Datasheet

LT8582
15
8582f
APPLICATIONS INFORMATION
Dual Inductor Inverting Converter Component
Selection – Coupled or Uncoupled Inductors
Figure 7. Dual Inductor Inverting Converter – The Component
Values Given Are Typical Values for a 1.5MHz, 5V to –12V
Inverting Topology Using Coupled Inductors
Table 3. Dual Inductor Inverting Design Equations
PARAMETERS/EQUATIONS
Step 1: Inputs Choose 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– 0.3V
Step 3: L
L
TYP
=
(V
IN
– 0.3V) •DC
f
OSC
•1A
(1)
L
MIN
=
(V
IN
–0.3V)•(2•DC–1)
1.7A • f
OSC
•(1–DC)
(2)
L
MAX
=
(V
IN
– 0.3V)•DC
f
OSC
•0.18A
(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
– 0.3V) •DC
f
OSC
•L
Step 5: I
OUT
I
OUT
=
3A–
I
RIPPLE
2
⎛
⎝
⎜
⎞
⎠
⎟
• (1– DC)
Step 6: D1 V
R
> V
IN
+ |V
OUT
|; I
AVG
> I
OUT
Step 7: C1 C1 ≥ 1µF; V
RATING
≥ V
IN
+ |V
OUT
|
Step 8: C
OUT
C
OUT
≥
I
RIPPLE
8 t f
OSC
t 0.005 t |V
OUT
|
Step 9: C
IN
C
IN
≥ C
VIN
+ C
PWR
≥
3A •DC
50 • f
Osc
•0.005• V
IN
+
I
RIPPLE
8•f
Osc
•0.005• V
IN
Step 10: R
FBX
R
FBX
=
|V
OUT
|+ 7mV
83.3µA
Step 11: R
T
R
T
=
81.6
f
OSC
–1;f
OSC
in MHz, R
T
in kΩ
Note 1: Above equations use numbers good for many applications but
for more exact results use the equations from the appendix with numbers
from the Electrical Characteristics.
Note 2: The final values for C
OUT
, and C
IN
may deviate from the above
equations in order to obtain desired load transient performance.
Due to its unique FBX pin, each channel of the LT8582 can
work in a dual inductor inverting configuration as shown in
Figure 7. Changing the connections of L2 and the Schottky
diode in the SEPIC topology results in generating negative
output voltages. This configuration results in very low
output voltage ripple due to inductor L2 in series with
the output. Output disconnect is inherently built into this
topology because of capacitor C1.
Table 3 is a step-by-step set of equations to calculate
component values for the LT8582 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 Output Current
I
RIPPLE
= Inductor Ripple Current
SSGNDSYNC
SWB
C1
2.2µF
SWA
LT8582
CHx
8582 F07
PG
RT
V
IN
SHDN
CLKOUT
V
C
FBX
GATE
V
OUT
–12V
550mA
V
IN
5V
100k
R
T
53.6K
14.7k
L1
4.7µH
L2
4.7µH
D1
30V, 2A
R
FBX
143k
C
IN
4.7µF
0.1µF 2.2nF
47pF
C
OUT2
10µF
s s