LTC3855 datasheet(13/44 Pages) LINER

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LTC3855

3855f

voltage below 0.6V (e.g., SGND). To select pulse-skipping

mode of operation, tie the MODE/PLLIN pin to INTVCC. To

select Burst Mode operation, float the MODE/PLLIN pin.

When a controller is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-third 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 current,

the error amplifier EA will decrease the voltage on the ITH

pin. When the ITH voltage drops below 0.5V, the internal

sleep signal goes high (enabling sleep mode) and both

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 external 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

(IREV) turns off the bottom external MOSFET just before

the inductor current reaches zero, preventing it from

reversing and going negative. Thus, the controller oper-

ates 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

theadvantagesofloweroutputrippleandlessinterference

with audio circuitry.

When the MODE/PLLIN pin is connected to INTVCC, the

LTC3855 operates in PWM pulse-skipping mode at light

loads.Atverylight loads,thecurrentcomparatorICMPmay

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.

Multichip Operations (PHASMD and CLKOUT Pins)

The PHASMD pin determines the relative phases between

the internal controllers as well as the CLKOUT signal as

shown in Table 1. The phases tabulated are relative to

zero phase being defined as the rising edge of the clock

of phase 1.

Table 1.

PHASMD

GND

FLOAT

INTVcc

Phase1

0°

Phase2

180°

240°

CLKOUT

60°

90°

120°

The CLKOUT signal can be used to synchronize additional

powerstagesinamultiphasepowersupplysolutionfeeding

a single, high current output or separate outputs. Input

capacitance ESR requirements and efficiency losses are

substantiallyreducedbecausethepeakcurrentdrawnfrom

the input capacitor is effectively divided by the number

of phases used and power loss is proportional to the

RMS current squared. A two stage, single output voltage

implementation can reduce input path power loss by 75%

and radically reduce the required RMS current rating of

the input capacitor(s).

Single Output Multiphase Operation

The LTC3855 can be used for single output multiphase

converters by making these connections

• Tie all of the ITH pins together

• Tie all of the VFB pins together

• Tie all of the TK/SS pins together

• Tie all of the RUN pins together

• Tie all of the ITEMP pins together

• Tie all of the ILIM pins together, or tie the ILIM pins to

the same potential

For three or more phases, tie the inputs of the unused dif-

ferential amplifier(s) to ground. Examples of single output

multiphase converters are shown in Figures 20 to 23.

operaTion

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