Datasheet

LTC3646/LTC3646-1
22
36461fa
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: V
IN
= 12V,
V
OUT
= 1.8V, I
OUT(MAX)
= 1A, I
OUT(MIN)
= 10mA.
Because efficiency is important at both high and low load
currents, Burst Mode operation and 1MHz operation is
chosen.
First the correct R
RT
resistor value for 1MHz switching
frequency must be chosen. Based on the equation dis-
cussed previously
,
the closest standard resistor value
for R
RT
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.
Next
C
OUT
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 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 INTV
CC
to select internal compensation. Since the switching and
Q current drawn from the 12V supply is not a significant
source of loss or heat, EXTV
CC
is disabled by tying the
pin to SGND.
SVIN
RUN
INTV
CC
MODE/SYNC
ITH
PGOOD
RT
EXTV
CC
PVIN
BOOST
SW
V
ON
V
FB
LTC3646-1
L
OUT
4.7µH
SGND PGND
C
BOOST
0.1µF
R1
150k
C
VCC
4.7µF
R
PG
100k
C
SVIN
1µF
R2
75k
3646 F09
C
F
1pF
C
OUT
15µF
V
OUT
12V
R
RT
90.9k
C
PVIN
22µF