LTC3829 datasheet(23/40 Pages) LINER

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LTC3829

3829f

applicaTions inForMaTion

where N is the number of output stages,

δ is the tem-

perature dependency of RDS(ON), RDR is the effective top

driverresistance(approximately2ΩatVGS=VMILLER),VIN

is the drain potential and the change in drain potential in

the particular application. VTH(IL) is the data sheet speci-

fied typical gate threshold voltage specified in the power

MOSFET data sheet at the specified drain current. CMILLER

is the calculated capacitance using the gate charge curve

from the MOSFET data sheet and the technique described

above.

BothMOSFETshaveI2RlosseswhilethetopsideN-channel

equation includes an additional term for transition losses,

which peak at the highest input voltage. 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.

The optional Schottky diodes conduct during the dead

time between the conduction of the two large power

MOSFETs. This prevents the body diode of the bottom

MOSFET from turning on, storing charge during the dead

time and requiring a reverse-recovery period which could

cost as much as several percent in efficiency. A 2A to 8A

Schottky is generally a good compromise for both regions

of operation due to the relatively small average current.

Larger diodes result in additional transition loss due to

their larger junction capacitance.

CIN and COUT Selection

In continuous mode, the source current of each top

N-channel MOSFET is a square wave of duty cycle VOUT/

VIN. A low ESR input capacitor sized for the maximum

RMS current must be used. The details of a close form

equation can be found in Application Note 77. Figure 10

showstheinputcapacitorripplecurrentfordifferentphase

configurations with the output voltage fixed and input volt-

age varied. The input ripple current is normalized against

the DC output current. The graph can be used in place of

tedious calculations. The minimum input ripple current

can be achieved when the product of phase number and

output voltage, N(VOUT), is approximately equal to the

input voltage VIN or:

where k

OUT

= , ,..., –

1 2

So the phase number can be chosen to minimize the input

capacitor size for the given input and output voltages. In

the graph of Figure 10, the local maximum input RMS

capacitor currents are reached when:

where k

OUT

2 1

1 2

–

, ,...,

Theseworst-caseconditionsarecommonlyusedfordesign

becauseevensignificantdeviationsdonotoffermuchrelief.

Note that capacitor manufacturer’s ripple current ratings

are often based on only 2000 hours of life. This makes

it advisable to further derate the capacitor or to choose

a capacitor rated at a higher temperature than required.

Several capacitors may also be paralleled to meet size or

height requirements in the design. Always consult the

capacitor manufacturer if there is any question.

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