LTC3370 datasheet(26/44 Pages) LINER

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LTC3372

Rev. A

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APPLICATIONS INFORMATION

voltage and maximum output current. Miller capacitance,

CMILLER, can be approximated from the gate charge curve

usually provided on the MOSFET manufacturers’ data

sheet. CMILLER is equal to the increase in gate charge

along the horizontal axis while the curve is approximately

flat divided by the specified change in VDS. This result is

then multiplied by the ratio of the application applied VDS

to the gate charge curve specified VDS. When the IC is

operating in continuous mode the duty cycles for the top

and bottom MOSFETs are given by:

Main Switch Duty Cycle =

VOUT

VIN

Synchronous Switch Duty Cycle =

VIN − VOUT

VIN

The MOSFET power dissipations at maximum output

current are given by:

PMAIN =

VOUT

VIN

(IMAX)

2(1+ δ)RDS(ON) +

(VIN)

2 IMAX

⎛

⎝

⎜

⎞

⎠

⎟(RDR)(CMILLER) •

VINTVCC − VTHMIN

VTHMIN

⎡

⎣

⎢

⎤

⎦

⎥(f)

PSYNC =

VIN − VOUT

VIN

(IMAX)

2(1+ δ)RDS(ON)

where δ is the temperature dependency of RDS(ON) and

RDR (approximately 2Ω) is the effective driver resistance

at the MOSFET’s Miller threshold voltage. VTHMIN is the

typical MOSFET minimum threshold voltage.

BothMOSFETshaveI2RlosseswhilethetopsideN-channel

equation includes an additional term for transition losses,

which are highest at high input voltages. For VIN < 20V

the high current efficiency generally improves with larger

MOSFETs, while for VIN > 20V the transition losses rapidly

increase to the point that the use of a higher RDS(ON)device

with lower CMILLERactuallyprovideshigherefficiency.The

synchronous MOSFET losses are greatest at high input

voltage when the top switch duty factor is low or during

a short-circuit when the synchronous switch is on close

to 100% of the period.

The term (1+ δ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs Temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

A Schottky diode can be placed in parallel with the bot-

tom MOSFET to conduct during the dead-time between

the conduction of the two power MOSFETs. This prevents

the body diode of the bottom MOSFET from turning on,

storing charge during the dead-time and requiring a re-

verse recovery period that could cost as much as 3% in

efficiency at high VIN. A 1A to 3A Schottky is generally a

good compromise for both regions of operation due to

the relatively small average current. Larger diodes result

in additional transition losses due to their larger junction

capacitance.

Another consideration is the losses due to the gate charge

of the MOSFETs. Each cycle, the bottom FET gate driver

draws a pulse of current from the INTVCC pin when turning

on the bottom FET gate. Another pulse of current is drawn

by the boost capacitor as the bottom FET turns on. This

energy is used by the floating high side driver to turn on

thetopMOSFET.TheINTVCCdecouplingcapacitorsmooths

the current flowing through the LDO. The resulting DC

current can be estimated as:

IGQ = f(QGT + QGB)

The LDO losses will then become:

PLDO = (VIN – VINTVCC) • IGQ

To avoid the LDO losses, program VOUT for 5V with the

VOUTPRG pin. With this setting, the INTVCC LDO is shut

down and the INTVCC pin is tied to VOUT with an internal

switch. This will provide significant power loss reductions

for high input voltages.

ThelossesinthegatedriverarealsoaffectedbytheMOSFET

gatecharge.Aconservativeestimateofthetheselossesis:

PGATE_DRIVE = IGQ • VINTVCC

High Voltage Buck Controller

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