LT8580
21
8580fa
For more information www.linear.com/LT8580
Figure 15. Boost Converter: The Component Values and Voltages
Given Are Typical Values for a 1.5MHz, 5V to 12V Boost
applicaTions inForMaTion
C
OUT
4.7µF
V
OUT
12V
200mA
L1
15µH
D1
R
FBX
130k
V
IN
5V
V
IN
SW
8580 F15
LT8580
10k
R
C
6.04k
R
T
56.2k
SHDN
GND
FBX
VCSYNC
SSRT
C
C
3.3nF
C
F
47pF
C
SS
0.22µF
C
IN
2.2µF
BOOST CONVERTER COMPONENT SELECTION
The LT8580 can be configured as a boost converter as in
Figure 15. This topology allows for positive output voltages
that are higher than the input voltage. A single feedback
resistor sets the output voltage. For output voltages higher
than 60V, see the Charge Pump Aided Regulators section.
Table 4 is a step-by-step set of equations to calculate com
-
ponent values for the LT8580 when operating as a boost
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 4.
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 4. Boost Design Equations
PARAMETERS/EQUATIONS
Step 1:
Inputs
Pick V
IN
, V
OUT
, and f
OSC
to calculate equations below
Step 2:
DC
DC
MAX
=
OUT
IN(MIN)
+
V
OUT
+ 0.5V– 0.4V
DC
MIN
=
V
OUT
– V
IN(MAX)
+ 0.5V
V
0.5V– 0.4V
Step 3:
L1
L
TYP
=
(V
IN(MIN)
– 0.4V)• DC
MAX
f
OSC
• 0.3A
(1)
L
MIN
=
IN(MIN)
MAX
1.25 • (DC
MAX
− 300ns• f
OSC
)• f
OSC
• (1– DC
MAX
)
(2)
L
MAX1
=
(V
IN(MIN)
– 0.4V)• DC
MAX
f
OSC
• 0.08A
(3)
L
MAX2
=
(V
IN(MAX)
– 0.4V)• DC
MIN
f
OSC
• 0.08A
(4)
•Solve equations 1 to 4 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 the lower of L
MAX1
and L
MAX2
.
Step 4:
I
RIPPLE
I
RIPPLE(MIN)
=
(V
IN(MIN)
– 0.4V)• DC
MAX
f
OSC
• L
1
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
OUT
; I
AVG
> I
OUT
Step 7:
C
OUT
C
OUT
≥
I
OUT
• DC
MAX
Step 8:
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 9:
R
FBX
R
FBX
=
V
OUT
−1.204V
Step
10:
R
T
R
T
=
f
–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
and C
IN
may deviate from the previous
equations in order to obtain desired load transient performance.
