Datasheet

LT3579/LT3579-1
15
35791fa
For more information www.linear.com/LT3579
APPLICATIONS INFORMATION
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
DUAL INDUCTOR INVERTING CONVERTER COMPONENT
SELECTION – COUPLED OR UN-COUPLED INDUCTORS
Due to its unique FB pin, the LT3579 can work in a Dual
Inductor Inverting configuration 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
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 Output Current
I
RIPPLE
= Inductor Ripple Current
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 ≅
|V
OUT
| + 0.5V
V
IN
+ | V
OUT
|+0.5V – 0.27V
Step 3: L
L
TYP
=
V
IN
– 0.27V
( )
• DC
f
OSC
• 1.8A
L
MIN
=
V
IN
– 0.27V
( )
• 2 • DC – 1
( )
4A • f
OSC
• 1– DC
( )
L
MAX
=
V
IN
– 0.27V
( )
• DC
f
OSC
• 0.5A
(1)
(2)
(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
– 0.27V
( )
• DC
f
OSC
• L
Step 5: I
OUT
I
OUT
= 6A –
I
RIPPLE
2






• 1– DC
( )
Step 6: D1
V
R
> V
IN
+| V
OUT
|; I
AVG
> I
OUT
Step 7: C1
4.7µF (typical); V
RATING
> V
IN
+ |V
OUT
|
Step 8: C
OUT
C
OUT
=
I
RIPPLE
8 • f
OSC
•0.005 • |V
OUT
|
Step 9: C
IN
C
IN
= C
PWR
+ C
VIN
C
IN
=
I
RIPPLE
8 • f
OSC
•0.005• V
IN
+
6A •DC
40 • f
OSC
•0.005• V
IN
Step 10: R
FB
R
FB
=
|V
OUT
| + 9mV
83.3µA
Step 11: R
T
R
T
=
87.6
f
OSC
–1; f
OSC
inMHz andR
T
in kΩ
Note: The maximum design target for peak switch current is 6A and
is used in this table. The final values for C
OUT
and C
IN
may deviate
from the above equations in order to obtain desired load transient
performance for a particular application.
V
IN
5V
R
T
72k
100k
C
OUT
10µF
×2
R
FB
144k
C
F
27pF
L1
3.3µH
L2
3.3µH
D1
30V, 2A
C1
4.7µF
SW1 SW2
V
IN
RT
V
C
C
C
1nF
R
C
20k
FAULT
SHDN
FB
CLKOUT
SSSYNC GND
GATE
LT3579
35791 F08
V
OUT
–12V
1.2A
C
SS
0.22µF
C
IN
22µF