Datasheet

LTC3859A
26
3859af
applicaTions inForMaTion
Tracking and Soft-Start
(TRACK/SS1, TRACK/SS2, SS3 Pins)
The start-up of each V
OUT
is controlled by the voltage on
the respective TRACK/SS pin (TRACK/SS1 for channel 1,
TRACK/SS2 for channel 2, SS3 for channel 3). When the
voltage on the TRACK/SS pin is less than the internal
0.8V reference (1.2V reference for the boost channel), the
LTC3859A regulates the V
FB
pin voltage to the voltage on the
TRACK/SS pin instead of the internal reference. Likewise,
the TRACK/SS pin for the buck channels can be used to
program an external soft-start function or to allow V
OUT
to track another supply during start-up.
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 6.
An internal 1µA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3859A will regulate the V
FB
pin (and hence V
OUT
)
according to the voltage on the TRACK/SS pin, allowing
V
OUT
to rise smoothly from 0V to its final regulated value.
The total soft-start time will be approximately:
t
SS _BUCK
= C
SS
•
0.8V
1µA
t
SS _BOOST
= C
SS
•
1.2V
1µA
Alternatively, the TRACK/SS1 and TRACK/SS2 pins for the
two buck controllers can be used to track two (or more) sup-
plies during start-up, as shown qualitatively in Figures 7a
and 7b. To do this, a resistor divider should be connected
from the master supply (V
X
) to the TRACK/SS pin of the
slave supply (V
OUT
), as shown in Figure 8. During start-up
V
OUT
will track V
X
according to the ratio set by the resis-
tor divider:
V
X
V
OUT
=
R
A
R
TRACKA
•
R
TRACKA
+ R
TRACKB
R
A
+ R
B
For coincident tracking (V
OUT
= V
X
during start-up),
R
A
= R
TRACKA
R
B
= R
TRACKB
7a. Coincident Tracking
7b. Radiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
3859A F07a
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT (V
OUT
)
TIME
3859A F07b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT (V
OUT
)
TIME
3859A F08
LTC3859A
V
FB1,2
TRACK/SS1,2
R
B
R
A
V
OUT
R
TRACKB
R
TRACKA
V
X
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
3859A F06
1/3 LTC3859A
TRACK/SS
SGND
C
SS