ltc3891 Linear Technology Corporation, ltc3891 Datasheet - Page 19

• Manufacturer Part Number: ltc3891
• Description: Ltc3891 - Low Iq, 60v Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The rising and falling UVLO thresholds are calculated using
the RUN pin thresholds:
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
Tracking and Soft-Start (TRACK/SS Pin)
The start-up of V
TRACK/SS pin. When the voltage on the TRACK/SS pin is
less than the internal 0.8V reference, the LTC3891 regulates
the V
instead of 0.8V. The TRACK/SS pin can be used to program
an external soft-start function or to allow V
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
according to the voltage on the TRACK/SS pin, allowing
V
The total soft-start time will be approximately:
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
OUT
V
V
t
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
SS
UVLO(RISING)
UVLO(FALLING)
FB
to rise smoothly from 0V to its final regulated value.
= C
pin voltage to the voltage on the TRACK/SS pin
SS
•
10µA
0.8V
=
OUT
=
1.21V 1+
C
1.16V 1+
SS
is controlled by the voltage on the
TRACK/SS
SGND
LTC3891
IN






voltage range.
3891 F05
R
R
R
R
A
B
FB
B
A






pin (and hence V
X
) to the TRACK/SS
OUT
to track
OUT
)
pin of the slave supply (V
During start-up V
set by the resistor divider:
For coincident tracking (V
R
R
V
Figure 6. Two Different Modes of Output Voltage Tracking
A
B
V
OUT
X
= R
= R
Figure 7. Using the TRACK/SS Pin for Tracking
=
TRACKA
TRACKB
R
TRACKA
R
A
R
R
(6b) Ratiometric Tracking
(6a) Coincident Tracking
TRACKB
TRACKA
OUT
•
will track V
V
x
R
V
TRACKA
OUT
TIME
TIME
OUT
R
R
OUT
B
A
R
= V
A
), as shown in Figure 7.
V
TRACK/SS
FB
+ R
+ R
LTC3891
X
X
according to the ratio
during start-up):
TRACKB
B
3891 F07
3891 F06a
3891 F06b
LTC3891
V
V
V
V
X(MASTER)
OUT(SLAVE)
X(MASTER)
OUT(SLAVE)
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