LT8607 datasheet(14/24 Pages) AD

14 / 24 page

LT8609S

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Inductor Selection and Maximum Output Current

The LT8609S 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 LT8609S safely tolerates oper-

ation 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 =

VOUT +VSW(BOT)

fSW

where fSW is the switching frequency in MHz, VOUT is

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

(~0.25V) 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 1/2 of in inductor ripple current:

IL(PEAK) =ILOAD(MAX)+

ΔL

where ∆IL is the inductor ripple current as calculated sev-

eral 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 LT8609S limits the peak switch current in order to

protect the switches and the system from overload faults.

The top switch current limit (ILIM) is typically 4.75A at low

duty cycles and decreases linearly to 4.0A 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 –

ΔIL

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

calculated as follows:

ΔIL =

VOUT

L • fSW

1–

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

where fSW is the switching frequency of the LT8609S, and

L is the value of the inductor. Therefore, the maximum out-

put current that the LT8609S will deliver depends on the

minimum 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 voltage used in

the desired application.

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 requir-

ing smaller load currents, the value of the inductor may

be lower and the LT8609S 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.

The internal circuitry of the LT8609S is capable of sup-

plying IOUT(MAX) up to 3A. Thermal limitations of the

LT8609S prevent continuous output of 3A loads due to

unsafe operating temperatures. In order to ensure safe

operating temperature, the average LT8609S current

must be kept below 2A, but will allow transient peaks up

to 3A or IOUT(MAX). If high average currents cause unsafe

heating of the part, the LT8609S will stop switching and

indicate a fault condition to protect the internal circuitry.

For more information about maximum output current and

discontinuous operation, see Analog Devices Application

Note 44.

Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),

a minimum inductance is required to avoid sub-harmonic

oscillation. See Analog Devices Application Note 19.

LT8607 датащи(PDF) 14 Page - Analog Devices

LT8607 датащи(HTML) 14 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

датащи скачать

ссылки URL