Datasheet

LT3579/LT3579-1
14
35791fa
For more information www.linear.com/LT3579
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
SEPIC CONVERTER COMPONENT SELECTION
– COUPLED OR UN-COUPLED INDUCTORS
The LT3579 can also be configured 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
IN
, V
OUT
and f
OSC
respectively). Refer to the Appendix
for further information on the design equations presented
in Table 2.
Variable Definitions:
V
IN
= Input Voltage
V
OUT
= Output Voltage
DC = Power Switch Duty Cycle
f
OSC
= Switching Frequency
I
OUT
= Maximum Output Current
I
RIPPLE
= Inductor Ripple Current
Table 2. SEPIC Design Equations
PARAMETERS/EQUATIONS
Step 1:
Inputs
Pick V
IN
, V
OUT
, and f
OSC
to calculate equations below.
Step 2: DC
D
C ≅
V
OUT
+ 0.5V
V
IN +
V
OUT
+ 0.5V – 0.27V
Step 3: L
L
TYP
=
V
IN
– 0.27V
( )
• DC
f
OSC
• 1.8A
L
MIN
=
V
IN
– 0.27V
( )
• 2 • DC – 1
( )
4A • f
OSC
• 1– DC
( )
L
MAX
=
V
IN
– 0.27V
( )
• DC
f
OSC
• 0.5A
(1)
(2)
(3)
• Solve equations 1, 2, and 3 for a range of L values.
• The minimum of the L value range is the higher of
L
TYP
and L
MIN
.
• The maximum of the L value range is L
MAX
.
• L = L1 = L2 for coupled inductors.
• L = L1||L2 for uncoupled inductors.
Step 4:
I
RIPPLE
I
RIPPLE
=
V
IN
– 0.27V
( )
• DC
f
OSC
• L
Step 5: I
OUT
I
OUT
= 6A –
I
RIPPLE
2






• 1– DC
( )
Step 6: D1
V
R
>
V
IN
+
V
OUT
;I
AVG
>
I
OUT
Step 7: C1
4.7µF (typical); V
RATING
> V
IN
Step 8: C
OUT
C
OUT
=
I
OUT
• DC
f
OSC
• 0.005 •V
OUT
Step 9: C
PWR
C
PWR
=
I
RIPPLE
8 • f
OSC
• 0.005 • V
IN
Step 10: C
VIN
C
VIN
=
6A
•
DC
40 • f
OSC
• 0.005 • V
IN
Step 11: R
FB
R
FB
=
V
OUT
– 1.215V
83.3µA
Step 12: R
T
R
T
=
87.6
f
OSC
–1; f
OSC
inMHz andR
T
in kΩ
Note: The maximum design target for peak switch current is 6A and
is used in this table. The final values for C
OUT
, C
PWR
, and C
VIN
may
deviate from the above equations in order to obtain desired load
transient performance for a particular application.
V
PWR
9V TO 16V
R
T
86.6k
100k
C
OUT
10µF
×3
C
PWR
4.7µF
R
FB
130k
C
F
47pF
C
SS
0.22µF
C
VIN
4.7µF
L1
6.8µH
D1
60V, 3A
C1
4.7µF
SW1 SW2
GATE
V
IN
RT
V
C
C
C
2.2nF
R
C
9.53k
FAULT
SHDN
FB
SSGNDSYNC
CLKOUT
LT3579
35791 F07
V
OUT
12V
1.6A (V
PWR
>9V)
1.9A (V
PWR
>12V)
L2
6.8µH
V
IN
3.3V
TO 5V