LT3724MPFE datasheet(12/24 Pages) LINER

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LT3724

3724fc

The basic LT3724 step-down (buck) application, shown

in the Typical Application on the front page, converts a

larger positive input voltage to a lower positive or negative

output voltage. This Application Information section assists

selection of external components for the requirements of

the power supply.

RSENSE Selection

The current sense resistor, RSENSE, monitors the inductor

current of the supply (See Typical Application on front

page). Its value is chosen based on the maximum required

output load current. The LT3724 current sense ampliﬁer

has a maximum voltage threshold of, typically, 150mV.

Therefore, the peak inductor current is 150mV/RSENSE.

The maximum output load current, IOUT(MAX), is the peak

inductor current minus half the peak-to-peak ripple cur-

rent, ΔIL.

Allowing adequate margin for ripple current and external

component tolerances, RSENSE can be calculated as fol-

lows:

RSENSE =

100mV

IOUT(MAX)

Typical values for RSENSE are in the range of 0.005Ω

to 0.05Ω.

Inductor Selection

The critical parameters for selection of an inductor are

minimum inductance value, volt-second product, satura-

tion current and/or RMS current.

The minimum inductance value is calculated as follows:

L VOUT •

VIN(MAX) –VOUT

fSW •VIN(MAX) • IL

fSW is the switch frequency (200kHz).

The typical range of values for ΔIL is (0.2 • IOUT(MAX)) to

(0.5 • IOUT(MAX)), where IOUT(MAX) is the maximum load

current of the supply. Using ΔIL = 0.3 • IOUT(MAX) yields a

good design compromise between inductor performance

versus inductor size and cost. Higher values of ΔIL will

increase the peak currents, requiring more ﬁltering on

the input and output of the supply. If ΔIL is too high,

the slope compensation circuit is ineffective and current

mode instability may occur at duty cycles greater than

50%. Lower values of ΔIL require larger and more costly

magnetics. A value of ΔIL = 0.3 • IOUT(MAX) produces a

±15% of IOUT(MAX) ripple current around the DC output

current of the supply.

Some magnetics vendors specify a volt-second product

in their datasheet. If they do not, consult the magnetics

vendor to make sure the speciﬁcation is not being exceeded

by your design. The volt-second product is calculated as

follows:

Volt-second (

μsec)=

(VIN(MAX) –VOUT)• VOUT

VIN(MAX) •fSW

The magnetics vendors specify either the saturation cur-

rent, the RMS current or both. When selecting an inductor

based on inductor saturation current, use the peak cur-

rent through the inductor, IOUT(MAX) + ΔIL/2. The inductor

saturation current speciﬁcation is the current at which

the inductance, measured at zero current, decreases by

a speciﬁed amount, typically 30%.

When selecting an inductor based on RMS current rating,

use the average current through the inductor, IOUT(MAX).

The RMS current speciﬁcation is the RMS current at which

the part has a speciﬁc temperature rise, typically 40°C,

above 25°C ambient.

After calculating the minimum inductance value, the volt-

second product, the saturation current and the RMS current

for your design, select an off-the-shelf inductor. A list of

magnetics vendors can be found at www.linear.com, or

contact the Linear Technology Application Department.

For more detailed information on selecting an inductor,

please see the “Inductor Selection” section of Linear

Technology Application Note 44.

Step-Down Converter: MOSFET Selection

The selection criteria of the external N-channel standard

level power MOSFET include on resistance(RDS(ON)), re-

verse transfer capacitance (CRSS), maximum drain source

voltage (VDSS), total gate charge (QG), and maximum

continuous drain current.

APPLICATIONS INFORMATION

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