LTC7803 datasheet(24/44 Pages) AD

24 / 44 page

LTC7817

Rev 0

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APPLICATIONS INFORMATION

In some applications, it may be desirable to change light

load operating mode based on the conditions present in

the system. For example, if a system is inactive, one might

select high efficiency Burst Mode operation by keeping

the PLLIN/MODE pin set to 0V. When the system wakes,

one might send an external clock to PLLIN/MODE, or tie

PLLIN/MODE to INTVCC to switch to low noise forced con-

tinuous mode. Such on-the-fly mode changes can allow

an individual application to benefit from the advantages

of each light load operating mode.

Power MOSFET Selection

Two external power MOSFETs must be selected for each

controller in the LTC7817: one N-channel MOSFET for the

top switch (main switch for the buck, synchronous for the

boost), and one N-channel MOSFET for the bottom switch

(main switch for the boost, synchronous for the buck).

The peak-to-peak gate drive levels are set by the INTVCC

regulation point of 5.1V. Consequently, logic level thresh-

old MOSFETs must be used in most applications. Pay

close attention to the BVDSS specification for the MOSFETs

as well; many of the logic level MOSFETs are limited to

30V or less.

Selection criteria for the power MOSFETs include the on

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

age 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:

Buck Main Switch Duty Cycle =

VOUT

VIN

Buck Sync Switch Duty Cycle =

VIN � VOUT

VIN

Boost Main Switch Duty Cycle

VOUT � VIN

VOUT

Boost Sync Switch Duty Cycle

VIN

VOUT

For a buck converter, the MOSFET power dissipations at

maximum output current are given by:

PMAIN_BUCK =

VOUT

VIN

IOUT(MAX)

(

)

+ �

(

)RDS(ON) +

(VIN)

2 IOUT(MAX)

�

÷(RDR)(CMILLER) •

VINTVCC � VTHMIN

VTHMIN

�

�(f)

PSYNC_BUCK =

VIN � VOUT

VIN

IOUT(MAX)

(

)

+ �

(

)RDS(ON)

Similarly, for a boost converter:

PMAIN_BOOST =

VOUT � VIN

(

)VOUT

VIN

IOUT(MAX)

(

)

•

+ �

(

)RDS(ON) +

OUT

VIN

�

��

�

÷÷

IOUT(MAX)

�

÷•

RDR

(

) CMILLER

(

) •

VINTVCC � VTHMIN

VTHMIN

�

�(f)

PSYNC_BOOST =

VOUT

VIN

IOUT(MAX)

(

)

+ �

(

)RDS(ON)

where

δ is the temperature dependency of RDS(ON)

(

δ ≈ 0.005/°C) and RDR is the effective driver resistance at

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