LTC3859AL_15 Datasheet, PDF (26/44 Pages) Linear Technology – Triple Output, Buck/Buck/Boost Synchronous Controller with 28A Burst Mode IQ

LTC3859AL
applications information
Tracking and Soft-Start
(TRACK/SS1, TRACK/SS2, SS3 Pins)
The start-up of each V_OUT is controlled by the voltage on
the respective TRACK/SS pin (TRACK/SS1 for channel 1,
TRACK/SS2 for channel 2, SS3 for channel 3). When the
voltage on the TRACK/SS pin is less than the internal
0.8V reference (1.2V reference for the boost channel), the
LTC3859AL regulates the V_FB pin voltage to the voltage
on the TRACK/SS pin instead of the internal reference.
Likewise, the TRACK/SS pin for the buck channels 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 5µA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3859AL 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 _ BUCK}
=
C_SS
•
0.8V
5µA
t_{SS _ BOOST}
=
C_SS
•
1.2V
5µA
Alternatively, the TRACK/SS1 and TRACK/SS2 pins for the
two buck controllers can be used to track two (or more) sup-
plies during start-up, as shown qualitatively 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 resis-
tor 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
1/3 LTC3859AL
TRACK/SS
C_SS
SGND
3859al F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
V_X(MASTER)
V_OUT(SLAVE)
TIME
7a. Coincident Tracking
3859al F07a
V_X(MASTER)
V_OUT(SLAVE)
TIME
7b. Radiometric Tracking
3859al F07b
Figure 7. Two Different Modes of Output Voltage Tracking
V_OUT
R_B
LTC3859AL
V_FB1,2
V_X
R_A
R_TRACKB
R_TRACKA
TRACK/SS1,2
3859al F08
Figure 8. Using the TRACK/SS Pin for Tracking
26
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3859alf