LTC4449 datasheet(20/108 Pages) LINER

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LTC3882-1

Rev A

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OPERATION

Soft-Start

The RUN pins are released for external control after the

part initializes and VINSNS is greater than the VIN_ON

threshold. If multiple LTC3882-1 ICs are used in an ap-

plication, shared RUN pins are held low until all units

initialize and VINSNS exceeds the VIN_ON threshold for

alldevices.AcommonSHARE_CLKsignalcanalsoensure

all connected devices use the same time reference for

initial start-up even if RUN pins cannot be shared due to

other design requirements. SHARE_CLK is not released

by each IC until the conditions for power sequencing have

been fully satisfied.

After a channel RUN pin rises above 2V and any specified

turn on delay (TON_DELAY) has expired, the LTC3882-1

performs an initial monotonic soft-start ramp on that chan-

nel. This is carried out with a digitally controlled ramp of

the regulated output voltage from 0V to the commanded

voltage set point over the programmed TON_RISE period,

allowing inrush current control. During the soft-start ramp,

the LTC3882-1 does not initiate PWM operation until the

commanded output exceeds the actual rail voltage. This

allows the regulator to start up into a pre-biased load

even when using gate drivers or power blocks that do not

support discontinuous operation. The soft-start feature

is disabled by setting the value of TON_RISE to any time

less than 0.25ms.

Time-Based Output Sequencing

The LTC3882-1 supports time-based on and off output

sequencing using a shared time reference (SHARE_CLK).

Followingavalidqualifiedcommandtoturnon,eachoutput

is enabled after waiting its programmed TON_DELAY. This

can be used to sequence outputs in a prescribed order

that can be preprogrammed as needed without hardware

modification. Channel off-sequencing is accomplished in

a similar way with the TOFF_DELAY command.

Output Ramping Control

The LTC3882-1 supports synchronized output on and off

ramping control using a shared time reference (SHARE_

CLK).Powerrailonandoffrelationshipssimilartothoseof

conventional analog tracking functions can be achieved by

using programmed delays and TON_RISE and TOFF_FALL

times. However, with LTC3882-1 digital control, on and

off ramping methods need not be the same, and ramping

configurations can be reprogrammed as needed without

hardware modification.

Programmable fault responses and fault sharing can

ensure that any desired time-based output sequencing

and ramping control is properly accomplished each time

the system powers up or down. Refer to the Applications

Information section for various LTC3882-1 hardware and

PMBus command configurations needed to fully support

synchronization for time-based sequencing and output

ramping when using multiple ICs.

Voltage-Based Output Sequencing

It is also possible to sequence outputs using cascaded

voltage events. To do this, the PGOOD status output from

one PWM channel can be used to control the RUN pin

of a downstream channel. This keeps the downstream

channel off unless acceptable output conditions exist on

the controlling channel.

Output Disable

BothPWMchannelsaredisabledanytimeVINSNSisbelow

the VIN_OFF threshold. The power stages are immediately

shut off to stop the transfer of energy to the load(s) as

quickly as possible.

APWMchannelmayalsobedisabledinresponsetocertain

internal fault conditions, an external fault propagated into

a FAULT pin, or loss of SHARE_CLK. In these cases the

power stage is immediately shut off to stop the transfer

of energy to the load as quickly as possible. Refer to the

following Fault Detection and Handling section for ad-

ditional details related to fault recovery.

Each PWM channel can be disabled with a PMBus OPERA-

TIONcommandatanytimeifenabledbyON_OFF_CONFIG.

This will force a controlled turn-off response with defined

delay (TOFF_DELAY) and ramp down rate (TOFF_FALL).

The controller will maintain the programmed mode of

operation for TOFF_FALL. In DCM, the controller will not

draw current from the load and fall time will be set by

output capacitance and load current.

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