LTC3835 datasheet(18/40 Pages) LINER

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LTC3890-3

38903f

For more information www.linear.com/3890-3

applicaTions inForMaTion

Power MOSFET and Schottky Diode

(Optional) Selection

Two external power MOSFETs must be selected for each

controller in the LTC3890-3: one N-channel MOSFET for

the top (main) switch, and one N-channel MOSFET for the

bottom (synchronous) switch.

Thepeak-to-peakdrivelevelsaresetbytheINTVCCvoltage.

This voltage is typically 5.1V during start-up (see EXTVCC

Pin Connection). Consequently, logic-level threshold

MOSFETs must be used in most applications. Pay close

attentiontotheBVDSSspecificationfortheMOSFETsaswell.

Selection criteria for the power MOSFETs include the

on-resistance, RDS(ON), Miller capacitance, CMILLER, input

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.

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