LTC1702A datasheet(12/24 Pages) LINER

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LT3844

3844fb

APPLICATIONS INFORMATION

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. A value of

ΔIL=0.3•IOUT(MAX)

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

output current of the supply. Lower values of

ΔIL require

larger and more costly magnetics. 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%. To satisfy slope compensation requirements the

minimum inductance is calculated as follows:

OUT

MAX

SENSE

•

–

•

•.

833

Some magnetics vendors specify a volt-second product

in their data sheet. 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) =

(–

) •

()

IN MAX

OUT

N MAX

() •

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.

Contact the Application group at Linear Technology for

further support.

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.

For maximum efﬁciency, minimize RDS(ON) and CRSS.

Low RDS(ON) minimizes conduction losses while low CRSS

minimizes transition losses. The problem is that RDS(ON) is

inversely related to CRSS. Balancing the transition losses

with the conduction losses is a good idea in sizing the

MOSFET. Select the MOSFET to balance the two losses.

Calculate the maximum conduction losses of the

MOSFET:

COND

OUT MAX

OUT

DS ON

⎛

⎝⎜

⎞

⎠⎟

()

(

)

()

(

)

Note that RDS(ON) has a large positive temperature depen-

dence. The MOSFET manufacturer’s data sheet contains a

curve, RDS(ON) vs Temperature.

Calculate the maximum transition losses:

PTRAN = (k)(VIN)2 (IOUT(MAX))(CRSS)(fSW)

where k is a constant inversely related to the gate driver

current, approximated by k = 2 for LT3844 applications.

The total maximum power dissipation of the MOSFET is

the sum of these two loss terms:

PFET(TOTAL) = PCOND + PTRAN

To achieve high supply efﬁciency, keep the PFET(TOTAL) to

less than 3% of the total output power. Also, complete

a thermal analysis to ensure that the MOSFET junction

temperature is not exceeded.

TJ = TA + PFET(TOTAL) • θJA

where

θJA is the package thermal resistance and TA is the

ambient temperature. Keep the calculated TJbelowthemax-

imum speciﬁed junction temperature, typically 150°C.

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