Datasheet

LT8582
14
8582f
APPLICATIONS INFORMATION
SEPIC Converter Component Selection – Coupled or
Uncoupled Inductors
Each channel of the LT8582 can also be configured as a
SEPIC as shown in Figure 6. This topology allows for posi-
tive 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. Therefore the
external PMOS is not required.
Table 2 is a step-by-step set of equations to calculate
component values for the LT8582 when operating as a
SEPIC converter. 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
Figure 6. SEPIC Converter – The Component Values Given
Are Typical Values for a 700kHz, 3V - 19V to 5V SEPIC
Topology Using Coupled Inductors
Table 2. SEPIC Design Equations
PARAMETERS/EQUATIONS
Step 1: Inputs Choose V
IN
, V
OUT
and f
OSC
to calculate equations
below.
Step 2: DC
DC ≅
V
OUT
+ 0.5V
V
IN
+ V
OUT
+ 0.5V– 0.3V
Step 3: L
L
TYP
=
(V
IN
– 0.3V) •DC
f
OSC
•1A
(1)
L
MIN
=
(V
IN
–0.3V)•(2•DC–1)
1.7A • f
OSC
•(1–DC)
(2)
L
MAX
=
(V
IN
–0.3V)•DC
f
OSC
•0.18A
(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
– 0.3V) •DC
f
OSC
•L
Step 5: I
OUT
I
OUT
=
3A–
I
RIPPLE
2
⎛
⎝
⎜
⎞
⎠
⎟
• (1– DC)
Step 6: D1 V
R
≥ V
IN
+ V
OUT
; I
AVG
≥ I
OUT
Step 7: C1 C1 ≥ 1µF; V
RATING
≥ V
IN
Step 8: C
OUT
C
OUT
≥
I
OUT
•DC
f
OSC
• 0.005 • V
OUT
Step 9: C
IN
C
IN
≥ C
VIN
+ C
PWR
≥
3A •DC
50 • f
Osc
• 0.005 • V
IN
+
I
RIPPLE
8•f
Osc
• 0.005 • V
IN
Step 10: R
FBX
R
FBX
=
V
OUT
– 1.204V
83.3µA
⎛
⎝
⎜
⎞
⎠
⎟
Step 11: R
T
R
T
=
81.6
f
OSC
–1;f
OSC
in MHz,R
T
in kΩ
Note 1: Above equations use numbers good for many applications but
for more exact results use the equations from the appendix with numbers
from the Electrical Characteristics.
Note 2: The final values for C
OUT
, and C
IN
may deviate from the above
equations in order to obtain desired load transient performance.
SSGND
SYNC
SWB
C1
2.2µF
SWA
LT8582
CHx
8582 F06
PG
RT
V
IN
SHDN
CLKOUT
V
C
FBX
GATE
V
OUT
5V
1A(V
IN
>12V)
V
IN
3V TO
19V
100k
R
T
107K
14.7k
L1
6.8µH
D1
40V, 2A
R
FBX
45.3k
C
IN
10µF
0.1µF
1.5nF
47pF
C
OUT
22µF
×2
s
s
L2
6.8µH