3724MPFE datasheet(13/24 Pages) LINER

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LT3724

3724fc

APPLICATIONS INFORMATION

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:

PCOND =(IOUT(MAX))

VOUT

VIN

(RDS(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 LT3724 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

ambienttemperature.KeepthecalculatedTJbelowthemaxi-

mum speciﬁed junction temperature, typically 150°C.

Note that when VIN is high, the transition losses may

dominate. A MOSFET with higher RDS(ON) and lower CRSS

may provide higher efﬁciency. MOSFETs with higher volt-

age VDSS speciﬁcation usually have higher RDS(ON) and

lower CRSS.

Choose the MOSFET VDSS speciﬁcation to exceed the

maximum voltage across the drain to the source of the

MOSFET, which is VIN(MAX) plus any additional ringing

on the switch node. Ringing on the switch node can be

greatly reduced with good PCB layout and, if necessary,

an RC snubber.

The internal VCC regulator operating range limits the maxi-

mum total MOSFET gate charge, QG, to 90nC. The QG vs

VGS speciﬁcation is typically provided in the MOSFET data

sheet. Use QG at VGS of 8V. If VCC is back driven from an

external supply, the MOSFET drive current is not sourced

from the internal regulator of the LT3724 and the QG of the

MOSFET is not limited by the IC. However, note that the

MOSFET drive current is supplied by the internal regulator

when the external supply back driving VCC is not available

such as during startup or short-circuit.

The manufacturer’s maximum continuous drain current

speciﬁcation should exceed the peak switch current,

IOUT(MAX) + ΔIL/2.

During the supply startup, the gate drive levels are set by

the VCC voltage regulator, which is approximately 8V. Once

the supply is up and running, the VCC can be back driven

by an auxiliary supply such as VOUT. It is important not to

exceed the manufacturer’s maximum VGS speciﬁcation.

A standard level threshold MOSFET typically has a VGS

maximum of 20V.

Step-Down Converter: Rectiﬁer Selection

The rectiﬁer diode (D1 on the Functional Diagram) in a

buck converter generates a current path for the inductor

current when the main power switch is turned off. The

rectiﬁer is selected based upon the forward voltage, re-

verse voltage and maximum current. A Schottky diode is

recommended. Its low forward voltage yields the lowest

power loss and highest efﬁciency. The maximum reverse

voltage that the diode will see is VIN(MAX).

In continuous mode operation, the average diode cur-

rent is calculated at maximum output load current and

maximum VIN:

IDIODE(AVG) =IOUT(MAX)

VIN(MAX) VOUT

VIN(MAX)

To improve efﬁciency and to provide adequate margin for

short-circuit operation, a diode rated at 1.5 to 2 times the

maximum average diode current, IDIODE(AVG), is recom-

mended.

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