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LT3684

3684f

frequency will be necessary to achieve safe operation at

high input voltages.

If the output is in regulation and no short-circuit or start-up

events are expected, then input voltage transients of up to

36V are acceptable regardless of the switching frequency.

In this mode, the LT3684 may enter pulse skipping opera-

tion where some switching pulses are skipped to maintain

output regulation. In this mode the output voltage ripple

and inductor current ripple will be higher than in normal

operation.

The minimum input voltage is determined by either the

LT3684’s minimum operating voltage of ~3.6V or by its

maximum duty cycle (see equation in previous section).

The minimum input voltage due to duty cycle is:

IN MIN

OUT

SW OFF MIN

DSW

()

1–

–

where VIN(MIN) is the minimum input voltage, and tOFF(MIN)

is the minimum switch off time (150ns). Note that higher

switching frequency will increase the minimum input

voltage. If a lower dropout voltage is desired, a lower

switching frequency should be used.

Inductor Selection

For a given input and output voltage, the inductor value

and switching frequency will determine the ripple current.

The ripple current

ΔIL increases with higher VIN or VOUT

and decreases with higher inductance and faster switch-

ing frequency. A reasonable starting point for selecting

the ripple current is:

ΔIL = 0.4(IOUT(MAX))

where IOUT(MAX) is the maximum output load current. To

guarantee sufﬁcient output current, peak inductor current

must be lower than the LT3684’s switch current limit (ILIM).

The peak inductor current is:

IL(PEAK) = IOUT(MAX) + ΔIL/2

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

the maximum output load current, and

ΔIL is the inductor

ripple current. The LT3684’s switch current limit (ILIM) is

at least 3.5A at low duty cycles and decreases linearly to

2.5A at DC = 0.8. The maximum output current is a func-

tion of the inductor ripple current:

IOUT(MAX) = ILIM – ΔIL/2

Be sure to pick an inductor ripple current that provides

sufﬁcient maximum output current (IOUT(MAX)).

The largest inductor ripple current occurs at the highest

VIN. To guarantee that the ripple current stays below the

speciﬁed maximum, the inductor value should be chosen

according to the following equation:

OUT

IN MAX

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝

⎜

⎞

⎠

⎟

()

∆

1–

⎟⎟

where VD is the voltage drop of the catch diode (~0.4V),

VIN(MAX) is the maximum input voltage, VOUT is the output

voltage, fSW is the switching frequency (set by RT), and L

is in the inductor value.

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

maximum load current and its saturation current should be

about 30% higher. For robust operation in fault conditions

(start-up or short circuit) and high input voltage (>30V),

the saturation current should be above 3.5A. To keep the

efﬁciency high, the series resistance (DCR) should be less

than 0.1

Ω, and the core material should be intended for

high frequency applications. Table 1 lists several vendors

and suitable types.

Table 1. Inductor Vendors

VENDOR

URL

PART SERIES

TYPE

Murata

www.murata.com

LQH55D

Open

TDK

www.componenttdk.com

SLF7045

SLF10145

Shielded

Toko

www.toko.com

D62CB

D63CB

D75C

D75F

Shielded

Open

Sumida

www.sumida.com

CR54

CDRH74

CDRH6D38

CR75

Open

Shielded

Open

APPLICATIONS INFORMATION

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