LTC3890
applicaTions inForMaTion
Tracking and Soft-Start (TRACK/SS Pins)
V
V
X(MASTER)
The start-up of each V
is controlled by the voltage on
OUT
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference,
OUT(SLAVE)
the LTC3890 regulates the V pin voltage to the voltage
FB
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
to track another supply during start-up.
OUT
3890 F07a
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.
TIME
(7a) Coincident Tracking
V
V
X(MASTER)
OUT(SLAVE)
The LTC3890 will regulate the V pin (and hence V
)
FB
OUT
according to the voltage on the TRACK/SS pin, allowing
V
to rise smoothly from 0V to its final regulated value.
OUT
The total soft-start time will be approximately:
0.8V
1µA
tSS = CSS
•
3890 F07b
1/2 LTC3890
TRACK/SS
TIME
(7b) Ratiometric Tracking
C
SS
SGND
Figure 7. Two Different Modes of Output Voltage Tracking
3890 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
V
V
OUT
x
Alternatively, the TRACK/SS pin can be used to track two
(ormore)suppliesduringstart-up,asshownqualitatively
in Figures 7a and 7b. To do this, a resistor divider should
1/2 LTC3890
R
B
V
FB
R
A
be connected from the master supply (V ) to the TRACK/
X
R
R
TRACKB
SS pin of the slave supply (V ), as shown in Figure 8.
OUT
TRACK/SS
During start-up V
will track V according to the ratio
OUT
X
3890 F08
TRACKA
set by the resistor divider:
RTRACKA + RTRACKB
RA + RB
VX
RA
Figure 8. Using the TRACK/SS Pin for Tracking
=
•
VOUT RTRACKA
For coincident tracking (V
= V during start-up):
X
OUT
R = R
A
TRACKA
TRACKB
R = R
B
3890fb
20
Linear Systems [ Linear Systems ]
