LT3581 LINER [Linear Technology], LT3581 Datasheet - Page 13

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

applicaTions inForMaTion
BOOST CONVERTER COMPONENT SELECTION
The LT3581 can be configured as a Boost converter as
in Figure 5. This topology allows for positive output volt-
ages that are higher than the input voltage. An external
PMOS (optional) driven by the GATE pin of the LT3581 can
achieve input or output disconnect during a fault event.
A single feedback resistor sets the output voltage. For
output voltages higher than 40V, see the Charge Pump
Aided Regulators section.
Table 1 is a step-by-step set of equations to calculate
component values for the LT3581 when operating as a
boost converter. Input parameters are input and output
voltage, and switching frequency (V
spectively). Refer to the Appendix for further information
on the design equations presented in Table 1.
Variable Definitions:
V
V
DC = Power Switch Duty Cycle
f
I
I
R
using PMOS)
4.7µF
Figure 5. Boost Converter – The Component Values and Voltages
Given Are Typical Values for a 2MHz, 5V to 12V Boost
OSC
OUT
RIPPLE
V
5V
C
IN
OUT
DSON_PMOS
IN
IN
= Input Voltage
= Switching Frequency
= Maximum Average Output Current
43.2k
= Output Voltage
R
= Inductor Ripple Current
T
R
100k
FAULT
1.5µH
L1
= R
V
FAULT
SHDN
RT
SYNC
IN
DSON
SW1 SW2
LT3581
GND
CLKOUT
of External PMOS (set to 0 if not
GATE
20V, 2A
SS
V
D1
FB
C
56pF
C
4.7µF
130k
C
0.1µF
R
OUT1
C
SS
FB
F
IN
, V
R
6.04k
OPTIONAL
GATE
OUT
R
10.5k
PMOS
C
C
1nF
C
3581 F05
and f
C
4.7µF
V
12V
I
OSC
OUT
OUT2
OUT
< 0.83A
re-
Table 1. Boost Design Equations
Step 1:
Inputs
Step 2:
DC
Step 3:
L1
Step 4:
I
Step 5:
I
Step 6:
D1
Step 7:
C
C
Step 8:
C
Step 9:
R
Step 10:
R
Step 11:
PMOS
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
above equations in order to obtain desired load transient performance.
RIPPLE
OUT
OUT1
OUT2
IN
FB
T
,
Pick V
• Pick L1 out of a range of inductor values where the minimum
• If PMOS is not used, then use just one capacitor where
• Refer to Input Capacitor Selection in Appendix for definition of
Only needed for input or output disconnect. See PMOS Selection
in the Appendix for information on sizing the PMOS, R
picking appropriae UVLO components.
value of the range is set by L
The maximum value of the range is set by L
on how to choose current rating for inductor value chosen.
C
C
OUT
VIN
IN
and C
= C
C
C
f
, V
45
OSC
L
L
L
OUT
IN
R
MIN
MAX
TYP
OUT1
OUT
•
≥
T
 
PWR
1
I
0 01
f
OUT
OSC
C
=
, and f
=
=
.
3 3
=
=
+ C
VIN
C
.
.
87 6
f
(
OSC
(
I
OUT
(
DC
RIPPLE
V
=
A DC
•
V
•
PARAMETERS/EQUATIONS
V
OUT2
OUT1
.
IN
+
IN
2 2
V
V
IN
0 005
R
OSC
f

 
•
O
.
R
OUT
C
2
.
≅
3 3
FB
f
– ;
– .
S S C
– .
OSC
PWR
– .
, C
A f
.
>
≥
.
1
to calculate equations below.
V
0 3
0 3
=
0 3
V
A
•
• 0 35
=
OUT
V
– .
OUT2
OUT
•
•
OUT
I
f
V
OSC
OSC
–
OUT
0 50
≥
V
(
V
V
1
.
V
OUT
V
)
)
A
TYP
IN
IN
+
)
I
;
and C
•
–
•
RIPPLE
83 3
•
in MHz and R in k
•
A
I
•
0 5
f
– . 0 3
+
DC
(
OSC
•
AVG
V
DC
2
– .
or L
DC
.
2 D D C
(
.
I
IN
1
8
OUT
1 215
•
V
µA
IN
–
•
+
•
MIN
>
V

 
f
– .
DC
OSC
may deviate from the
L
0 5
• R R
I
)
0 3
•
OUT
1
.
, whichever is higher.
–
•
I
)
V
(
RIP
DSON PMOS
V
DC
1
• .
1
V
0 005
)
MAX
–
P P LE
T
LT3581
DC
_
. See appendix
)
•
Ω
V
GATE
IN

 
(1)
(2)
(3)
and
3581f