Datasheet

LTC3891
19
3891fa
The rising and falling UVLO thresholds are calculated using
the RUN pin thresholds:
V
UVLO(RISING)
=
1.21V 1+
R
B
R
A






V
UVLO(FALLING)
=
1.16V 1+
R
B
R
A






The resistor values should be carefully chosen such that
the absolute maximum ratings of the RUN pin do not get
violated over the entire V
IN
voltage range.
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 LTC3891 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 5.
An internal 10μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3891 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
10µA
APPLICATIONS INFORMATION
pin of the slave supply (V
OUT
), as shown in Figure 7.
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
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
LTC3891
TRACK/SS
C
SS
SGND
3891 F05
Alternatively, the TRACK/SS pin can be used to track
another supply during start-up, as shown qualitatively in
Figures 6a and 6b. To do this, a resistor divider should be
connected from the master supply (V
X
) to the TRACK/SS
TIME
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
3891 F06a
TIME
3891 F06b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
(6a) Coincident Tracking
(6b) Ratiometric Tracking
Figure 6. Two Different Modes of Output Voltage Tracking
LTC3891
V
OUT
V
x
V
FB
TRACK/SS
3891 F07
R
B
R
A
R
TRACKA
R
TRACKB
Figure 7. Using the TRACK/SS Pin for Tracking