Datasheet

LTC3857-1
20
38571fc
APPLICATIONS INFORMATION
(7a) Coincident Tracking
(7b) Ratiometric Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V
OUT
is controlled by the voltage on
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference, the
LTC3857-1 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.
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 LTC3857-1 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
1µ A
TIME
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
38571 F07a
TIME
38571 F07b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
1/2 LTC3857-1
V
OUT
V
x
V
FB
TRACK/SS
38571 F08
R
B
R
A
R
TRACKA
R
TRACKB
1/2 LTC3857-1
TRACK/SS
C
SS
SGND
38571 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies 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 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