Datasheet

LTC3861
26
3861fa
For more information www.linear.com/LTC3861
applicaTions inFormaTion
For example, Figure 12a shows the start-up of V
OUT2
controlled 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 12a and
12b. When the voltage on the TRACK/SS2 pin is less
than the internal 0.6V reference, the LTC3861 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 12c):
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
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
VSNSP2
V
OUT2
V
OUT1
VSNSP1
TRACK/SS2
R2B
R2A
3861 F12a
R1B
R1A
R
TRACKA
R
TRACKB
Figures 12b and 12c. Tw o Different Modes of
Output Voltage Tracking
Figure 12a. Using the TRACK/SS Pin
TIME
(12b) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3861 F12b_c
(12c) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE