LT8613 datasheet(21/30 Pages) AD

21 / 30 page

LT8645S/LT8646S

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

rating of the inductor must be higher than the load current

plus 1/2 of in inductor ripple current:

IL(PEAK) = ILOAD(MAX) +

ΔIL

(7)

where ∆IL is the inductor ripple current as calculated in

Equation 9 and ILOAD(MAX) is the maximum output load

for a given application.

As a quick example, an application requiring 2A output

should use an inductor with an RMS rating of greater than

2A and an ISAT of greater than 3A. 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 0.02Ω, and the core material

should be intended for high frequency applications.

The LT8645S/LT8646S limits the peak switch current in

ordertoprotecttheswitchesandthesystemfromoverload

faults. The top switch current limit (ILIM) is 14A at low duty

cycles and decreases linearly to 11.5A at DC = 0.9. The

inductorvaluemustthenbesufficienttosupplythedesired

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

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

IOUT(MAX) = ILIM –

ΔIL

(8)

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

calculated as follows:

ΔIL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(9)

where fSW is the switching frequency of the LT8645S/

LT8646S, and L is the value of the inductor. Therefore, the

maximum output current that the LT8645S/LT8646S 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 cur-

rent does not allow sufficient maximum output current

(IOUT(MAX)) given the switching frequency, and maximum

input voltage used in the desired application.

When operating at high VIN (greater than 40V) and at a

frequency and duty cycle that would require a switch on-

time of less than 100ns, choose an inductor such that the

∆IL is greater than 1.5A in order to prevent duty cycle jitter.

Inordertoachievehigherlightloadefficiency,moreenergy

must be delivered to the output during the single small

pulses in Burst Mode operation such that the LT8645S/

LT8646Scanstayinsleepmodelongerbetweeneachpulse.

This can be achieved by using a larger value inductor (i.e.,

4.7µH), and should be considered independent of switch-

ing 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.SeecurveinTypicalPerformanceCharacteristics.

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 LT8645S/LT8646S 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 (See Equation 10). See Application Note 19

for more details.

LMIN =

VIN(2 •DC – 1)

3 • fSW

(10)

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

switching frequency.

Input Capacitors

The VIN of the LT8645S/LT8646S should be bypassed

with at least three ceramic capacitors for best perfor-

mance. Two small ceramic capacitors of 0.47µF can be

placed close to the part; one on each side of the device

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