LT8608 datasheet(13/24 Pages) LINER

13 / 24 page

LT8608/LT8608B

Rev C

For more information www.analog.com

APPLICATIONS INFORMATION

Inductor Selection and Maximum Output Current

The LT8608 is designed to minimize solution size by al-

lowing the inductor to be chosen based on the output load

requirements of the application. During overload or short

circuit conditions the LT8608 safely tolerates operation

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 =

OUT + VSW (BOT )

where fSW is the switching frequency in MHz, VOUT is

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

(~0.35V) and L is the inductor value in μH.

Toavoidoverheatingandpoorefficiency,aninductormust

be chosen with an RMS current rating that is greater than

the maximum expected output load of the application. In

addition, the saturation current (typically labeled ISAT) rat-

ing of the inductor must be higher than the load current

plus 1/2 of in inductor ripple current:

L(PEAK ) = ILOAD(MAX ) +

∆L

where ∆IL is the inductor ripple current as calculated

several paragraphs below and ILOAD(MAX) is the maximum

output load for a given application.

As a quick example, an application requiring 0.5A output

should use an inductor with an RMS rating of greater

than 0.5A and an ISAT of greater than 0.8A. 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 LT8608 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 2.1A at low

duty cycles and decreases linearly to 1.55A at D = 0.8. The

inductorvaluemustthenbesufficienttosupplythedesired

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

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

OUT(MAX ) = ILIM –

∆IL

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

calculated as follows:

∆IL =

OUT

L • f

1–

OUT

IN(MAX )

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

where fSW is the switching frequency of the LT8608, and

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

output current that the LT8608 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.

For more information about maximum output current and

discontinuous operation, see Analog Device’s 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 Application Note 19.

Input Capacitor

BypasstheinputoftheLT8608circuitwithaceramiccapaci-

tor of X7R or X5R type. Y5V types have poor performance

over temperature and applied voltage, and should not be

used. A 4.7μF to 10μF ceramic capacitor is adequate to

bypasstheLT8608andwilleasilyhandletheripplecurrent.

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 significant inductance due to

long wires or cables, additional bulk capacitance may be

necessary. This can be provided with a low performance

electrolytic capacitor.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage

ripple at the LT8608 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 4.7μF capacitor is capable of this task, but only if it is

placed close to the LT8608 (see the PCB Layout section).

Asecondprecautionregardingtheceramicinputcapacitor

concernsthemaximuminputvoltageratingoftheLT8608.

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