LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 14

• Manufacturer Part Number: LT3581
• Description: 3.3A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

LT3581
applicaTions inForMaTion
3V TO 16V
The LT3581 can also be configured as a SEPIC as shown in
Figure 6. This topology allows for positive output voltages
that are lower, equal, or higher than the input voltage. Out-
put 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 LT3581 when operating as a
SEPIC converter. Input parameters are input and output
voltage, and switching frequency (V
spectively). Refer to the Appendix for further information
on the design equations presented in Table 2.
Variable Definitions:
V
V
DC = Power Switch Duty Cycle
f
I
I
Figure 6. SEPIC Converter – The Component Values and Voltages
Given Are Typical Values for a 700kHz, Wide Input Range (3V to
16V) SEPIC Converter with 5V Out

SEPIC CONVERTER COMPONENT SELECTION
(COUPLED OR UN-COUPLED INDUCTORS)
OSC
OUT
RIPPLE
IN
OUT
V
= Input Voltage
IN
ENABLE
= Switching Frequency
= Maximum Average Output Current
= Output Voltage
= Inductor Ripple Current
124k
R
T
R
C
22µF
100k
FAULT
IN
3.3µH
L1
V
FAULT
SHDN
RT
SYNC
IN
SW1 SW2
LT3581
GND
CLKOUT
GATE
SS
V
1µF
FB
C1
C
•
L2
3.3µH
C
1µF
30V, 2A
SS
D1
IN
R
7.87k
C
C
2.2nF
C
22µF
, V
45.3k
C
OUT
2
R
FB
OUT
C
100pF
3581 F06
V
5V
I
I
F
OUT
OUT
and f
OUT
< 0.9A (V
< 1.5A (V
OSC
IN
IN
= 3V)
= 12V)
re-
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:
R
Step 11:
R
Note 1: The maximum design target for peak switch current is 3.3A and is
used in this table.
Note 2: The final values for C
equations in order to obtain desired load transient performance.
RIPPLE
OUT
OUT
IN
FB
T
Pick V
• Pick L out of a range of inductor values where the minimum
• Pick L1 = L2 = L for coupled inductors.
• Pick L1L2 = L for un-coupled inductors.
• L = L1 = L2 for coupled inductors.
• L = L1L2 for un-coupled inductors.
• Refer to Input Capacitor Selection in Appendix for definition
value of the range is set by L
The maximum value of the range is set by L
Appendix on how to choose current rating for inductor value
chosen.
of C
VIN
IN
C
45
, V
IN
R
and C
L
L
L
•
OUT
≥
T
MIN
MAX
TYP
I
f
OUT
OSC
C
=
3 3
, and f
DC
VIN
PWR
.
87 6
=
=
f
V
=
OSC
I
C
RIPPLE
=
R
A DC
•
PARAMETERS/EQUATIONS
(
OUT
≅
(
.
OUT
+
(
C
.
V
0 005
V
R

 
•
>
V
OSC
1 1
IN
C
IN
2 2
.
V
IN
3 3
FB
– ;
, C
f
V
PWR
IN
.
O
≥
.
f
≥
– .
IN
1
– .
S S C
OSC
– .
=
IN
A f
to calculate equations below.
+
=
A
µF V
f
0 3
0 3
+
OSC
0 3
•
•
f
and C1 may deviate from the above
• 0 35
V
V
OSC
(
–
≥
V
V
;
OUT
•
V
OUT
V
OSC
OUT
V
V
OUT
IN
IN
V
1
.
I
TYP
I
•
RATING
)
RIPPLE
OUT
)
83 3
A
)
in MHz and R in k
0 005
•
– . 0 3
+
•
f
+
2
•
OSC
– .
.
•
;
A
+
.
(
or L
DC
8
DC
0 5
I
1 215
2 D D C
(
AVG
•
0 5
µA
.
1
•
•
.
DC
•
V
f
–
MIN

 
V
≥
OSC
•
L
V
)
•
DC
>
V
– .
V
•
I
, whichever is higher.
(
OUT
V
IN
RIP
I
• .
0 3
–
DC
1
OUT
)
0 005
–
1
P P LE
MAX
T
)
V
DC
. See
)
•
Ω
V
IN
(1)
(2)
(3)
3581f