LT3682 datasheet(13/24 Pages) LINER

13 / 24 page

LT3682

3682f

where fSW is the switching frequency in MHz, VOUT is the

output voltage, VD is the catch diode drop (~0.5V) and L

is the inductor value in μH.

The inductor’s RMS current rating must be greater than the

maximum load current and its saturation current should be

about 30% higher. To keep the efﬁciency high, the series

resistance (DCR) should be less than 0.1Ω, and the core

material should be intended for high frequency applications.

Table 1 lists several vendors and suitable types.

For robust operation in fault conditions (start-up or short

circuit) and high input voltage (>30V), the saturation

current should be chosen high enough to ensure that the

inductor peak current does not exceed 3.5A. For example,

an application running from an input voltage of 36V us-

ing a 10μH inductor with a saturation current of 2.5A will

tolerate the mentioned fault conditions.

The optimum inductor for a given application may differ

from the one indicated by this simple design guide. A larger

value inductor provides a higher maximum load current and

reduces the output voltage ripple. If your load is lower than

the maximum load current, then you can relax the value of

the inductor and operate with higher ripple current. This

allows you to use a physically smaller inductor, or one with

a lower DCR resulting in higher efﬁciency. Be aware that

if the inductance differs from the simple rule above, then

the maximum load current will depend on input voltage.

In addition, low inductance may result in discontinuous

mode operation, which further reduces maximum load

current. For details of maximum output current and

discontinuous mode 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 oscillations:

MIN

OUT

() •

The current in the inductor is a triangle wave with an av-

erage value equal to the load current. The peak inductor

and switch current is:

SW PEAK

L PEAK

OUT MAX

()

(

)

where IL(PEAK) is the peak inductor current, IOUT(MAX) is

the maximum output load current, and

ΔIL is the induc-

tor ripple current. The LT3682 limits its switch current in

order to protect itself and the system from overload faults.

Therefore, the maximum output current that the LT3682 will

deliver depends on the switch current limit, the inductor

value and the input and output voltages.

When the switch is off, the potential across the inductor

is the output voltage plus the catch diode drop. This gives

the peak-to-peak ripple current in the inductor:

ΔI

OUT

−+

() •(

)

•

where fSW is the switching frequency of the LT3682, DC

is the duty cycle and L is the value of the inductor.

To maintain output regulation, the inductor peak current

must be less than the LT3682’s switch current limit ILIM.

If SYNC pin is grounded ILIM is at least 1.45A at low duty

cycles and decreases to 1.1A at DC = 90%. If SYNC pin

is tied to 0.8V or more or if it is tied to a clock source for

synchronization ILIM is at least 1.18A at low duty cycles

and decreases to 0.85A at DC = 90%. The maximum output

current is also a function of the chosen inductor value

and can be approximated by the following expressions

depending on the SYNC pin conﬁguration:

For SYNC pin grounded:

ADC

OUT MAX

LIM

()

.•(

•

)

=−

−

ΔΔ

145

1 024

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