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LT3844

3844fb

APPLICATIONS INFORMATION

two losses. Calculate the maximum conduction losses of

the MOSFET:

PDC

COND

MAX

OUT MAX

MAX

DS ON

−

⎛

⎝⎜

⎞

⎠⎟

()

•

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

dence. The MOSFET manufacturer’s data sheet contains

a curve, RDS(ON) vs Temperature. Calculate the maximum

transition losses:

TRAN

OUT

OUT MAX

RSS

()(

)

()()( )

−

()

(

MAX )

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 TJ below the

maximum speciﬁed junction temperature, typically 150°C.

Note that when VOUT is high (>20V), the transition losses

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

CRSS may provide higher efﬁciency. MOSFETs with higher

voltage 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 VOUT plus the forward voltage of the

rectiﬁer, typically less than 1V.

The internal VCC regulator is capable of sourcing up to

40mA which limits the maximum total MOSFET gate

charge, QG, to 40mA / fSW. 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 LT3844 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 start-up or short-circuit.

The manufacturer’s maximum continuous drain current

speciﬁcation should exceed the peak switch current which is

the same as the inductor peak current, IL(MAX) + ΔIL/2.

During the supply start-up, 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.

Boost Converter: Rectiﬁer Selection

The rectiﬁer is selected based upon the forward voltage,

reverse voltage and maximum current. A Schottky diode

is recommended for its low forward voltage and yields the

lowest power loss and highest efﬁciency. The maximum

reverse voltage that the diode will see is VOUT. The average

diode current is equal to the maximum output load current,

IOUT(MAX). A diode rated at 1.5 to 2 times the maximum

average diode current is recommended. Remember boost

converters are not short-circuit protected.

Boost Converter: Output Capacitor Selection

In boost mode, the output capacitor requirements are

more demanding due to the fact that the current waveform

is pulsed instead of continuous as in a buck converter.

The choice of component(s) is driven by the acceptable

ripple voltage which is affected by the ESR, ESL and bulk

capacitance. The total output ripple voltage is:

ΔVI

ESR

OUT

OUT MAX

OUT

MAX

−

⎛

⎝⎜

⎞

⎠⎟

()

•

where the ﬁrst term is due to the bulk capacitance and the

second term due to the ESR.

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