LT3685E 데이터 시트보기 (PDF) - Linear Technology

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LT3685E

36V, 2A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT3685

APPLICATIONS INFORMATION

VIN(MAX)

=

VOUT + VD

fSW tON(MIN)

–

VD

+

VSW

where VIN(MAX) is the maximum operating input voltage,

VOUT is the output voltage, VD is the catch diode drop

(~0.5V), VSW is the internal switch drop (~0.5V at max

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

tON(MIN) is the minimum switch on time (~150ns). Note that

a higher switching frequency will depress the maximum

operating input voltage. Conversely, a lower switching

frequency will be necessary to achieve safe operation at

high input voltages.

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

up, or overload events are expected, then input voltage

transients of up to 60V are acceptable regardless of the

switching frequency. In this mode, the LT3685 may enter

pulse skipping operation 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. Above 38V switching

will stop.

The minimum input voltage is determined by either the

LT3685’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:

VIN(MIN)

=

1–

VOUT + VD

fSW tOFF(MIN)

–

VD

+

VSW

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 switching

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 LT3685’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 LT3685’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:

L

=

⎛

⎝⎜

VOUT + VD ⎞

fSWΔIL ⎠⎟

⎛

⎜ 1–

⎝

VOUT + VD

VIN(MAX)

⎞

⎟

⎠

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 , and the core material should be intended for

high frequency applications. Table 1 lists several vendors

and suitable types.

3685fb

10

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