LTC7802EUFDM datasheet(12/34 Pages) AD

12 / 34 page

LTC7802

Rev. 0

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OPERATION

the TRACK/SS voltage rises linearly from 0V to 0.8V (and

beyond), the output voltage VOUT rises smoothly from

zero to its final value.

Alternatively, the TRACK/SS pins can be used to make the

start-up of VOUT track that of another supply. Typically

this requires connecting to the TRACK/SS pin through an

external resistor divider from the other supply to ground

(see the Applications Information section).

Light Load Operation: Burst Mode Operation, Pulse-

Skipping, or Forced Continuous Mode (MODE Pin)

The LTC7802 can be set to enter high efficiency Burst

Mode operation, constant frequency pulse-skipping

mode or forced continuous conduction mode at low load

currents.

To select Burst Mode operation, tie the MODE pin to

ground. To select forced continuous operation, tie the

MODE pin 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 ground

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 controllers are enabled for Burst Mode opera-

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

approximately 25% of its maximum value even though

the voltage on the ITH pin might indicate a lower value.

If the average inductor current is higher than the load

current, 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 LTC7802 draws. If

one channel is in sleep mode and the other channel is shut

down, the LTC7802 draws only 15μA of quiescent current.

If both channels are in sleep mode, the LTC7802 draws

only 18μA of quiescent current. When VOUT on channel 1

is 3.2V or higher, the majority of this quiescent current is

supplied by the SENSE1– pin, which further reduces the

input-referred quiescent current 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 MOSFET on the next cycle

of the internal oscillator.

When a controller is enabled for Burst Mode operation,

the inductor current is not allowed to reverse. The reverse

current comparator (IR) turns off the bottom 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 the inductor current is

allowed to reverse at light loads or under large transient

conditions. The peak inductor current is determined 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, continuous 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 LTC7802 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 top MOSFET to stay off for the same

number of cycles (i.e., skipping pulses). The inductor cur-

rent is not allowed to reverse (discontinuous operation).

This mode, like forced continuous operation, exhibits low

output ripple as well as low audio noise and reduced RF

interference as compared to Burst Mode operation. It pro-

vides 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.

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