Data Sheet

LT8615

analog.com

Rev 0

20 of 31

The LT8615 is capable of maximum duty cycle approaching 100%, and the VIN to VOUT dropout is limited by the RDS(ON)

of the top switch. In this mode, the LT8615 skips switch cycles, resulting in a lower switching frequency than

programmed by RT.

For applications that cannot allow deviation from the programmed switching frequency at low VIN/VOUT ratios, use

the following formula to set switching frequency:

VIN(MIN) =

VOUT + VSW(BOT)

1 − fSW × tOFF(MIN)

− VSW(BOT) + VSW(TOP)

where, VIN(MIN) is the minimum input voltage without skipped cycles, VOUT is the output voltage, VSW(TOP) and VSW(BOT)

are the internal switch drops (~0.28V, ~0.28V, respectively, at max. load), fSW is the switching frequency (set by RT),

and tOFF(MIN) is the minimum switch offtime. Note that higher switching frequency increases the minimum input

voltage, below which cycles are dropped to achieve higher duty cycle.

Inductor Selection and Maximum Output Current

The LT8615 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 LT8615 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:

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where, fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.28V),

and L is the inductor value in μH.

To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating 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:

IL(PEAK) = ILOAD(MAX) +

1

2

∆IL

where, ∆IL is the inductor ripple current as calculated several paragraphs below and ILOAD(MAX) is the maximum output

load for a given application.

As a quick example, an application requiring 1A output should use an inductor with an RMS rating of greater than 1A

and an ISAT of greater than 1.3A. To keep the efficiency high, the series resistance (DCR) should be less than 0.04Ω,

and the core material should be intended for high frequency applications.

The LT8615 limits the peak switch current to protect the switches and the system from overload faults. The top switch

current limit (ILIM) is typically 7.3A at low duty cycles and decreases linearly to 6.1A at D = 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:

IOUT(MAX) = ILIM −

∆IL

2

The peak-to-peak ripple current in the inductor can be calculated as follows: