LT3581IMSE-TRPBF Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT3581IMSE-TRPBF

3.3A Boost/Inverting DC/DC Converter with Fault Protection

Linear Technology 

LT3581

APPLICATIONS INFORMATION

SEPIC CONVERTER COMPONENT SELECTION

(COUPLED OR UN-COUPLED INDUCTORS)

VIN

3V TO 16V

L1

3.3µH

C1

1µF

CIN

22µF

SW1 SW2

ENABLE

RT

124k

RFAULT

100k

VIN

FB

LT3581

FAULT

GATE

SHDN CLKOUT

RT

VC

SYNC

SS

GND

D1

30V, 2A

L2

• 3.3µH

COUT

22µF

×2

RFB

45.3k

CSS

RC

1µF

7.87k

CC

2.2nF

VOUT

5V

IOUT < 0.9A (VIN = 3V)

IOUT < 1.5A (VIN = 12V)

CF

100pF

3581 F06

Figure 6. SEPIC Converter – The Component Values and Voltages

Given Are Typical Values for a 700kHz, Wide Input Range (3V to

16V) SEPIC Converter with 5V Out

The LT3581 can also be configured as a SEPIC as shown in

Figure 6. This topology allows for positive output voltages

that are lower, equal, or higher than the input voltage. Out-

put disconnect is inherently built into the SEPIC topology,

meaning no DC path exists between the input and output

due to capacitor C1. This implies that a PMOS controlled

by the GATE pin is not required in the power path.

Table 2 is a step-by-step set of equations to calculate

component values for the LT3581 when operating as a

SEPIC 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 2.

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

Table 2. SEPIC Design Equations

PARAMETERS/EQUATIONS

Step 1: Pick VIN, VOUT, and fOSC to calculate equations below.

Inputs

Step 2:

DC

DC ≅

VOUT + 0.5V

VIN + VOUT + 0.5V – 0.3V

Step 3:

L

Step 4:

IRIPPLE

Step 5:

IOUT

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 L 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.

• Pick L1 = L2 = L for coupled inductors.

• Pick L1L2 = L for un-coupled inductors.

( ) IRIPPLE =

VIN – 0.3V • DC

fOSC • L

• L = L1 = L2 for coupled inductors.

• L = L1L2 for un-coupled inductors.

IOUT

=





3.3A

–

IRIPPLE

2





•

(1–

DC)

Step 6:

D1

Step 7:

C1

Step 8:

COUT

VR > VIN + VOUT; IAVG > IOUT

C1 ≥ 1µF; VRATING ≥ VIN

COUT

≥

fOSC

IOUT • DC

•0.005• VOUT

Step 9:

CIN

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.

Step 10:

RFB

RFB

=

VOUT – 1.215V

83.3µA

Step 11:

RT

RT

=

87.6

fOSC

– 1;

fOSC

in MHz

and RT

in kΩ

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 COUT, CIN and C1 may deviate from the above

equations in order to obtain desired load transient performance.

3581fb

14

For more information www.linear.com/LT3581

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