LTC3861-1 Datasheet, PDF (25/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
For example, Figure 11 shows the start-up of V_OUT2
controlled by the voltage on the TRACK/SS2 pin. Nor-
mally this pin is used to allow the start-up of V_OUT2 to
track that of V_OUT1 as shown qualitatively in Figures
12a and 12b. When the voltage on the TRACK/SS2 pin
is less than the internal 0.6V reference, the LTC3861-1
regulates the FB2 voltage to the TRACK/SS2 pin voltage
instead of 0.6V. The start-up of V_OUT2 may ratiometrically
track that of V_OUT1, according to a ratio set by a resistor
divider (Figure 12b):
V_OUT1
V_OUT2
=
R2A
R_TRACKA
•
R_TRACKA
R2B
+ R_TRACKB
+ R2A
For coincident tracking (V_OUT1 = V_OUT2 during start-up),
R2A = R_TRACKA
R2B = R_TRACKB
The ramp time for V_OUT2 to rise from 0V to its final
value is:
t_SS2
=
t_SS1 •
0.6
V_OUT1F
• ^{RTRACKA + RTRACKB}
R_TRACKA
For coincident tracking,
t_SS2
=
t_SS1
•
V_OUT2F
V_OUT1F
where V_OUT1F and V_OUT2F are the final, regulated values
of V_OUT1 and V_OUT2. V_OUT1 should always be greater than
V_OUT2 when using the TRACK/SS2 pin for tracking. If no
tracking function is desired, then the TRACK/SS2 pin may
be tied to a capacitor to ground, which sets the ramp time
to final regulated output voltage. It is only possible to track
another supply that is slower than the internal soft-start
ramp. At the completion of tracking, the TRACK/SS pin
must be >620mV, so as not to affect regulation accuracy
and to ensure the part is in CCM mode.
V_OUT1
V_OUT2
R_TRACKB
R_TRACKA
LTC3861-1
R1B
R2B
FB1
FB2
R1A
R2A
TRACK/SS2
38611 F11
Figure 11. Using the TRACK/SS Pin
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(12a) Coincident Tracking
TIME
38611 F12b
(12b) Ratiometric Tracking
Figure 12. Two Different Modes of Output Voltage Tracking
38611f
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