Datasheet

LTC3827-1
17
38271fe
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
LTC3827-1 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 LTC3827-1 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 fi nal 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
TRACKB
R
A
+ R
B
For coincident tracking (V
OUT
= V
X
during start-up),
R
A
= R
TRACKA
R
B
= R
TRACKB
INTV
CC
Regulators
The LTC3827-1 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 LTC3827-1’s internal circuitry. The V
IN
LDO
regulates the voltage at the INTV
CC
pin to 5.25V and the
Figure 7. Using the TRACK/SS Pin for Tracking
Figure 6. Two Different Modes of Output
Voltage Tracking
TIME
(6a) Coincident Tracking
V
X
(MASTER)
V
OUT
(SLAVE)
OUTPUT VOLTAGE
38271 F06A
V
X
(MASTER)
V
OUT
(SLAVE)
TIME
38271 F06B
(6b) Ratiometric Tracking
OUTPUT VOLTAGE
1/2 LTC3827-1
V
OUT
V
x
V
FB
TRACK/SS
38271 F07
R
B
R
A
R
TRACKA
R
TRACKB
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
1/2 LTC3827-1
TRACK/SS
C
SS
SGND
38271 F05