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LT8642-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Inductor Selection and Maximum Output Current

The LT8642-1 is designed to minimize solution size by

allowing the inductor to be chosen based on the output

load requirements of the application. During overload or

short-circuit conditions the LT8642-1 safely tolerates

operation with a saturated inductor through the use of a

high speed peak-current mode architecture.

A good first choice for the inductor value is given by

Equation 5.

VOUT + VSW(BOT)

fSW

⎛

⎝⎜

⎞

⎠⎟

•0.5

(5)

where fSW is the switching frequency in MHz, VOUT is

the output voltage, VSW(BOT) is the bottom switch drop

(~0.1V) and L is the inductor value in µH.

To avoid overheating and poor efficiency, an inductor must

be chosen with an RMS current rating that is greater than

the maximum expected output load of the application.

In addition, the saturation current (typically labeled ISAT)

rating of the inductor must be higher than the load current

plus 1/2 of in inductor ripple current (Equation 6)

IL(PEAK) =ILOAD(MAX) +

ΔIL

(6)

where ∆IL is the inductor ripple current as calculated in

Equation 8 and ILOAD(MAX) is the maximum output load

for a given application.

As a quick example, an application requiring 3A output

should use an inductor with an RMS rating of greater than

3A and an ISAT of greater than 4A. During long duration

overload or short-circuit conditions, the inductor RMS

rating requirement is greater to avoid overheating of the

inductor. To keep the efficiency high, the series resistance

(DCR) should be less than 0.01Ω, and the core material

should be intended for high frequency applications.

The LT8642-1 limits the peak switch current in order

to protect the switches and the system from overload

faults. The top switch current limit (ILIM) is 18A at low

duty cycles and decreases linearly to 13.5A at DC = 0.8.

The inductor value must then be sufficient to supply the

desired maximum output current (IOUT(MAX)), which is

a function of the switch current limit (ILIM) and the rip-

ple current (Equation 7).

IOUT(MAX) =ILIM −

ΔIL

(7)

The peak-to-peak ripple current in the inductor can be

calculated with Equation 8.

ΔIL =

VOUT

L • fSW

• 1

−

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(8)

where fSW is the switching frequency of the LT8642-1,

and L is the value of the inductor. Therefore, the maximum

output current that the LT8642-1 will deliver depends on

the switch current limit, the inductor value, and the input

and output voltages. The inductor value may have to be

increased if the inductor ripple current does not allow

sufficient maximum output current (IOUT(MAX)) given the

switching frequency, and maximum input voltage used in

the desired application.

The optimum inductor for a given application may differ

from the one indicated by this design guide. A larger value

inductor provides a higher maximum load current and

reduces the output voltage ripple. For applications requir-

ing smaller load currents, the value of the inductor may be

lower and the LT8642-1 may operate with higher ripple cur-

rent. This allows use of a physically smaller inductor, or one

with a lower DCR resulting in higher efficiency. Be aware

that low inductance may result in discontinuous mode

operation, which further reduces maximum load current.

For more information about maximum output current and

discontinuous operation, see Analog Devices Application

Note 44.

For duty cycles greater than 50% (VOUT/VIN > 0.5), a min-

imum inductance is required to avoid subharmonic oscil-

lation (see Equation 9). See Analog Devices Application

Note 19 for more details.

LMIN =

VIN(2•DC−1)

5• fSW

(9)

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