LT8640 datasheet(19/28 Pages) AD

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LT8638S

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

where fSW is the switching frequency in MHz, VOUT is

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

(~0.08V) 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 8mΩ, and the core material

should be intended for high frequency applications.

The LT8638S limits the peak switch current in order

to protect the switches and the system from overload

faults. The top switch current limit (ILIM) is 20A at low

duty cycles and decreases linearly to 15A 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 ripple

current (Equation 7).

IOUT(MAX) =ILIM –

ΔIL

(7)

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

calculated using Equation 8.

ΔIL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(8)

where fSW is the switching frequency of the LT8638S, and

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

output current that the LT8638S 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.

In order to achieve higher light load efficiency, more

energy must be delivered to the output during the sin-

gle small pulses in Burst Mode operation such that the

LT8638S can stay in sleep mode longer between each

pulse. This can be achieved by using a larger value induc-

tor (i.e., 4.7µH), and should be considered independent

of switching frequency when choosing an inductor. For

example, while a lower inductor value would typically be

used for a high switching frequency application, if high

light load efficiency is desired, a higher inductor value

should be chosen. See curve in Typical Performance

Characteristics.

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 LT8638S may operate with higher ripple

current. 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

minimum inductance is required to avoid subharmonic

oscillation (Equation 9). See Application Note 19 for

more details.

LMIN =

VIN(2•DC−1)

•fSW

(9)

where DC is the duty cycle ratio (VOUT/VIN) and fSW is the

switching frequency.

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