LTC7802EUFDM-3.3 datasheet(19/34 Pages) AD

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LTC7802-3.3

Rev. 0

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Selecting the Light-Load Operating Mode

The LTC7802-3.3 can be set to enter high efficiency Burst

Mode operation, constant frequency pulse-skipping mode

or forced continuous conduction mode at light load cur-

rents. To select Burst Mode operation, tie the MODE to

ground. To select forced continuous operation, tie the

MODE pin to INTVCC. To select pulse-skipping mode,

tie the MODE pin to INTVCC through a 100k resistor.

An internal 100k resistor from the MODE pin to ground

selects Burst Mode if the pin is floating. When synchro-

nized to an external clock through the PLLIN/SPREAD

pin, the LTC7802-3.3 operates in pulse-skipping mode

if it is selected, or in forced continuous mode otherwise.

Table 2. summarizes the use of the MODE pin to select

the light load operating mode.

Table 2.

MODE PIN

LIGHT-LOAD

OPERATING MODE

MODE WHEN

SYNCHRONIZED

0V or Floating

Burst Mode

Forced Continuous

100k to INTVCC

Pulse-Skipping

INTVCC

Forced Continuous

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 discon-

tinuous operation. In addition, when the load current is

very light, the inductor current will begin bursting at fre-

quencies 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 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

Two external power MOSFETs must be selected for each

controller in the LTC7802-3.3: 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 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

APPLICATIONS INFORMATION

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