LTC7803 datasheet(12/36 Pages) AD

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LTC7803

Rev 0

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OPERATION

to INTVCC. To select pulse-skipping mode, tie the MODE

pin to a DC voltage greater than 1.2V and less than

INTVCC – 1.3V. An internal 100k resistor to GND invokes

Burst Mode operation when the MODE pin is floating and

pulse-skipping mode when the MODE pin is tied to INTVCC

through an external 100k resistor.

When the controller is enabled for Burst Mode opera-

tion, the minimum peak current in the inductor is set to

approximately 25% of the maximum sense voltage even

though the voltage on the ITH pin indicates a lower value.

If the average inductor current is higher than the load cur-

rent, the error amplifier, EA, will decrease the voltage on

the ITH pin. When the ITH voltage drops below 0.425V,

the internal sleep signal goes high (enabling sleep mode)

and both external MOSFETs are turned off. The ITH pin is

then disconnected from the output of the EA and parked

at 0.45V.

In sleep mode, much of the internal circuitry is turned off,

reducing the quiescent current that the LTC7803 draws

to only 14μA. When VOUT is 3.2V or higher, the majority

of this quiescent current is supplied by the SENSE– pin,

which further reduces the input-referred quiescent cur-

rent by the ratio of VIN/VOUT multiplied by the efficiency.

In sleep mode, the load current is supplied by the output

capacitor. As the output voltage decreases, the EA’s out-

put begins to rise. When the output voltage drops enough,

the ITH pin is reconnected to the output of the EA, the

sleep signal goes low, and the controller resumes normal

operation by turning on the top external MOSFET on the

next cycle of the internal oscillator.

When the controller is enabled for Burst Mode opera-

tion, the inductor current is not allowed to reverse. The

reverse current comparator, IR, turns off the bottom exter-

nal MOSFET just before the inductor current reaches zero,

preventing it from reversing and going negative. Thus, the

controller operates in discontinuous operation.

In forced continuous operation or clocked by an external

clock source to use the phase-locked loop (see Frequency

Selection and Phase-Locked Loop section), the inductor

current is allowed to reverse at light loads or under large

transient conditions. The peak inductor current is deter-

mined by the voltage on the ITH pin, just as in normal

operation. In this mode, the efficiency at light loads is

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.

When the MODE pin is connected for pulse-skipping

mode, the LTC7803 operates in PWM pulse-skipping

mode at light loads. In this mode, constant frequency

operation is maintained down to approximately 1% of

designed maximum output current. At very light loads, the

current comparator, ICMP, may remain tripped for several

cycles and force the external top MOSFET to stay off for

the same number of cycles (i.e., skipping pulses). The

inductor current is not allowed to reverse (discontinu-

ous operation). This mode, like forced continuous opera-

tion, exhibits low output ripple as well as low audio noise

and reduced RF interference as compared to Burst Mode

operation. It provides higher low current efficiency than

forced continuous mode, but not nearly as high as Burst

Mode operation.

Unlike forced continuous mode and pulse-skipping mode,

Burst Mode cannot be synchronized to an external clock.

Therefore, if Burst Mode is selected and PLLIN/SPREAD

pin is clocked to use the phase-locked loop, the LTC7803

switches from Burst Mode to forced continuous mode.

Frequency Selection, Spread Spectrum and Phase-

Locked Loop (FREQ and PLLIN/SPREAD Pins)

The selection of switching frequency is a trade-off between

efficiency and component size. Low frequency opera-

tion increases efficiency by reducing MOSFET switching

losses, but requires larger inductance and/or capacitance

to maintain low output ripple voltage.

The free running switching frequency of the LTC7803 is

selected using the FREQ pin. If the PLLIN/SPREAD pin is

not being driven by an external clock source, the FREQ

pin can be tied to GND, tied to INTVCC or programmed

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