LT8640 datasheet(21/32 Pages) AD

21 / 32 page

LT8640S/LT8643S/

LT8640SA/LT8643SA

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

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) +

ΔIL

(8)

where ∆IL is the inductor ripple current as calculated in

Equation 10 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 0.02Ω, and the core material

should be intended for high frequency applications.

The LT8640S/LT8643S/LT8640SA/LT8643SA limits the

peak switch current in order to protect the switches and

the system from overload faults. The top switch current

limit (ILIM) is 10A at low duty cycles and decreases lin-

early to 7A 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.

IOUT(MAX) =ILIM –

ΔIL

(9)

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

calculated as follows:

ΔIL =

VOUT

L • fSW

• 1–

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

(10)

where fSW is the switching frequency of the LT8640S/

LT8643S/LT8640SA/LT8643SA, and L is the value of the

inductor. Therefore, the maximum output current that

the LT8640S/LT8643S/LT8640SA/LT8643SA 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 volt-

age used in the desired application.

In order 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 LT8640S/

LT8643S/LT8640SA/LT8643SA can stay in sleep mode

longer between each pulse. This can be achieved by using

a larger value inductor (i.e., 4.7µH), and should be consid-

ered 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 dif-

fer 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 induc-

tor may be lower and the LT8640S/LT8643S/LT8640SA/

LT8643SA 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 11). See Application Note 19

for more details.

LMIN =

VIN 2•DC–1

(

)

3.5• fSW

(11)

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

switching frequency.

LT8640 Datasheet(PDF) 21 Page - Analog Devices

LT8640 Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL