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LTC3646/LTC3646-1

36461fc

For more information www.linear.com/LTC3646

Figure 9

APPLICATIONS INFORMATION

Design Example

As a design example, consider using the LTC3646-1 in an

application with the following specifications: VIN = 12V,

VOUT = 1.8V, IOUT(MAX) = 1A, IOUT(MIN) = 10mA.

Because efficiency is important at both high and low load

currents, Burst Mode operation and 1MHz operation is

chosen.

First the correct RRT resistor value for 1MHz switching

frequency must be chosen. Based on the equation dis-

cussed previously, the closest standard resistor value

for RRT is 90.9k.

Next, determine the inductor value for approximately 35%

ripple current using:

L =

1.8V

1MHz • 350mA











 1−

1.8V

12V











 = 4.37µH

A standard 4.7µH inductor would work well for this ap-

plication.

NextCOUTisselectedbasedontherequiredoutputtransient

performance and the required ESR to satisfy the output

voltage ripple. Using a 15µF ceramic capacitor with an

ESR of 5mΩ will result in approximately 5mV of ripple.

Decoupling the PVIN pin with a 22µF capacitor and the

SVIN pin with a 1µF capacitor should be adequate for

most applications. A 0.1µF boost capacitor should also

work for most applications.

To save board space the ITH pin is connected to INTVCC

to select internal compensation. Since the switching and

Q current drawn from the 12V supply is not a significant

source of loss or heat, EXTVCC is disabled by tying the

pin to SGND.

SVIN

RUN

INTVCC

MODE/SYNC

ITH

PGOOD

EXTVCC

PVIN

BOOST

VON

VFB

LTC3646-1

LOUT

4.7µH

SGND

PGND

CBOOST

0.1µF

150k

CVCC

4.7µF

RPG

100k

CSVIN

1µF

75k

3646 F09

1pF

COUT

15µF

VOUT

12V

RRT

90.9k

CPVIN

22µF

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