LTC3857 Datasheet, PDF (20/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857
Applications Information
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 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 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
1/2 LTC3857
TRACK/SS
C_SS
SGND
3857 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 qualita-
tively 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_A
+ R_TRACKB
+ R_B
For coincident tracking (V_OUT = V_X during start-up):
R_A = R_TRACKA
R_B = R_TRACKB
20
V_X(MASTER)
V_OUT(SLAVE)
TIME
3857 F07a
(7a) Coincident Tracking
V_X(MASTER)
V_OUT(SLAVE)
TIME
3857 F07b
(7b) Ratiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
V_x V_OUT
R_TRACKB
R_TRACKA
R_B 1/2 LTC3857
V_FB
R_A
TRACK/SS
3857 F08
Figure 8. Using the TRACK/SS Pin for Tracking
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