LTC7803 datasheet(24/42 Pages) AD

24 / 42 page

LTC7811

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

In forced continuous mode, the inductor current is

allowed to reverse on the buck channels at light loads

and switches at the same frequency regardless of load.

In this mode, the efficiency at light loads is considerably

lower than in Burst Mode operation. However, continu-

ous operation has the advantage of lower output voltage

ripple and less interference to audio circuitry. In forced

continuous mode, the output ripple is independent of load

current for the buck channels. The inductor current for

the boost channel cannot reverse since it does not have

a synchronous switch and therefore always operates in

discontinuous operation at light load.

In pulse-skipping mode, constant frequency operation

is maintained down to approximately 1% of designed

maximum output current. At very light loads, the PWM

comparator may remain tripped for several cycles and

force the main switch (top MOSFET for the bucks, bottom

MOSFET for the boost) to stay off for the same number of

cycles (i.e., skipping pulses). The inductor current is not

allowed to reverse (discontinuous operation). This mode,

like forced continuous operation, exhibits low output rip-

ple as well as low audio noise and reduced RF interference

as compared to Burst Mode operation. It provides higher

light load efficiency than forced continuous mode, but not

nearly as high as Burst Mode operation. Consequently,

pulse-skipping mode represents a compromise between

light load efficiency, output ripple and EMI.

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

MODE pin set to 0V. When the system wakes, one might

send an external clock to PLLIN/SPREAD, or tie MODE to

INTVCC to switch to low noise forced continuous 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

External power MOSFETs must be selected for each con-

troller in the LTC7811: N-channel MOSFETs for the buck

top main switches, and N-channel MOSFETs for the bot-

tom switches (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

LTC7803 датащи(PDF) 24 Page - Analog Devices

LTC7803 датащи(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

датащи скачать

ссылки URL