LTC3860_15 Datasheet, PDF (21/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
APPLICATIONS INFORMATION
Dual-Channel Operation
The LTC3860 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 channel 1 should be sensed using the}
remote sense differential amplifier. The SGND pins and
exposed pad and all local small-signal GND should then
be a Kelvin connection to the negative terminal of the
channel 2 output. This will provide the best possible
regulation on channel 2 without adversely affecting
channel 1.
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 4 shows the I_LIM and EA configuration for dual-
channel operation.
Table 4. 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 LTC3860
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 current
source charges the capacitor, creating a linear voltage ramp
at the TRACK/SS pin, and causing the output supply to
rise smoothly from its prebiased value to its final regulated
value. The total soft-start time is approximately:
t_SS
=
C_SS
•
600mV
2.5µA
Alternatively, the TRACK/SS pin can be used to track
another supply during start-up.
For example, Figure 11a shows the start-up of V_OUT2 con-
trolled by the voltage on the TRACK/SS2 pin. Normally this
pin is used to allow the start-up of V_OUT2 to track that of
V_OUT1 as shown qualitatively in Figures 11a and 11b. When
the voltage on the TRACK/SS2 pin is less than the internal
0.6V reference, the LTC3860 regulates the FB2 voltage to
V_OUT1
V_OUT2
R_TRACKB
R_TRACKA
R1B ^LTC3860
R2B
FB1 FB2
R1A
R2A
TRACK/SS2
3860 F11a
Figure 11a. Using the TRACK/SS Pin
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(11b) Coincident Tracking
TIME
3860 F11b_c
(11c) Ratiometric Tracking
Figure 11b and 11c. Two Different Modes of Output Voltage Tracking
3860fc
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