lt3579iufd-trpbf Linear Technology Corporation, lt3579iufd-trpbf Datasheet - Page 14

• Manufacturer Part Number: lt3579iufd-trpbf
• Description: Lt3579/lt3579-1 6a Boost/inverting Dc/dc Converter With Fault Protection Features
• Manufacturer: Linear Technology Corporation

LT3579/LT3579-1
APPLICATIONS INFORMATION
Figure 7. SEPIC Converter – The Component Values Given Are
Typical Values for a 1MHz, 9V–16V to 12V SEPIC Topology Using
Coupled Inductors
The LT3579 can also be confi gured as a SEPIC as in
Figure 7. This topology allows for positive output voltages
that are lower, equal, or higher than the input voltage. Output
disconnect is inherently built into the SEPIC topology,
meaning no DC path exists between the input and output
due to capacitor C1. This implies that a PMOS controlled
by the GATE pin is not required in the power path.
Table 2 is a step-by-step set of equations to calculate
component values for the LT3579 when operating as a
SEPIC converter using coupled inductors. Input parameters
are input and output voltage, and switching frequency
(V
for further information on the design equations presented
in Table 2.
Variable Defi nitions:
V
V
DC = Power Switch Duty Cycle
f
I
I
4.7μF
14
SEPIC CONVERTER COMPONENT SELECTION
– COUPLED OR UN-COUPLED INDUCTORS
9V TO 16V
TO 5V
C
OSC
OUT
RIPPLE
3.3V
IN
OUT
VIN
V
IN
IN
V
= Input Voltage
, V
PWR
= Switching Frequency
= Maximum Output Current
= Output Voltage
OUT
= Inductor Ripple Current
100k
R
86.6k
T
C
4.7μF
and f
6.8μH
PWR
L1
SHDN
FAULT
V
RT
SYNC
IN
OSC
SW1 SW2
LT3579
GND
respectively). Refer to the Appendix
CLKOUT
4.7μF
GATE
C1
V
SS
FB
C
C
0.22μF
SS
60V, 3A
L2
6.8μH
D1
C
47pF
F
R
130k
R
9.53k
FB
C
C
2.2nF
C
3759 F07
V
12V
1.6A (V
1.9A (V
C
10μF
OUT
OUT
3
PWR
PWR
>9V)
>12V)
Table 2. SEPIC Design Equations
Step 1:
Inputs
Step 2: DC
Step 3: L
Step 4:
I
Step 5: I
Step 6: D1
Step 7: C1
Step 8: C
Step 9: C
Step 10: C
Step 11: R
Step 12: R
Note: The maximum design target for peak switch current is 6A and
is used in this table. The fi nal values for C
deviate from the above equations in order to obtain desired load
transient performance for a particular application.
RIPPLE
OUT
OUT
PWR
VIN
FB
T
Pick V
• Solve equations 1, 2, and 3.
• Choose the higher value between L
• L = L1 = L2 for coupled inductors.
• L = L1⏐⏐L2 for un-coupled inductors.
L should never exceed L
IN
L
L
L
R
, V
MIN
MAX
TYP
T
OUT
=
I
DC
OUT
=
=
C
C
4 7 .
87 6
=
f
I
V
, and f
OSC
RIPPLE
C
PWR
VIN
(
R
≅
(
PARAMETERS/EQUATIONS
(
OUT
.
V
V
=
μF typical V
V
R
>
IN
V
IN
IN
=
⎛
⎜
⎝
FB
IN
– ;
f
4
V
=
6
(
OSC
f
OSC
=
– .
– .
OSC
40
IN
1
– .
A f
+
A
=
=
8
f
0 27
0 27
OSC
•
0 27
V
+
–
f
•
(
•
OSC
V
to calculate equations below.
•
OUT
• 0 5 .
V
V
OSC
V
OUT
MAX
f
f
1 8
OSC
IN
OSC
OUT
I
OUT
• 0 005
RIPPLE
I
V
V
.
) ;
OUT
OUT
V
6
83 3
in MHz and R in k
– . 0 27
) )
+
)
2
I
.
A
•
)
f
A DC
RIPPLE
– .
A
OSC
.
•
•
•
•
;
.
+
•
, C
0 5
(
RATING
I
•
DC
(
1
•
1 215
0 005
0 005
.
D
AVG
0 5
μA
2
.
.
PWR
–
⎞
⎟ •
⎠
DC
.
C C
V
•
•
TYP
V
•
DC
L
V
DC
V
– .
)
>
(
, and C
OUT
1
V
•
>
0 27
•
•
)
I
and L
OUT
–
DC
–
V
V
V
DC
IN
IN
1
IN
T
)
V
MIN
VIN
)
Ω
may
for L.
35791f
(1)
(2)
(3)