Datasheet

LTC3613
26
3613fa
APPLICATIONS INFORMATION
Design Example
Consider a step-down converter with V
IN
= 6V to 24V, V
OUT
= 1.2V, I
OUT(MAX)
= 15A, and f = 350kHz (see Figure 9).
The regulated output voltage is determined by:
V
OUT
=0.6V• 1+
R
FB2
R
FB1
⎛
⎝
⎜
⎞
⎠
⎟
Using a 20k resistor from V
OSNS
+
to V
OSNS
–
, the top
feedback resistor is also 20k.
The frequency is programmed by:
R
T
kΩ
[]
=
41550
fkHz
[]
–2.2=
41550
350
–2.2≈116.5k
Select the nearest standard value of 115k.
The minimum on-time occurs for maximum V
IN
and should
be greater than 65ns, which is the best that the LTC3613
can achieve. The minimum on-time for this application is:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
• f
=
1.2V
24V•350kHz
≈143ns
Set the inductor value to give 40% ripple current at maxi-
mum V
IN
:
L=
1.2V
350kHz• 40%•15A
• 1–
1.2V
24V
⎛
⎝
⎜
⎞
⎠
⎟
≈0.54µH
Select 0.56H, which is the nearest standard value.
PV
IN
SV
IN
V
OUT
PGOOD
RUN
V
RNG
SENSE
–
SENSE
+
R
PGD
100k
INTV
CC
R
ITH
28k
R
T
115k
R
DCR
3.09k
LTC3613
R
FB2
20k
R
FB1
20k
C
OUT1
330µF
2.5V
×2
C
OUT2
100µF
×2
L1
0.56µH
SW
D
B
C
SS
0.1µF
R
DIV1
52.3k
R
DIV2
10k
C
ITH1
220pF
C
ITH2
100pF
3613 F10
V
IN
6V TO 24V
V
OUT
1.2V
15A
C
DCR
0.1µF
C
B
0.1µF
C
VCC
4.7µF
C
IN1
: SANYO 25SVPD82M
C
OUT1
: SANYO 2R5TPE330M9
D
B
: CENTRAL CMDSH-3
L1: VISHAY IHLP4040DZ-056µH
TRACK/SS
ITH
RT
MODE/PLLIN
EXTV
CC
SGND
BOOST
INTV
CC
INTV
CC
PGND
V
OSNS
+
V
OSNS
–
C
IN2
10µF
C
IN1
82µF
25V
+
+
350kHz
LOAD CURRENT (A)
0.1
40
EFFICIENCY (%)
60
70
80
100
1 10 100
3613 F10a
20
30
50
90
10
0
V
IN
= 12V
V
OUT
= 1.2V
PULSE-SKIPPING
MODE
FORCED
CONTINUOUS
MODE
Figure 9. 1.2V, 15A, 350kHz Step-Down Converter