LTC7802EUFDM datasheet(18/34 Pages) AD

18 / 34 page

LTC7802

Rev. 0

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In general, the requirements of each application will dictate

the appropriate choice for light-load operating mode. In

Burst Mode operation, the inductor current is not allowed

to reverse. The reverse current comparator turns off the

bottom MOSFET just before the inductor current reaches

zero, preventing it from reversing and going negative.

Thus, the regulator operates in discontinuous operation.

In addition, when the load current is very light, the inductor

current will begin bursting at frequencies lower than the

switching frequency and enter a low current sleep mode

when not switching. As a result, Burst Mode operation has

the highest possible efficiency at light load.

In forced continuous mode, the inductor current is

allowed to reverse 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, continuous operation has the advan-

tage of lower output voltage ripple and less interference

to audio circuitry. In forced continuous mode, the output

ripple is independent of load current.

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 top MOSFET 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 sys-

tem. 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

Two external power MOSFETs must be selected for each

controller in the LTC7802: one N-channel MOSFET for

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

the bottom (synchronous) switch. The peak-to-peak

gate drive levels are set by the INTVCC regulation point of

5.1V. Consequently, logic level threshold 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

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 cur-

rent are given by:

PMAIN_BUCK =

VOUT

VIN

IOUT(MAX)

(

)2 1+δ

(

)RDS(ON) +

(VIN)2

IOUT(MAX)

⎛

⎝⎜

⎞

⎠⎟

(RDR)(CMILLER)•

VINTVCC − VTHMIN

VTHMIN

⎡

⎣

⎢

⎤

⎦

⎥(f)

PSYNC_BUCK =

VIN − VOUT

VIN

IOUT(MAX)

(

)2 1+δ

(

)RDS(ON)

APPLICATIONS INFORMATION

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