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LT8620

8620fa

For more information www.linear.com/LT8620

APPLICATIONS INFORMATION

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. During long duration

overload or short-circuit conditions, the inductor RMS

routing requirement is greater to avoid overheating of the

inductor. 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 LT8620 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 3.8A at low

duty cycles and decreases linearly to 2.8A at DC = 0.8. The

inductorvaluemustthenbesufficienttosupplythedesired

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

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

IOUT(MAX) =ILIM –

∆IL

(8)

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

calculated as follows:

∆IL =

VOUT

L • fSW

• 1– VOUT

VIN(MAX)













(9)

where fSW is the switching frequency of the LT8620, and

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

output current that the LT8620 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.

Inordertoachievehigherlightloadefficiency,moreenergy

must be delivered to the output during the single small

pulses in Burst Mode operation such that the LT8620 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 considered independent of switching frequency

when choosing an inductor. For example, while a lower

inductor value would typically be used for a high switch-

ing frequency application, if high light load efficiency is

desired, a higher inductor value should be chosen.

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 LT8620 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 Linear Technology’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

Bypass the input of the LT8620 circuit with a ceramic ca-

pacitor of X7R or X5R type placed as close as possible to

the VIN and PGND pins. 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

bypasstheLT8620andwilleasilyhandletheripplecurrent.

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 LT8620 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 LT8620 (see the PCB Layout section).

A second precaution regarding the ceramic input capaci-

tor concerns the maximum input voltage rating of the

LT8620. A ceramic input capacitor combined with trace

or cable inductance forms a high quality (under damped)

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