LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 15

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

applicaTions inForMaTion
DUAL INDUCTOR INVERTING CONVERTER COMPONENT
SELECTION (COUPLED OR UN-COUPLED INDUCTORS)
V
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
V
further information on the design equations presented
in Table 3.
Variable Definitions:
V
V
DC = Power Switch Duty Cycle
f
I
I
5V
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
OSC
OUT
RIPPLE
IN
ENABLE
OUT
IN
OUT
= Input Voltage
43.2k
= Switching Frequency
= Maximum Average Output Current
= Output Voltage
R
and f
T
= Inductor Ripple Current
R
C
3.3µF
100k
FAULT
IN
OSC
3.3µH
L1
V
FAULT
SHDN
RT
SYNC
IN
respectively). Refer to the Appendix for
SW1 SW2
LT3581
GND
CLKOUT
GATE
SS
V
FB
1µF
C1
C
C
100nF
SS
D1
20V
1A
3.3µH
L2
R
11k
C
C
1nF
C
C
4.7µF
143k
R
OUT
FB
C
47pF
3581 F07
F
V
–12V
I
OUT
OUT
< 625mA
IN
,
Table 3. Dual Inductor Inverting Design Equations
Step 1: Inputs Pick V
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 L out of a range of inductor values where the
• 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
minimum value of the range is set by L
whichever is higher. The maximum value of the range
is set by L
rating for inductor value chosen.
definition of C
C
45
IN
R
IN
L
L
L
•
≥
T
, V
TYP
MIN
MAX
I
f
OUT
OSC
C
=
C
OUT
DC
C
3 3
VIN
MAX
OUT
OUT
V
1 1
.
87 6
f
=
=
OSC
I
R
=
, and f
≥
RIPPLE
=
A DC
•
≅
PARAMETERS/EQUATIONS
.
, C
(
+
(
. See Appendix on how to choose current
>
VIN
(
0 005

 
≥
•
V
V
R
V
µF V
V
C
.
IN
3 3
V
IN
IN
2 2
IN
– ;
IN
FB
PWR
f
8
and C
IN
O
.
.
;
1
OSC
and C1 may deviate from the above
f
– .
– .
S S C
•
+
OSC
– .
+
=
A
A f
=
RATING
0 3
f
0 3
•
|
|
0 3
OSC
f
|
OSC
•
• 0 35
(
–
≥
|
to calculate equations below.
V
V
V
PWR
V
V
V
OUT
•
OUT
OUT
OSC
IN
IN
OUT
V
V
I
V
.
1
RIPPLE
83 3
(
)
)
in MHz and R in k
A
I
.
)
– . 0 3
0 005
+
f
RIPPLE
•
2
|
•
OSC
| ;
≥
|
•
.
.
•
|
+
A
8
+
(
DC
+
DC
0 5
I
V
2 D D C
(
µA
AVG
•
0 5
1
IN
.
5
•
•
V
f

 
.
OSC
–
mV
•
V
L
)
+
•
V
DC
•
|
>
I
– .
(
|
V
RIP
• .
DC
1
V
LT3581
I
–
OUT
0 3
TYP
)
OUT
0 005
OUT
–
P P LE
1
T
)
DC
V
or L
|
)
|
)
•

MIN
Ω
V
IN
,
(1)
(2)
(3)
3581f