LT8607 datasheet(14/24 Pages) AD

14 / 24 page

LT8607/LT8607B

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Inductor Selection and Maximum Output Current

The LT8607 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 LT8607 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:

VOUT + VSW(BOT)

fSW

• 2

where fSW is the switching frequency in MHz, VOUT is

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

(~0.125V) 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 0.25A output

should use an inductor with an RMS rating of greater

than 0.5A and an ISAT of greater than 0.7A. 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 LT8607 limits the peak switch current in order to

protect the switches and the system from overload faults.

The top switch current limit (ILIM) is at least 1.2A at low

duty cycles and decreases linearly to at least 0.9A 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 LT8607, and

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

output current that the LT8607 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 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 LT8607 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.

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.

Input Capacitor

Bypass the input of the LT8607 circuit with a ceramic

capacitor of X7R or X5R type. Y5V types have poor per-

formance over temperature and applied voltage, and

should not be used. A 4.7µF to 10µF ceramic capacitor

is adequate to bypass the LT8607 and will easily handle

the ripple current. Note that larger input capacitance is

required when a lower switching frequency is used. If

the input power source has high impedance, or there is

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