Data Sheet

LT8638-2

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Rev.0

23 of 35

IL(PEAK) = ILOAD(MAX) +

1

2

∆IL

(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 LT8638-2 limits the peak switch current 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 −

∆IL

2

(7)

The peak-to-peak ripple current in the inductor can be calculated using Equation 8.

∆IL =

VOUT

L × fSW

× (1 −

VOUT

VIN(MAX)

)

(8)

where, fSW is the switching frequency of the LT8638-S, and L is the value of the inductor.

Therefore, the maximum output current the LT8638-2 delivers depends on the switch current limit, inductor value,

and 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.

To achieve higher light load efficiency, more energy must be delivered to the output during the single small pulses

in Burst Mode operation such that the LT8638-2 can stay in sleep mode longer between each pulse. This can be

achieved using a larger value inductor (that is, 4.7µH), and should be considered independent of switching frequency

when choosing an inductor. For example, while a lower inductor value is typically used for a high switching frequency

application, if high light load efficiency is desired, a higher inductor value should be chosen. See the 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 requiring

smaller load currents, the value of the inductor may be lower and the LT8638-2 may operate with higher ripple

current. This allows the 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, refer to application note 44.

For duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid subharmonic oscillation

(Equation 9). Refer to application note 19 for more details.

LMIN =

VIN(2 × DC − 1)

5 × fSW

(9)