LTC3834 Datasheet, PDF (16/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834
APPLICATIONS INFORMATION
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 LTC3834 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 3.
An internal 1μA current source charges up the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3834 will regulate the V_FB pin (and hence V_OUT)
LTC3834
TRACK/SS
C_SS
SGND
3834 F03
Figure 3. Using the TRACK/SS Pin to Program Soft-Start
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
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 4a and 4b. 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 5.
During start-up V_OUT will track V_X according to the ratio
set by the resistor divider:
V_{X =} R_{A •} R_TRACKA + R_TRACKB
V_OUT R_TRACKA
R_A + R_B
For coincident tracking (V_OUT = V_X during start-up),
R_A = R_TRACKA
R_B = R_TRACKB
V_X (MASTER)
V_X (MASTER)
V_OUT (SLAVE)
V_OUT (SLAVE)
16
TIME
3834 F04A
(4a) Coincident Tracking
TIME
3834 F04B
(4b) Ratiometric Tracking
Figure 4. Two Different Modes of Output Voltage Tracking
V_x
V_OUT
RB
RA
R_TRACKB
R_TRACKA
LTC3834
V_FB
TRACK/SS
3834 F05
Figure 5. Using the TRACK/SS Pin for Tracking
3834fb