Datasheet
LT3581
15
3581fa
For more information www.linear.com/LT3581
Due to its unique FB pin, the LT3581 can work in a Dual
Inductor Inverting configuration as in Figure 7. Changing
the connections of L2 and the Schottky diode in the SEPIC
topology results in generating negative output voltages.
This solution results in very low output voltage ripple
due to inductor L2 being in series with the output. Output
disconnect is inherently built into this topology due to the
capacitor C1.
Table 3 is a step-by-step set of equations to calculate
component values for the LT3581 when operating as a
dual inductor inverting 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 3.
Variable Definitions:
V
IN
= Input Voltage
V
OUT
= Output Voltage
DC = Power Switch Duty Cycle
f
OSC
= Switching Frequency
I
OUT
= Maximum Average Output Current
I
RIPPLE
= Inductor Ripple Current
applicaTions inForMaTion
Figure 7. Dual Inductor Inverting Converter – The Component
Values and Voltages Given Are Typical Values for a 2MHz, 5V to
–12V Inverting Topology Using Coupled Inductors
DUAL INDUCTOR INVERTING CONVERTER COMPONENT
SELECTION (COUPLED OR UN-COUPLED INDUCTORS)
Table 3. Dual Inductor Inverting Design Equations
PARAMETERS/EQUATIONS
Step 1: Inputs Pick V
IN
, V
OUT
, and f
OSC
to calculate equations below.
Step 2: DC
DC
VV
VV
VV
OUT
IN OUT
≅
+
++
||.
||.–.
05
05 03
Step 3: L
L
VVDC
fA
L
VV
TYP
IN
OSC
MIN
IN
=
()
•
•
=
()
••
–.
–.
03
1
03 2
DDC
Af DC
L
VVDC
f
OSC
MAX
IN
O
–
.–
–.
1
22 1
03
()
••
()
=
()
•
SSC
A
•
035.
(1)
(2)
(3)
• Pick L out of a range of inductor values where the
minimum value of the range is set by L
TYP
or L
MIN
,
whichever is higher. The maximum value of the range
is set by L
MAX
. See Appendix on how to choose current
rating for inductor value chosen.
• Pick L1 = L2 = L for coupled inductors.
• Pick L1L2 = L for un-coupled inductors.
Step 4: I
RIPPLE
I
VV
DC
fL
RIPPLE
IN
OSC
=
()
•
•
–.03
• L = L1 = L2 for coupled inductors.
• L = L1L2 for un-coupled inductors.
Step 5: I
OUT
IA
I
DC
OUT
RIPPLE
=
•
()
33
2
1.– –
Step 6: D1
VV VII
RIN OUT AVG OUT
>+ >||;
Step 7: C1
CµFV
VV
RATING IN OUT
11
≥≥+;|
|
Step 8: C
OUT
C
I
fV
OUT
RIPPLE
OSC OUT
≥
••
()
80005.||
Step 9: C
IN
CC C
ADC
fV
I
IN VINPWR
OSCIN
RIP
≥+ ≥
•
•• •
+
33
45 0 005
.
.
PPLE
OSCIN
fV
80005••.•
• Refer to Input Capacitor Selection in Appendix for
definition of C
VIN
and C
PWR
.
Step 10: R
FB
R
VmV
µA
FB
OUT
=
+||
.
5
83 3
Step 11: R
T
R
f
finMHz andR in k
T
OSC
OS
CT
=
87 6
1
.
–; Ω
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
OUT
, C
IN
and C1 may deviate from the above
equations in order to obtain desired load transient performance.
L2
3.3µH
D1
20V
1A
V
IN
5V
R
FAULT
100k
R
T
43.2k
L1
3.3µH
3581 F07
C
SS
100nF
C
OUT
4.7µF
C
IN
3.3µF
V
OUT
–12V
I
OUT
< 625mA
SW1 SW2
FB
CLKOUT
GATE
V
C
SS
V
IN
RT
GND
SYNC
FAULT
SHDN
ENABLE
LT3581
C
F
47pF
R
FB
143k
•
•
C
C
1nF
R
C
11k
C1
1µF