LTC3859AL datasheet(16/44 Pages) LINER

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LTC7812

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OPERATION

the TRACK/SS pin is less than the 0.8V internal reference

for the buck and the 1.2V internal reference for the boost,

the LTC7812 regulates the VFB voltage to the TRACK/SS

pinvoltageinsteadofthecorrespondingreferencevoltage.

ThisallowstheTRACK/SSpintobeusedtoprogramasoft-

start by connecting an external capacitor from the TRACK/

SS pin to SGND. An internal 5µA pull-up current charges

this capacitor creating a voltage ramp on the TRACK/SS

pin. As the TRACK/SS voltage rises linearly from 0V to

0.8V/1.2V (and beyond up to INTVCC), the output voltage

VOUT rises smoothly from zero to its final value.

Alternatively the TRACK/SS pin for buck channel 1 can be

used to cause the start-up of VOUT to track that of another

supply. Typically, this requires connecting to the TRACK/

SS pin an external resistor divider from the other supply to

ground (see the Applications Information section).

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping, or Forced Continuous Conduction)

(PLLIN/MODE Pin)

The LTC7812 can be enabled to enter high efficiency Burst

Modeoperation,constantfrequencypulse-skippingmode

or forced continuous conduction mode at low load cur-

rents.ToselectBurstModeoperation,tiethePLLIN/MODE

pin to ground. To select forced continuous operation, tie

the PLLIN/MODE pin to INTVCC. To select pulse-skipping

mode, tie the PLLIN/MODE pin to a DC voltage greater

than 1.2V and less than INTVCC – 1.3V.

When a controller is enabled for Burst Mode operation, the

minimum peak current in the inductor is set to approxi-

mately 25% of the maximum sense voltage (30% for the

boost) even though the voltage on the ITH pin indicates 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 0.450V.

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

reducing the quiescent current that the LTC7812 draws. If

the buck channel 1 is in sleep mode and the boost channel

2 is shut down, the LTC7812 draws only 28µA of quies-

cent current. If both channels are in sleep mode, it draws

only 33µA of quiescent current. In sleep mode, the load

current is supplied by the output capacitor. As the output

voltage decreases, the EA’s output 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

externalMOSFETonthenextcycleoftheinternaloscillator.

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 external

MOSFET (the top external MOSFET for the boost) 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 when clocked by an

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

the 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 cur-

rent 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.

WhenthePLLIN/MODEpinisconnectedforpulse-skipping

mode, the LTC7812 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 (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 provides higher low current efficiency than

forced continuous mode, but not nearly as high as Burst

Mode operation.

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