Datasheet

LTC3835
16
3835fd
APPLICATIONS INFORMATION
Tracking and Soft-Start (TRACK/SS Pin)
The start-up of V
OUT
is controlled by the voltage on the
TRACK/SS pin. When the voltage on the TRACK/SS pin is
less than the internal 0.8V reference, the LTC3835 regulates
the V
FB
pin voltage to the voltage on the TRACK/SS pin
instead of 0.8V. The TRACK/SS pin can be used to program
an external soft-start function or to allow V
OUT
to “track”
another supply during start-up.
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
LTC3835
TRACK/SS
C
SS
SGND
3835 F03
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 3.
An internal 1µA current source charges up the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3835 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
= C
SS
•
0.8V
1µA
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 4a and 4b. 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 5.
During start-up V
OUT
will track V
X
according to the ratio
set by the resistor 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
TIME
(4a) Coincident Tracking
V
X
(MASTER)
V
OUT
(SLAVE)
OUTPUT VOLTAGE
3835 F04A
V
X
(MASTER)
V
OUT
(SLAVE)
TIME
3835 F04B
(4b) Ratiometric Tracking
OUTPUT VOLTAGE
Figure 5. Using the TRACK/SS Pin for Tracking
LTC3835
V
OUT
V
x
V
FB
TRACK/SS
3835 F05
R
B
R
A
R
TRACKA
R
TRACKB
Figure 4. Two Different Modes of Output Voltage Tracking