LTC7803 datasheet(24/36 Pages) AD

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LTC7803

Rev 0

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In addition, when the load current is very light, the induc-

tor 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 opera-

tion 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.

Minimum On-Time Considerations

Minimum on-time, tON(MIN), is the smallest time duration

that the LTC7803 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

tON(MIN) <

VOUT

VIN• f

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTC7803 is approximately

40ns. However, as the peak sense voltage decreases the

minimum on-time gradually increases up to about 60ns.

This is of particular concern in forced continuous applica-

tions with low ripple current at light loads. If the duty cycle

drops below the minimum on-time limit in this situation,

a significant amount of cycle skipping can occur with cor-

respondingly larger current and voltage ripple.

Fault Conditions: Current Limit and Current Foldback

The LTC7803 includes current foldback to help limit load

current when the output is shorted to ground. If the out-

put voltage falls below 70% of its nominal output level,

then the maximum sense voltage is progressively low-

ered from 100% to 40% of its maximum selected value.

Under short-circuit conditions with very low duty cycles,

the LTC7803 will begin cycle skipping in order to limit the

short-circuit current. In this situation the bottom MOSFET

will be dissipating most of the power but less than in

normal operation. The short-circuit ripple current is deter-

mined by the minimum on-time, tON(MIN), of the LTC7803

(≈40ns), the input voltage and inductor value:

ΔIL(SC) = tON(MIN)

VIN

⎛

⎝⎜

⎞

⎠⎟

The resulting average short-circuit current is:

IL(SC) = 40% •ILIM(MAX) –

ΔIL(SC)

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