LTC3826-1 Datasheet, PDF (17/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

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
LTC3826-1 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 1μA current source charges up the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3826-1 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
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies 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 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_A
+
+
R_TRACKB
R_B
For coincident tracking (V_OUT = V_X during start-up),
R_A = R_TRACKA
R_B = R_TRACKB
INTV_CC Regulators
The LTC3826-1 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTV_CC pin from either the V_IN supply pin or the
EXTV_CC pin, respectively, depending on the connection
of the EXTV_CC pin. INTV_CC powers the gate drivers and
much of the LTC3826-1’s internal circuitry. The V_IN LDO
regulates the voltage at the INTV_CC pin to 5.25V and the
LTC3826-1
1/2 LTC3826-1
TRACK/SS
C_SS
SGND
38261 F05
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
V_X (MASTER)
V_OUT (SLAVE)
TIME
38261 F06A
(6a) Coincident Tracking
V_X (MASTER)
V_OUT (SLAVE)
TIME
38261 F06B
(6b) Ratiometric Tracking
Figure 6. Two Different Modes of Output Voltage Tracking
V_x V_OUT
R_TRACKB
R_TRACKA
RB 1/2 LTC3826-1
V_FB
RA
TRACK/SS
38261 F07
Figure 7. Using the TRACK/SS Pin for Tracking
38261fb
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