LTM4677 datasheet(21/116 Pages) AD

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LTC3883/LTC3883-1

Rev. F

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OPERATION

the decay time of the faulted output determines the retry

delay. If the natural decay time of the output is too long,

it is possible to remove the voltage requirement of the

MFR_RETRY_DELAY command by asserting bit 0 of

MFR_CHAN_CONFIG_LTC3883.Alternatively,thecontrol-

lercanbeconfiguredsothatitremainslatched-offfollow-

ing a fault and clearing requires user intervention such

as toggling RUN or commanding the part OFF then ON.

LIGHT LOAD CURRENT OPERATION

The LTC3883 has three modes of operation including high

efficiency Burst Mode operation, discontinuous conduc-

tion mode or forced continuous conduction mode. Mode

selection is done using the MFR_PWM_MODE_LTC3883

command (discontinuous conduction is always the start-

up mode, forced continuous is the default running mode).

In 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

approximately 0.5V, the internal Burst Mode operation as-

serts and both external MOSFETS are turned off. In Burst

Mode operation, the load current is supplied by the output

capacitor. As the output voltage decreases, the EA output

begins to rise. When the output voltage drops sufficiently,

Burst Mode operation is deasserted, and the controller

resumes normal operation by turning on the top external

MOSFET on the next PWM cycle.

If a controller is enabled for Burst Mode operation, the

inductor current is not allowed to reverse. The reverse

currentcomparator,IREV,turnsoffthebottomgateexternal

MOSFET just before the inductor current reaches zero,

preventing it from reversing and going negative. Thus,

the controller can operate 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 solely

by the voltage on the ITH pin. In this mode, the efficiency

at light loads is lower than in Burst Mode operation. How-

ever, continuous mode exhibits lower output ripple and

less interference with audio circuitry. Forced continuous

conduction mode may result in reverse inductor current,

which can cause the input supply to boost. The VIN_OV_

FAULT_LIMIT can detect this and turn off the offending

channel. However, this fault is based on an ADC read and

can take up to 80ms to detect. If there is a concern about

the input supply boosting, keep the part in discontinuous

conduction or Burst Mode operation.

If the part is set to Burst Mode operation, as the inductor

average current increases, the controller will automati-

cally modify the operation from Burst Mode operation,

to discontinuous mode to continuous mode.

SWITCHING FREQUENCY AND PHASE

The switching frequency of the LTC3883’s controller can

be established with internal clock references or with an

external time-base. The LTC3883 can be configured for

an external clock input through the programmed value in

NVM, a PMBus command or setting the RBOTTOM resistor

of the FREQ_CFG pin to 0Ω and the RTOP to open. The

PMBus command FREQUENCY_SWITCH is set to exter-

nal clock. The MFR_PWM_CONFIG_LTC3883 command

determines the relative phasing. The RCONFIG input can

set the relative phasing with respect to the falling edge of

SYNC. The master should be selected to be out of phase

with the slave. The RUN pin must be low before the

FREQUENCY and MFR_PWM_CONFIG_LTC3883 com-

mands can be written to the LTC3883. The relative phas-

ing of all devices in a PolyPhase rail should be optimally

phased. The relative phasing of each rail is 360/n where

n is the number of phases in the rail.

If the LTC3883 is configured as the oscillator output on

SYNC,theswitchingfrequencysourcecanbeselectedwith

either external configuration resistors or through serial

busprogramming.TheFREQ_CFGconfigurationresistor

pin can be used to select the FREQUENCY_SWITCH

and MFR_PWM_CONFIG_LTC3883 values as outlined

in Table 14. Otherwise, the FREQUENCY_SWITCH and

MFR_PWM_CONFIG_LTC3883 PMBus commands can

be used to select PWM switching frequency and the PWM

channel phase relationship. The phase and frequency

relationships are completely independent of each other

providing the numerous application options for the user.

If the LTC3883 is configured to drive the SYNC pin using

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