LT3581IMSE-TRPBF Ver la hoja de datos (PDF) - Linear Technology
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LT3581IMSE-TRPBF Datasheet PDF : 36 Pages
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LT3581
APPLICATIONS INFORMATION
BOOST CONVERTER COMPONENT SELECTION
VIN
5V
CIN
4.7µF
L1
1.5µH
D1
20V, 2A
SW1 SW2
RFAULT
100k
VIN
FB
LT3581
FAULT
GATE
SHDN
RT
CLKOUT
VC
RT
43.2k
SYNC
SS
GND
COUT1
4.7µF
RFB
130k
CF
56pF
CSS
0.1µF
OPTIONAL
PMOS
RGATE
6.04k
VOUT
12V
IOUT < 0.83A
COUT2
4.7µF
RC
10.5k
CC
1nF
3581 F05
Figure 5. Boost Converter – The Component Values and Voltages
Given Are Typical Values for a 2MHz, 5V to 12V Boost
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 (VIN, VOUT and fOSC re-
spectively). Refer to the Appendix for further information
on the design equations presented in Table 1.
Variable Definitions:
VIN = Input Voltage
VOUT = Output Voltage
DC = Power Switch Duty Cycle
fOSC = Switching Frequency
IOUT = Maximum Average Output Current
IRIPPLE = Inductor Ripple Current
RDSON_PMOS = RDSON of External PMOS (set to 0 if not
using PMOS)
Table 1. Boost Design Equations
PARAMETERS/EQUATIONS
Step 1: Pick VIN, VOUT, and fOSC to calculate equations below.
Inputs
Step 2:
DC
DC ≅ VOUT – VIN + 0.5V
VOUT + 0.5V – 0.3V
Step 3:
L1
Step 4:
IRIPPLE
LTYP
=
( VIN
– 0.3V) •
fOSC • 1A
DC
(1)
LMIN
=
(
VIN – 0.3V) • (2 • DC –
2.2A • fOSC • (1– DC)
1)
(2)
LMAX
=
(VIN – 0.3V) • DC
fOSC • 0.35A
(3)
• Pick L1 out of a range of inductor values where the minimum
value of the range is set by LTYP or LMIN, whichever is higher.
The maximum value of the range is set by LMAX. See appendix
on how to choose current rating for inductor value chosen.
( ) IRIPPLE =
VIN – 0.3V • DC
fOSC • L1
Step 5:
IOUT
IOUT
=
3.3A
–
IRIPPLE
2
•
(1–
DC)
Step 6:
D1
VR > VOUT; IAVG > IOUT
Step 7:
COUT1,
COUT2
Step 8:
CIN
Step 9:
RFB
COUT1 = COUT2 ≥
IOUT • DC
fOSC0.01• VOUT – 0.50 • IOUT • RDSON _PMOS
• If PMOS is not used, then use just one capacitor where
COUT = COUT1 + COUT2.
CIN ≥ CVIN + CPWR ≥
45
•
3.3A • DC
fOSC • 0.005
•
VIN
+
8
•
IRIPPLE
fOSC • 0.005
•
VIN
• Refer to Input Capacitor Selection in Appendix for definition of
CVIN and CPWR.
RFB
=
VOUT – 1.215V
83.3µA
Step 10:
RT
RT
=
87.6
fOSC
– 1;
fOSC
in MHz
and RT
in kΩ
Step 11:
PMOS
Only needed for input or output disconnect. See PMOS Selection
in the Appendix for information on sizing the PMOS, RGATE and
picking appropriae UVLO components.
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 COUT1, COUT2 and CIN may deviate from the
above equations in order to obtain desired load transient performance.
For more information www.linear.com/LT3581
3581fb
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