LT3579 LINER [Linear Technology], LT3579 Datasheet - Page 15

• Manufacturer Part Number: LT3579
• Description: 6A 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
Figure 8. Dual Inductor Inverting Converter – The Component
Values Given Are Typical Values for a 1.2MHz, 5V to –12V Inverting
Topology Using Coupled Inductors
Due to its unique FB pin, the LT3579 can work in a Dual
Inductor Inverting confi guration as in Figure 8. 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 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 LT3579 when operating as a Dual
Inductor Inverting converter using coupled inductors. Input
parameters are input and output voltage, and switching
frequency (V
Appendix for further information on the design equations
presented in Table 3.
Variable Defi nitions:
V
V
DC = Power Switch Duty Cycle
f
I
I
V
OSC
OUT
RIPPLE
5V
IN
IN
OUT
= Input Voltage
= Switching Frequency
= Maximum Output Current
= Output Voltage
C
22μF
= Inductor Ripple Current
IN
R
72k
T
100k
IN
, V
3.3μH
L1
V
SHDN
FAULT
SYNC
RT
OUT
IN
SW1 SW2
LT3579
and f
GND
CLKOUT
OSC
4.7μF
C1
GATE
SS
FB
V
C
respectively). Refer to the
C
0.22μF
SS
D1
30V, 2A
3.3μH
L2
C
27pF
F
R
144k
R
20k
FB
3759 F08
C
C
1nF
C
V
–12V
1.2A
C
10μF
OUT
OUT
2
Table 3. Dual Inductor Inverting 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: The maximum design target for peak switch current is 6A and
is used in this table. The fi nal values for C
from the above equations in order to obtain desired load transient
performance for a particular application.
RIPPLE
OUT
OUT
IN
FB
T
Pick V
• Solve equations 1, 2, and 3.
• Choose the higher value between L
• L = L1 = L2 for coupled inductors.
• L = L1⏐⏐L2 for un-coupled inductors.
C
C
L should never exceed L
IN
IN
=
=
L
L
L
R
IN
C
MIN
MAX
4 7 .
TYP
8
T
, V
PWR
•
DC
=
I
μF typical V
C
OUT
f
OUT
OSC
=
=
V
OUT
=
87 6
f
I
OSC
RIPPLE
LT3579/LT3579-1
R
+
≅
(
, and f
(
(
I
PARAMETERS/EQUATIONS
(
RIPPLE
C
.
V
V
=
>
V
• .
V
IN
IN
=
VIN
R
IN
0 005
IN
⎛
⎜
⎝
V
– ;
f
4
FB
OSC
f
6
IN
– .
8
– .
OSC
1
A f
– .
+
A
OSC
=
•
0 27
0 27
+
=
) ;
0 27
|
•
–
f
f
V
(
OSC
|
|
•
OSC
|
• 0 5 .
•
OSC
V
V
V
to calculate equations below.
OUT
V
1 8
IN
V
MAX
OUT
RATING
OUT
I
OUT
OUT
V
RIPPLE
V
.
IN
V
83 3
in MHz and R in k
– . 0 27
• .
) )
I
)
2
|
A
•
)
f
RIPPLE
A
0 005
+
•
.
OSC
+
•
| ;
and C
|
•
.
|
(
+
0 5
(
DC
1
40
+
D
I
μA
2
.
AVG
0 5
–
>
C C
⎞
⎟ •
⎠
9
•
•
.
V
•
V
mV
DC
L
V
IN
DC
TYP
•
f
V
)
(
IN
OSC
– .
>
1
|
•
may deviate
)
V
6
0 27
I
–
+
–
DC
and L
OUT
OUT
A
• .
DC
|
1
T
• •
V
0 005
)
DC
OUT
V
|
)
MIN
15
Ω
|
for L.
•
V
35791f
IN
(1)
(2)
(3)