LT1913IDD 查看數據表（PDF） - Linear Technology

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LT1913IDD

25V, 3.5A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT1913

APPLICATIONS INFORMATION

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 25V are acceptable regardless of the

switching frequency. In this mode, the LT1913 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.

The minimum input voltage is determined by either the

LT1913’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)

=

VOUT + VD

1– 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 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 LT1913’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 LT1913’s switch current limit (ILIM) is

5.5A at low duty cycles and decreases linearly to 4.5A at

DC = 0.8. The maximum output current is a function 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

fSW
IL











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. To keep the efficiency high, the series

resistance (DCR) should be less than 0.05 , 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

Murata www.murata.com

TDK

www.componenttdk.com

Toko

www.toko.com

Sumida www.sumida.com

NEC

www.nec.com

PART SERIES

LQH55D

SLF10145

D75C

D75F

CDRH74

CR75

CDRH8D43

MPLC073

MPBI0755

TYPE

Open

Shielded

Shielded

Open

Shielded

Open

Shielded

Shielded

Shielded

Of course, such a simple design guide will not always re-

sult in the optimum inductor for your application. A larger

value inductor provides a slightly higher maximum load

current and will reduce the output voltage ripple. If your

1913f

11

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