LTC3861-1 Datasheet, PDF (24/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
Output Current Sharing
When multiple LTC3861-1s are daisychained to drive a
common load, accurate output current sharing is essential
to achieve optimal performance and efficiency. Otherwise,
if one stage is delivering more current than another, then
the temperature between the two stages will be different,
and that could translate into higher switch R_DS(ON), lower
efficiency, and higher RMS ripple. When the COMP and
I_AVG pins of multiple LTC3861-1s are tied together, the
amount of output current delivered from each LTC3861-1
is actively balanced by the I_AVG loop. The SGND pins of
the multiple LTC3861-1s must be kelvined to the same
point for optimal current sharing.
Dual-Channel Operation
The LTC3861-1 can control two independent power supply
outputs with no channel-to-channel interaction or jitter.
The following recommendations will ensure maximum
performance in this mode of operation:
n _{The output of each channel should be sensed using}
the differential sense amplifier. The SGND pins and
exposed pad and all local small-signal GND should
then be a Kelvin connection to the negative terminal of
each channel output. This will provide the best possible
regulation of each channel without adversely affecting
the other channel.
n _{Due to internal logic used to determine the mode of}
operation, separate current limit resistors should be
used for each channel in dual-channel operation, even
when the values are the same.
Table 5 shows the I_LIM and EA configuration for dual-
channel operation.
Table 5. Dual-Channel Configuration
CH1
CH2
EA1
EA2
Independent Independent On
On
I_LIM1
Resistor
to GND
I_LIM2
Resistor
to GND
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of the supply output is controlled by the volt-
age on the TRACK/SS pin for that channel. The LTC3861-1
regulates the FB pin voltage to the lower of the voltage
on the TRACK/SS pin and the internal 600mV reference.
The TRACK/SS pin can therefore be used to program an
external soft-start function or allow the output supply to
track another supply during start-up.
External soft-start is enabled by connecting a capacitor
from the TRACK/SS pin to SGND. An internal 2.5µA cur-
rent source charges the capacitor, creating a linear voltage
ramp at the TRACK/SS pin, and causing the output sup-
ply to rise smoothly from its prebiased value to its final
regulated value. The total soft-start time is approximately:
t_SS(milliseconds)
=
C_SS µF
•
600mV
2.5µA
Alternatively, the TRACK/SS pin can be used to track
another supply during start-up.
38611f
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