LT8580 datasheet(22/32 Pages) LINER

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LT8580

8580fa

For more information www.linear.com/LT8580

Figure 16. SEPIC Converter: The Component Values and Voltages

Given Are Typical Values for a 1MHz, 9V to 16V Input to 12V

Output SEPIC Converter

applicaTions inForMaTion

COUT

4.7µF

VOUT

12V

240mA

22µH

1µF

RFBX

130k

VIN

9V TO 16V

VIN

8580 F16

LT8580

487k

16.2k

84.5k

SHDN

GND

FBX

SYNC

1nF

22pF

CSS

0.22µF

CIN

4.7µF

22µH

•

SEPIC CONVERTER COMPONENT SELECTION

(COUPLED OR UNCOUPLED INDUCTORS)

The LT8580 can also be configured as a SEPIC, as shown

in Figure 16. This topology allows for positive output

voltages that are lower, equal or higher than the input volt-

age. Output disconnect is inherently built into the SEPIC

topology, meaning no DC path exists between the input

and output due to capacitor C1.

Table 5 is a step-by-step set of equations to calculate com-

ponent values for the LT8580 when operating as a SEPIC

converter. Input parameters are input and output voltage,

and switching frequency (VIN, VOUT and fOSC, respectively).

Refer to the Applications Information section for further

information on the design equations presented in Table 5.

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 5. SEPIC Design Equations

PARAMETERS/EQUATIONS

Step 1:

Inputs

Pick VIN, VOUT and fOSC to calculate equations below

Step 2:

DCMAX =

VOUT +0.5V

VIN(MIN) + VOUT +0.5V– 0.4V

DCMIN =

VOUT +0.5V

VIN(MAX) + VOUT +0.5V– 0.4V

Step 3:

LTYP =

(VIN(MIN) – 0.4V)•DCMAX

OSC • 0.3A

(1)

MIN =

(VIN(MIN) – 0.4V)•(2 •DCMAX – 1)

1.25 •(DCMAX −300ns • fOSC)• fOSC •(1–DCMAX)

(2)

MAX =

(VIN(MIN) – 0.4V)•DCMAX

OSC • 0.08 A

(3)

•Solve equations 1, 2 and 3 for a range of L values

•The minimum of the L value range is the higher of LTYP and LMIN

•The maximum of the L value range is LMAX

•L = L1 = L2 for coupled inductors

•L = L1|| L2 for uncoupled inductors

Step 4:

IRIPPLE

IRIPPLE(MIN) =

(VIN(MIN) – 0.4V)•DCMAX

fOSC •L

IRIPPLE(MAX) =

(VIN(MAX) – 0.4V)•DCMIN

fOSC •L

Step 5:

IOUT

IOUT(MIN) = 1A −

IRIPPLE(MIN)

⎛

⎝

⎜

⎞

⎠

⎟ • 1−DCMAX

(

)

IOUT(MAX) = 1A −

IRIPPLE(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ • 1−DCMIN

(

)

Step 6:

VR > VIN + VOUT; IAVG > IOUT

Step 7:

C1 ≥ 1µF; VRATING ≥ VIN

Step 8:

COUT

COUT ≥

IOUT(MIN) •DCMAX

fOSC • 0.005 • VOUT

Step 9:

CIN

CIN ≥ CVIN +CPWR ≥

1A •DCMAX

40 • fOSC • 0.005 • VIN(MIN)

IRIPPLE(MAX)

8 • fOSC • 0.005 • VIN(MAX)

•Refer to the Capacitor Selection Section for definition of CVIN and CPWR

Step 10:

RFBX

RFBX =

VOUT −1.204V

83.3µA

Step 11:

RT =

85.5

OSC

–1; f

OSC in MHz and RT 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 COUT, CIN and C1 may deviate from the

previous equations in order to obtain desired load transient performance.

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