LTC1702A datasheet(13/24 Pages) LINER

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LT3844

3844fb

APPLICATIONS INFORMATION

Note that when VIN is high and fSW is high, 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 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 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,

IOUT(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.

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:

DIODE AVG

OUT MAX

IN MAX

OUT

IN 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 recommended.

Step-Down Converter: Input Capacitor Selection

A local input bypass capacitor is required for buck convert-

ers because the input current is pulsed with fast rise and

fall times. The input capacitor selection criteria are based

on the bulk capacitance and RMS current capability. The

bulk capacitance will determine the supply input ripple

voltage. The RMS current capability is used to keep from

overheating the capacitor.

The bulk capacitance is calculated based on maximum

input ripple,

ΔVIN:

IN BULK

OUT MAX

OUT

IN MIN

()

•

••

ΔVIN is typically chosen at a level acceptable to the user.

100mV to 200mV is a good starting point. Aluminum elec-

trolytic capacitors are a good choice for high voltage, bulk

capacitance due to their high capacitance per unit area.

The capacitor’s RMS current is:

CIN RMS

OUT

()

(–

)

()

If applicable, calculate it at the worst-case condition,

VIN = 2VOUT. The RMS current rating of the capacitor

is speciﬁed by the manufacturer and should exceed the

calculated ICIN(RMS). Due to their low ESR (equivalent

series resistance), ceramic capacitors are a good choice

for high voltage, high RMS current handling. Note that the

ripple current ratings from aluminum electrolytic capacitor

manufacturers are based on 2000 hours of life. This makes

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