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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

For low VIN applications that cannot allow deviation from

the programmed oscillator frequency, use the following

formula to set the switching frequency:

VIN(MIN) =

VSW(BOT) + VOUT

1– tOFF(MIN) • fOSC

+ VSW(TOP) – VSW(BOT)

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 (~0.54V, ~0.264V,

respectively at maximum load), fOSC is the oscillating fre-

quency (set by RT), and tOFF(MIN) is the minimum switch-

ing off-time. Note that higher switching frequency will

increase the minimum input voltage below which cycles

will be dropped to achieve higher duty cycle.

Inductor Selection and Maximum Output Current

The LT8619 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 LT8619 safely tolerates opera-

tion with a saturated inductor through the use of a high

speed peak-current mode architecture.

A good first choice for the inductor value is:

L = 2

VOUT + VSW(BOT)

fOSC

where fOSC is the switching frequency in MHz, VOUT is

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

(~0.264V) 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 applica-

tion. In addition, the saturation current (typically labeled

ISAT) rating of the inductor must be higher than the load

current plus one-half of inductor ripple current:

ISAT >ILOAD(MAX) +

ΔIL(MAX)

APPLICATIONS INFORMATION

where ILOAD(MAX) is the maximum output load for a given

application and ∆IL(MAX) is the inductor ripple current as

calculated in the following equation:

ΔIL(MAX) =

fOSC •L

VOUT 1–

VOUT

VIN(MAX)

⎡

⎣

⎢

⎤

⎦

⎥

As a quick example, an application requiring 1A output

current should use an inductor with an RMS rating of

greater than 1A and an ISAT of greater than 1.5A. During

long duration overload or short-circuit conditions, the

inductor RMS rating requirement is greater to avoid over-

heating of the inductor. To push for high efficiency, select

an inductor with low series resistance (DCR), preferably

below 0.04Ω, and the core material should be intended

for high frequency application. However, achieving this

requires a large size inductor. An inductor with DCR

around 0.1Ω is generally a good compromise for both

efficiency and board area, at the expense of trimming 1%

to 2% from the efficiency number.

The LT8619 limits the peak switch current in order to pro-

tect the switches and the system from overload faults. The

top switch current limit (ILIM) is at least 1.5A. The induc-

tor value must then be sufficient to supply the desired

maximum output current (ILOAD(MAX)), which is a function

of the switch current limit (ILIM) and the ripple current:

ILOAD(MAX) = ILIM –

ΔIL

Therefore, the maximum output current that the LT8619

will deliver depends on the switch current limit, the induc-

tor value, and the input and output voltages. The inductor

value may have to be increased if the inductor ripple cur-

rent does not allow sufficient maximum output current

(ILOAD(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 single small

pulses in Burst Mode operation such that the LT8619 can

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