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LTC3882-1

Rev A

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APPLICATIONS INFORMATION

While these considerations may or may not be important

for slave channels, VOUT must be connected back to the

slave channel VSENSE pin(s) in order for the IOUT telemetry

of those phases to be accurate. So in general, sound Kelvin

VOUT sensing techniques for all LTC3882-1 channels is

recommended.

start-up voltage ramp. The maximum rate at which the

LTC3882-1 can move the output in this fashion is 100µs/

step. Soft-start is disabled by setting TON_ RISE to any

value less than 0.250ms. The LTC3882-1 will perform the

necessary math internally to assure the voltage ramp is

controlled to the desired slope. However, the voltage slope

cannot be any faster than the fundamental limits of the

power stage. The smaller TON_RISE becomes, the more

noticeable an output voltage stair-step may become.

The LTC3882-1 also supports soft turn off in the same

manner it controls turn on. TOFF_FALL is processed when

the RUN pin goes low or if the part is commanded off.

If the part faults off or FAULT is pulled low and the part

is programmed to respond to this, the PWM instantly

commands the output off. The output will then decay as

a function of load current.

The LTC3882-1 can produce a controlled ramp off as long

as the power stage is configured to run in CCM and the

TOFF_FALL time is sufficiently slow that the power stage

can achieve the desired slope. The TOFF_FALL time can

only be met if the power stage can sink sufficient current

under closed loop control to assure the output is at 0V

by the end of the fall time. If TOFF_FALL is shorter than

the time required to discharge the load capacitance, the

output will not reach 0V. In this case, the power stage will

still be commanded off at the end of TOFF_FALL and VOUT

will decay at a rate determined by the load. If the control-

ler is set to run DCM, the controller will not pull negative

current and the output will only be pulled low by the load,

not the power stage. The maximum fall time is limited to

1.3 seconds. The smaller TOFF_FALL becomes, the more

noticeable an output voltage stair-step may become.

Time-Based Output Sequencing and Ramping

TheLTC3882-1TON_DELAYandTOFF_DELAYcommands

can be used in combination with the rise and fall time com-

mandscoveredintheprevioussectiontoimplementawide

range of versatile sequencing and ramping schemes. The

key to time-based sequencing and ramping is the ability of

LTC3882-1 master phases to move their outputs up and

down according to PMBus command values as shown in

Figure 29 and Figure 30.

Figure 27. Voltage Measured Directly Across RSENSE

Figure 28. Voltage Measured at ISENSE Pins

500ns/DIV

VRSENSE

20mV/DIV

38821 F27

VESL(STEP)

500ns/DIV

VISENSE

20mV/DIV

38821 F28

Soft-Start and Stop

The LTC3882-1 uses digital ramp control to create both

soft-start and soft-stop.

The LTC3882-1 must enter the run state prior to soft-start.

The RUN pins are released after the part initializes and

VINSNS is determined to be greater than the VIN_ON

threshold.

Onceintherunstate,soft-startisperformedafteranyaddi-

tionalprescribeddelay(nextsection)byactivelyregulating

the load voltage while digitally ramping the target voltage

from 0V to the commanded voltage set point. Rise time of

thevoltagerampcanbeprogrammedusingtheTON_RISE

command to minimize inrush currents associated with

LTC4449 Datasheet(PDF) 51 Page - Linear Technology

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