LT8580 datasheet(21/32 Pages) LINER

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LT8580

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

COUT

4.7µF

VOUT

12V

200mA

15µH

RFBX

130k

VIN

8580 F15

LT8580

10k

6.04k

56.2k

SHDN

GND

FBX

SYNC

3.3nF

47pF

CSS

0.22µF

CIN

2.2µF

BOOST CONVERTER COMPONENT SELECTION

The LT8580 can be configured as a boost converter as in

Figure15.Thistopologyallowsforpositiveoutputvoltages

that are higher than the input voltage. A single feedback

resistorsetstheoutputvoltage.Foroutputvoltageshigher

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,

andswitchingfrequency(VIN,VOUTandfOSCrespectively).

Refer to the Applications Information section for further

information on the design equations presented in Table 4.

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 4. Boost Design Equations

PARAMETERS/EQUATIONS

Step 1:

Inputs

Pick VIN, VOUT, and fOSC to calculate equations below

Step 2:

DCMAX =

VOUT – VIN(MIN) +0.5V

VOUT +0.5V– 0.4V

DCMIN =

VOUT – VIN(MAX) +0.5V

VOUT +0.5V– 0.4V

Step 3:

TYP =

(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)

LMAX1 =

(VIN(MIN) – 0.4V)•DCMAX

fOSC • 0.08A

(3)

LMAX2 =

(VIN(MAX) – 0.4V)•DCMIN

fOSC • 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 LTYP and LMIN

•The maximum of the L value range is the lower of LMAX1 and LMAX2.

Step 4:

IRIPPLE IRIPPLE(MIN) =

(VIN(MIN) – 0.4V)•DCMAX

fOSC •L1

IRIPPLE(MAX) =

(VIN(MAX) – 0.4V)•DCMIN

fOSC •L1

Step 5:

IOUT

IOUT(MIN) = 1A −

IRIPPLE(MIN)

⎛

⎝

⎜

⎞

⎠

⎟ • (1−DCMAX )

IOUT(MAX) = 1A −

IRIPPLE(MAX)

⎛

⎝

⎜

⎞

⎠

⎟ • (1−DCMIN)

Step 6:

VR > VOUT; IAVG > IOUT

Step 7:

COUT

COUT ≥

IOUT •DCMAX

fOSC • 0.005 • VOUT

Step 8:

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 9:

RFBX

RFBX =

VOUT −1.204V

83.3µA

Step

10:

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 and CIN may deviate from the previous

equations in order to obtain desired load transient performance.

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