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LT8494

8494fa

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APPLICATIONS INFORMATION

Adequate Load Current: Small value inductors result in

increased ripple currents and thus, due to the limited peak

switch current, decrease the average current that can be

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

load current, L should be at least:

L >

DC• VIN

2 f

( ) ILIM – VOUT •IOUT

VIN • η









For boost topologies, or:

L >

DC• VIN

2 f

( ) ILIM – VOUT •IOUT

VIN • η

–IOUT









for the SEPIC topologies.

where:

L = L1||L2 for the uncoupled SEPIC topology

DC = switch duty cycle (see previous section)

ILIM = switch current limit, typically about 2.35A

at 50% duty cycle (see the Typical Performance

Characteristics section)

η = power conversion efficiency (typically 85% to

90% for boost and 80% to 85% for SEPIC at high

currents)

f = switching frequency

Negative values of L indicate that the output load cur-

rent IOUT exceeds the switch current limit capability of

the LT8494.

Avoiding Subharmonic Oscillations: The internal slope

compensation circuit of LT8494 helps prevent the subhar-

monic oscillations 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:

L >

VIN – VCESAT

(

)• 2DC–1

(

)

0.76 • 1.5•DC+1

(

)•f• 1–DC

(

)

for boost and coupled inductor SEPIC, or:

L1||L2 >

VIN – VCESAT

(

)• 2DC–1

(

)

0.76 • 1.5•DC+1

(

)•f• 1–DC

(

)

for the uncoupled inductor SEPIC topologies.

Maximum Inductance: Excessive inductance can reduce

current ripple to levels that are difficult for the current

comparator (A2 in the Block Diagram) to cleanly discrimi-

nate, thus causing duty cycle jitter and/or poor regulation.

The maximum inductance can be calculated by:

LMAX =

VIN – VCESAT

IMIN(RIPPLE)

•

where LMAX is L1||L2 for uncoupled SEPIC topologies and

IMIN(RIPPLE) is typically 150mA.

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 and average input inductor cur-

rents (continuous conduction mode only) are given by:

IL1(PEAK) =

VOUT •IOUT

VIN • η

+ VIN •DC

2•L1• f

IL1(AVG) =

VOUT •IOUT

VIN • η

for the boost and uncoupled inductor SEPIC topology.

For uncoupled SEPIC topologies, the peak and average

currents of the output inductor L2 are given by:

IL2(PEAK) =IOUT +

VOUT • 1–DC

(

)

2•L2• f

IL2(AVG) =IOUT

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