LTC3882-1
51
Rev A
APPLICATIONS INFORMATION
While these considerations may or may not be important
for slave channels, V
OUT
must be connected back to the
slave channel V
SENSE
pin(s) in order for the I
OUT
telemetry
of those phases to be accurate. So in general, sound Kelvin
V
OUT
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 V
OUT
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
The LTC3882-1 TON_DELAY and TOFF_DELAY commands
can be used in combination with the rise and fall time com-
mands covered in the previous section to implement a wide
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 27. Voltage Measured Directly Across R
SENSE
Figure 28. Voltage Measured at I
SENSE
Pins
500ns/DIV
V
RSENSE
20mV/DIV
38821 F27
V
ESL(STEP)
500ns/DIV
V
ISENSE
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.
Once in the run state, soft-start is performed after any addi-
tional prescribed delay (next section) by actively regulating
the load voltage while digitally ramping the target voltage
from 0V to the commanded voltage set point. Rise time of
the voltage ramp can be programmed using the TON_RISE
command to minimize inrush currents associated with
