LT3579 datasheet(13/40 Pages) LINER

13 / 40 page

LT3579/LT3579-1

35791f

APPLICATIONS INFORMATION

Figure 6. Boost Converter – The Component Values Given Are

Typical Values for a 1MHz, 5V to 12V Boost

BOOST CONVERTER COMPONENT SELECTION

OPTIONAL

VIN

CIN

22μF

86.6k

COUT1

10μF

RGATE

6.3k

2.2nF

47pF

CSS

0.1μF

RFB

130k

VOUT

12V

1.7A

VIN

COUT

10μF

2.2μH

30V, 4A

SW1 SW2

GATE

VIN

FAULT

SHDN

GND

SYNC

CLKOUT

LT3579

37591 F06

100k

200k

The LT3579 can be conﬁgured as a Boost converter as in

Figure 6. This topology allows for positive output voltages

that are higher than the input voltage. An external PMOS

(optional) driven by the GATE pin of the LT3579 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 topology

in the Charge Pump Aided Regulators section.

Table 1 is a step-by-step set of equations to calculate

component values for the LT3579 when operating as a

Boost 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 1.

Variable Deﬁnitions:

VIN = Input Voltage

VOUT = Output Voltage

DC = Power Switch Duty Cycle

fOSC = Switching Frequency

IOUT = Maximum 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:

Inputs

Pick VIN, VOUT, and fOSC to calculate equations below.

Step 2:

OUT

≅

–.

.– .

027

Step 3:

TYP

OSC

MIN

= ()

•

= (

–.

027

)) ••

()

••

()

= ()

•

027

OSC

MAX

–

–.

OSC • 05

(1)

(2)

(3)

• Solve equations 1, 2, and 3.

• Choose the higher value between LTYP and LMIN for L1.

L1 should never exceed LMAX.

Step 4:

IRIPPLE

RIPPLE

OSC

= ()

•

–.

027

Step 5:

IOUT

OUT

RIPPLE

⎛

⎝

⎜

⎞

⎠

⎟ •

()

––

Step 6:

OUT

AVG

OUT

;

Step 7:

COUT,

COUT1

IDC

OUT

OSC

OUT

•

001

0 5

.•

– . •

•

DSON PMOS

()

Step 8:

CIN

PWR

VIN

RIPPLE

OSC

0 005

•• .

•

••

•

OSC

0 005

Step 9:

RFB

μA

OUT

–.

1 215

83 3

Step 10:

fin MHz and R in k

OSC

87 6

–;

Step 11:

PMOS

Only needed for input or output disconnect. See PMOS

Selection in the Appendix for information on sizing the PMOS

and the biasing resistor, RGATE.

Note: The maximum design target for peak switch current is 6A and

is used in this table. The ﬁnal values for COUT and CIN may deviate

from the above equations in order to obtain desired load transient

performance for a particular application.

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