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| LTC3646-1_15 Datasheet, PDF (22/28 Pages) Linear Technology – 40V, 1A Synchronous Step-Down Converter | |||
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LTC3646/LTC3646-1
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
=


ï£
1.8V
1MHz ⢠350mA
ï£ï£¬1â
1.8V
12V


=
4.37µH
A standard 4.7µH inductor would work well for this ap-
plication.
Next COUT is selected based on the required output transient
performance and the required ESR to satisfy the output
voltage ripple. Using a 15µF ceramic capacitor with an
ESR of 5m⦠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.
12V
CSVIN
1µF
CVCC
4.7µF
RPG
100k
RRT
90.9k
SVIN
PVIN
RUN
LTC3646-1
BOOST
INTVCC
MODE/SYNC
SW
ITH
PGOOD
VON
RT
VFB
EXTVCC
SGND
PGND
CPVIN
22µF
CBOOST LOUT
0.1µF 4.7µH
R1
CF
150k
1pF
VOUT
COUT
R2
15µF
75k
3646 F09
Figure 9
22
For more information www.linear.com/LTC3646
36461fb
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