LTC4449 datasheet(21/108 Pages) LINER

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LTC3882-1

Rev A

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OPERATION

Finally, each PWM channel can be commanded off by

pulling the associated RUN pin low. Pulling the RUN pin

low can force the channel to perform a controlled turn off

or immediately disable the power stage, depending on the

programming of the ON_OFF_CONFIG command.

Minimum Output Disable Times

When a PMBus OPERATION command is used to turn off

an LTC3882-1 channel, a minimum output disable time of

120ms is imposed regardless of how quickly the channel

is commanded back on. If bit 4 of MFR_CHAN_CONFIG is

clear,aPMBuscommandtoturnthechanneloffalsopulses

the RUN pin low. Once the RUN pin is pulled low internally

or externally, a minimum output disable time (RUN forced

low) of TOFF_DELAY + TOFF_FALL + 136ms is enforced.

If MFR_RESTART_DELAY is greater than this mandatory

minimum, the larger value of MFR_RESTART_DELAY is

used. In either case the LTC3882-1 holds its own RUN

pin low during the entire disable period. These minimum

off times allow a consistent channel restart with coher-

ent monitor ADC values and make the LTC3882-1 highly

compatible with other LTC PMBus digital power system

management products.

Output Short Cycle

An output short cycle condition is created when a mas-

ter channel is commanded back on while waiting for

TOFF_DELAY or TOFF_FALL to expire. Any time this

occurs, the LTC3882-1 asserts the Short Cycle bit in

STATUS_MFR_SPECIFIC. Device response at that point

is governed by bits in MFR_CHAN_CONFIG_LTC3882-1

and SMBALERT_MASK. Refer to the detailed descriptions

of those commands for additional details. Generally, the

LTC3882-1 should be controlled so that short cycle condi-

tions are not created during normal operation.

Light Load Current Operation

The LTC3882-1 has two modes of PWM operation: discon-

tinuous conduction mode (DCM) and forced continuous

conduction mode (CCM). Mode selection is made with

the MFR_PWM_MODE command.

In DCM, the inductor current is not allowed to reverse.

The reverse current comparator IREV disables the external

bottom MOSFET (synchronous rectifier) when the induc-

tor current reaches approximately 0A, preventing it from

going substantially negative. The external gate driver or

power block must have short delays to a high impedance

output, relative to the PWM cycle, to support DCM.

Efficiency at light loads in CCM is lower than in DCM.

Continuous conduction mode exhibits less interference

with audio circuitry but may result in reverse inductor

current, for instance at light loads or under large transient

conditions.

Switching Frequency and Phase

There is a high degree of flexibility for setting the PWM

operating frequency of the LTC3882-1. The switching

frequency of the PWM can be established with an in-

ternal oscillator or an external time base. The internal

phase-locked loop (PLL) synchronizes PWM control to

this timing reference with proper phase relation, whether

the clock is provided internally or externally. The device

can also be configured to provide the master clock to

other ICs through PMBus command, EEPROM setting,

or external configuration resistors as outlined in applica-

tion Table 10. For PMbus or EEPROM configuration, the

LTC3882-1 is designated as a clock master by clearing

bit 4 of MFR_CONFIG_ALL_LTC3882-1. As clock master,

the LTC3882-1 will drive its open-drain SYNC pin at the

selected rate with a pulse width of 125ns. An external

pull-up resistor between SYNC and VDD33 is required in

this case. Only one device connected to SYNC should be

designated to drive the pin. If more than one LTC3882-1

sharing SYNC is programmed as clock master, just one of

the devices is automatically elected to provide the clock.

The others disable their SYNC outputs and indicate this

with bit 10 of MFR_PADS_LTC3882-1.

The LTC3882-1 will automatically accept an external SYNC

input, disabling is own SYNC drive if necessary, as long

as the external clock frequency is greater than 1/2 of the

programmedinternaloscillator.Whetherconfiguredtodrive

SYNC or not, the LTC3882-1 can continue PWM operation

at the selected frequency (FREQUENCY_SWITCH) using

its own internal oscillator, if an external clock signal is

subsequently lost.

The MFR_PWM_CONFIG_LTC3882-1 command can be

usedtoconfigurethephaseofeachchannel.Desiredphase

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