Datasheet

LTC3861-1
25
38611fa
For more information www.linear.com/LTC3861-1
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
V
RA
R
RR
RB R
OUT
OUT TRACKA
TRACKA TRACKB1
2
2
22
=
+
+
•
AA
For coincident tracking (V
OUT1
= V
OUT2
during start-up),
R2A = R
TRACKA
R2B = R
TRACKB
applicaTions inFormaTion
The ramp time for V
OUT2
to rise from 0V to its final
value is:
tt
V
RR
R
SS SS
OUT F
TRACKA TRACKB
TRACKA
21
1
06
=
+
•
.
•
For coincident tracking,
tt
V
V
SS SS
OUT F
OUT F
21
2
1
= •
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.
LTC3861-1
FB2
V
OUT2
V
OUT1
FB1
TRACK/SS2
R2B
R2A
38611 F11
R1B
R1A
R
TRACKA
R
TRACKB
Figure 11. Using the TRACK/SS Pin
TIME
(12a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
38611 F12b
(12b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
Figure 12. Tw o Different Modes of Output Voltage Tracking