ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 20

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

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
TRACK/SS pin of the slave supply (V
Figure 8. During start-up V
to the ratio set by the resistor divider:
For coincident tracking (V
LTC3890
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each V
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference,
the LTC3890 regulates the V
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
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 LTC3890 will regulate the V
according to the voltage on the TRACK/SS pin, allowing
V
The total soft-start time will be approximately:
APPLICATIONS INFORMATION
20
OUT
R
R
t
V
SS
A
B
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
V
OUT
to rise smoothly from 0V to its fi nal regulated value.
X
= R
= R
= C
=
TRACKA
TRACKB
OUT
SS
R
TRACKA
•
to track another supply during start-up.
R
0.8V
1μA
A
•
OUT
R
C
SS
TRACKA
OUT
is controlled by the voltage on
TRACK/SS
SGND
1/2 LTC3890
OUT
FB
R
= V
A
pin voltage to the voltage
FB
+
+
will track V
3890 F06
X
R
R
pin (and hence V
during start-up):
B
TRACKB
OUT
), as shown in
X
according
X
) to the
OUT
)
INTV
The LTC3890 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV
pin depending on the connection of the EXTV
INTV
internal circuitry. The V
Figure 7. Two Different Modes of Output Voltage Tracking
CC
CC
CC
powers the gate drivers and much of the LTC3890’s
Regulators
pin from either the V
Figure 8. Using the TRACK/SS Pin for Tracking
R
R
TRACKB
TRACKA
(7b) Ratiometric Tracking
(7a) Coincident Tracking
V
IN
x
LDO and the EXTV
V
TIME
TIME
OUT
R
R
B
A
IN
supply pin or the EXTV
V
TRACK/SS
1/2 LTC3890
FB
3890 F07a
3890 F07b
3890 F08
V
V
V
V
X(MASTER)
OUT(SLAVE)
X(MASTER)
OUT(SLAVE)
CC
LDO regulate
CC
pin.
3890f
CC