LT3581IMSE-TRPBF View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3581IMSE-TRPBF

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

Linear Technology 

LT3581

APPLICATIONS INFORMATION

DUAL INDUCTOR INVERTING CONVERTER COMPONENT

SELECTION (COUPLED OR UN-COUPLED INDUCTORS)

L1

3.3µH

C1

1µF

VIN

5V

CIN

3.3µF

SW1 SW2

ENABLE

RT

43.2k

RFAULT

100k

VIN

FB

LT3581

FAULT

GATE

SHDN CLKOUT

RT

VC

SYNC

SS

GND

L2

3.3µH

D1

20V

1A

COUT

4.7µF

RFB

143k

VOUT

–12V

IOUT < 625mA

CSS

100nF

RC

11k

CC

1nF

CF

47pF

3581 F07

Figure 7. Dual Inductor Inverting Converter – The Component

Values and Voltages Given Are Typical Values for a 2MHz, 5V to

–12V Inverting Topology Using Coupled Inductors

Due to its unique FB pin, the LT3581 can work in a Dual

Inductor Inverting configuration as in Figure 7. Changing

the connections of L2 and the Schottky diode in the SEPIC

topology results in generating negative output voltages.

This solution results in very low output voltage ripple

due to inductor L2 being in series with the output. Output

disconnect is inherently built into this topology due to the

capacitor C1.

Table 3 is a step-by-step set of equations to calculate

component values for the LT3581 when operating as a

dual inductor inverting converter. Input parameters are

input and output voltage, and switching frequency (VIN,

VOUT and fOSC respectively). Refer to the Appendix for

further information on the design equations presented

in Table 3.

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 3. Dual Inductor Inverting Design Equations

PARAMETERS/EQUATIONS

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

Step 2: DC

DC

≅

VIN

| VOUT | + 0.5V

+ | VOUT | +0.5V – 0.3V

Step 3: L

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.

Step 4: IRIPPLE

( ) IRIPPLE =

VIN – 0.3V • DC

fOSC • L

• L = L1 = L2 for coupled inductors.

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

Step 5: IOUT

IOUT

=





3.3A

–

IRIPPLE

2





•

(1–

DC)

Step 6: D1

VR > VIN + | VOUT | ; IAVG > IOUT

Step 7: C1

Step 8: COUT

C1 ≥ 1µF; VRATING ≥ VIN + | VOUT |

( ) COUT

≥

8 • fOSC

IRIPPLE

0.005 • | VOUT |

Step 9: CIN

Step 10: RFB

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 | + 5mV

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

For more information www.linear.com/LT3581

15

Share Link: