LTC3829 datasheet(13/40 Pages) LINER

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LTC3829

3829f

Light Load Current Operation (Burst Mode Operation,

Stage Shedding or Continuous Conduction)

The LTC3829 can be enabled to enter high efficiency

Burst Mode operation, Stage Shedding mode or forced

continuous conduction mode. To select forced continuous

operation, tie the MODE pin to a DC voltage below 0.6V

(e.g., SGND). To select Stage Shedding mode of opera-

tion, tie the MODE pin to INTVCC. To select Burst Mode

operation, float the MODE pin.

When the controller is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-sixth of the maximum sense voltage even though the

voltage on the ITH pin indicates a lower value. The peak

current can be programmed through the ISET pin. 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 0.5V (can

also be programmed by the ISET pin), the internal sleep

signal goes high (enabling sleep mode) and the external

MOSFETs are turned off. 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 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, IREV, turns off the bottom external

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. In this mode, the efficiency at light loads is

lower than in Burst Mode operation. However, continuous

mode has the advantages of lower output ripple and less

interference with audio circuitry.

When the MODE pin is connected to INTVCC,theLTC3829

operates in Stage Shedding mode at light loads. The

controller will turn off channels 2 and 3 and increase

the current gain of the first channel to ensure smooth

transition. The threshold where the controller goes into

Stage Shedding mode is when the ITH voltage drops

below 0.5V, but it can be programmed by ISET pin. The

inductor current is not allowed to reverse in this mode

(discontinuousoperation).Atverylightloads,thecurrent

comparator 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). This mode ex-

hibits 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.

2-Chip Operations (CLKOUT Pin)

The LTC3829’s three channels are 120° out of phase

providing multiphase operation. This configuration can

provide enough power for most high current applications.

However, for even higher power applications, the LTC3829

can be configured for PolyPhase® and 2-chip operation.

The LTC3829 features a CLKOUT pin which enables two

LTC3829s to operate out of phase. The CLKOUT signal is

60°outofphasewithrespecttophase1ofthecontroller.In

StageSheddingmode,however,theCLKOUTsignalis180°

out of phase with respect to phase 1 of the controller.

Frequency Selection and Phase-Locked Loop

(FREQ and PLLIN Pins)

Theselectionofswitchingfrequencyisatrade-offbetween

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.

If the PLLIN pin is not being driven by an external clock

source,theFREQpincanbeusedtoprogramthecontroller’s

operating frequency from 250kHz to 770kHz. There is a

precision 10µA current flowing out of the FREQ pin so that

the user can program the controller’s switching frequency

with a single resistor to SGND. A curve is provided later in

theApplicationsInformationsectionshowingtherelation-

ship between the voltage on the FREQ pin and switching

frequency.

operaTion (Refer to Functional Diagram)

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