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LT3580

3580fg

APPLICATIONS INFORMATION

provided to a load (IOUT). In order to provide adequate

load current, L should be at least:

DC • V

2(f) I

LIM −

OUT |• IOUT

IN • η

⎛

⎝⎜

⎞

⎠⎟

for boost, topologies, or:

DC • V

2(f) I

LIM −

OUT •IOUT

IN • η

− I

OUT

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

for the SEPIC and inverting topologies.

where:

L = L1||L2 for uncoupled dual inductor topologies

DC = switch duty cycle (see previous section)

ILIM = switch current limit, typically about 2.4A at 50%

duty cycle (see the Typical Performance Characteristics

section).

η = power conversion efficiency (typically 88% for

boost and 75% for dual inductor topologies at high

currents).

f = switching frequency

Negative values of L indicate that the output load current

IOUT exceeds the switch current limit capability of the

LT3580.

Avoiding Subharmonic Oscillations: The LT3580’s internal

slope compensation circuit will prevent subharmonic oscil-

lations that can occur when the duty cycle is greater than

50%, provided that the inductance exceeds a minimum

value. In applications that operate with duty cycles greater

than 50%, the inductance must be at least:

IN •2 • DC – 1

()

−DC)•(f)

for boost, coupled inductor SEPIC, and coupled inductor

inverting topologies, or:

L1 L2

IN •2 • DC – 1

()

−DC)•(f)

for the uncoupled inductor SEPIC and uncoupled inductor

inverting topologies.

Maximum Inductance: Excessive inductance can reduce

current ripple to levels that are difficult for the current com-

parator (A3 in the Block Diagram) to cleanly discriminate,

thus causing duty cycle jitter and/or poor regulation. The

maximum inductance can be calculated by:

MAX =

IN –VCESAT

MIN

−RIPPLE

•

where LMAX is L1||L2 for uncoupled dual inductor topolo-

gies and IMIN-RIPPLE is typically 95mA.

Current Rating: Finally, the inductor(s) must have a rating

greater than its peak operating current to prevent inductor

saturation resulting in efficiency loss. In steady state, the

peak input inductor current (continuous conduction mode

only) is given by:

−PEAK =

OUT •IOUT

IN • η

IN •DC

2• L1• f

for the boost, uncoupled inductor SEPIC and uncoupled

inductor inverting topologies.

For uncoupled dual inductor topologies, the peak output

inductor current is given by:

−PEAK =IOUT +

OUT •1– DC

()

2• L2 • f

For the coupled inductor topologies:

OUT 1+

OUT

η•V

⎡

⎣

⎢

⎤

⎦

⎥ +

IN •DC

2• L • f

Note: Inductor current can be higher during load transients.

It can also be higher during start-up if inadequate soft-start

capacitance is used.

Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multilayer ceramic capacitors are an excellent choice, as

they have an extremely low ESR and are available in very

small packages. X5R or X7R dielectrics are preferred, as

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