LT8580
23
8580fa
For more information www.linear.com/LT8580
applicaTions inForMaTion
Figure 17. Dual Inductor Inverting Converter: The Component
Values and Voltages Given Are Typical Values for a 750kHz
Wide Input (5V to 40V) to –15V Inverting Topology Using
Coupled Inductors
C
OUT
4.7µF
V
OUT
–15V
90mA (V
IN
= 5V)
210mA (V
IN
= 12V)
420mA (V
IN
= 40V)
L1
22µH
L2
22µH
C1
1µF
D1
R
FBX
182k
V
IN
5V TO 40V
V
IN
SW
8580 F17
LT8580
10k
R
C
13.7k
R
T
113k
SHDN
GND
FBX
VCSYNC
SSRT
C
C
10nF
C
F
47pF
C
SS
0.22µF
C
IN
4.7µF
• •
DUAL INDUCTOR INVERTING CONVERTER COMPONENT
SELECTION (COUPLED OR UNCOUPLED INDUCTORS)
Due to its unique FBX pin, the LT8580 can work in a dual
inductor inverting configuration as in Figure 17. Chang-
ing 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 6 is a step-by-step set of equations to calculate
component values for the LT8580 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 Applications
Information section for further information on the design
equations presented in Table 6.
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
Table 6. 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
MAX
=
V
OUT
+ 0.5V
V
IN(MIN)
+ V
OUT
+ 0.5V– 0.4V
DC
MIN
=
V
OUT
+ 0.5V
V
IN(MAX)
+ V
OUT
+ 0.5V– 0.4V
Step 3:
L
L
TYP
=
(V
IN(MIN)
– 0.4V)• DC
MAX
f
OSC
• 0.3A
(1)
L
MIN
=
(V
IN(MIN)
– 0.4V)• (2 • DC
MAX
– 1)
1.25 • (DC
MAX
− 300ns• f
OSC
)• f
OSC
• (1– DC
MAX
)
(2)
L
MAX
=
(V
IN(MIN)
– 0.4V)• DC
MAX
f
OSC
• 0.08A
(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(MIN)
=
(V
IN(MIN)
– 0.4V)• DC
MAX
f
OSC
• L
I
RIPPLE(MAX)
=
(V
IN(MAX)
– 0.4V)• DC
MIN
f
OSC
• L
Step 5:
I
OUT
I
OUT(MIN)
= 1A −
I
RIPPLE(MIN)
2
⎛
⎝
⎜
⎞
⎠
⎟
• 1−DC
MAX
( )
I
OUT(MAX)
= 1A −
I
RIPPLE(MAX)
2
⎛
⎝
⎜
⎞
⎠
⎟
• 1−DC
MIN
( )
Step 6:
D1
V
R
> V
IN
+ |V
OUT
|; I
AVG
> I
OUT
Step 7:
C1
C1 ≥ 1µF; V
RATING
≥ V
IN(MAX)
+ |V
OUT
|
Step 8:
C
OUT
C
OUT
≥
I
RIPPLE(MAX)
8 • f
OSC
(0.005 • V
OUT
)
Step 9:
C
IN
C
IN
≥ C
VIN
+C
PWR
≥
1A • DC
MAX
40 • f
OSC
• 0.005 • V
IN(MIN)
+
I
RIPPLE(MAX)
8 • f
OSC
• 0.005 • V
IN(MAX)
•Refer to the Capacitor Selection Section for definition of C
VIN
and C
PWR
Step 10:
R
FBX
R
FBX
=
V
OUT
+ 3mV
83.3µA
Step 11:
R
T
R
T
=
f
OSC
–1; f
OSC
in MHz and R
T
in kΩ
Note 1: This table uses 1A for the peak switch current. Refer to the
Electrical Characteristics Table and Typical Performance Characteristics
plots for the peak switch current at an operating duty cycle.
Note 2: The final values for C
OUT
, C
IN
and C1 may deviate from the
previous equations in order to obtain desired load transient performance.
