛

⎝⎜

⎞

⎠⎟

122

1

1

2

.•

Inductor Selection

The LTC3426 can utilize small surface mount inductors

due to its 1.2MHz switching frequency. A 1.5

µH or 2.2µH

inductor will be the best choice for most LTC3426 appli-

cations. Larger values of inductance will allow greater

output current capability by reducing the inductor ripple

current. Increasing the inductance above 3.3

µH will in-

crease component size while providing little improve-

ment in output current capability. The inductor current

ripple is typically set for 20% to 40% of the maximum

materials reduce frequency dependent power losses com-

pared to cheaper powdered iron types, improving effi-

ciency. The inductor should have low DCR (DC resistance)

handle the peak inductor current without saturating.

Several inductor manufacturers are listed in Table 1.

Table 1. Inductor Manufacturers

TDK

www.tdk.com

Sumida

www.sumida.com

Murata

www.murata.com

Output and Input Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used to minimize the output voltage ripple. Multilayer

ceramic capacitors are an excellent choice as they have

extremely low ESR and are available in small footprints. A

15

µF to 30µF output capacitor is sufficient for most

applications. X5R and X7R dielectric materials are pre-

ferred for their ability to maintain capacitance over wide

voltage and temperature ranges.

Low ESR input capacitors reduce input switching noise

and reduce the peak current drawn from the input supply.

It follows that ceramic capacitors are also a good choice

for input decoupling and should be located as close as