LTC3891 datasheet(20/38 Pages) LINER

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LTC3892/

LTC3892-1/LTC3892-2

38921fc

For more information www.linear.com/LTC3892

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

IOUT(MAX)

(

)2 1+δ

(

)RDS(ON) +

(VIN)2

IOUT(MAX)















 (RDR)(CMILLER)•

VDRVCC − VTHMIN

VTHMIN













(f)

PSYNC =

VIN − VOUT

VIN

IOUT(MAX)

(

)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.

Both MOSFETs have I2R losses while the main N-channel

equations include 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

withlowerCMILLERactuallyprovideshigherefficiency.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.

Optional Schottky diodes placed across the synchronous

MOSFET conduct during the dead-time between the con-

duction of the two power MOSFETs. This prevents the

body diode of the synchronous MOSFET from turning

on, storing charge during the dead-time and requiring a

reverse 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.

CIN and COUT Selection

The selection of CIN is simplified by the 2-phase architec-

ture and its impact on the worst-case RMS current drawn

throughtheinputnetwork(battery/fuse/capacitor).Itcanbe

shown that the worst-case capacitor RMS current occurs

when only one controller is operating. The controller with

the highest (VOUT)(IOUT) product needs to be used in the

formula shown in Equation 1 to determine the maximum

RMS capacitor current requirement. Increasing the out-

put current drawn from the other controller will actually

decrease the input RMS ripple current from its maximum

value. The opt-of-phase technique typically reduces the

input capacitor’s RMS ripple current by a factor of 30%

to 70% when compared to a single phase power supply

solution.

Incontinuousmode,thesourcecurrentofthetopMOSFET

is a square wave of duty cycle (VOUT)/(VIN). To prevent

large voltage transients, a low ESR capacitor sized for the

maximum RMS current of one channel must be used. The

maximum RMS capacitor current is given by:

CIN Required IRMS ≈

IMAX

VIN

VOUT

(

) VIN −VOUT

(

)









1/2

(1)

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