Datasheet

LTC3890-3
20
38903f
For more information www.linear.com/3890-3
applicaTions inForMaTion
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfied, the capacitance is adequate for filtering. The
output ripple (∆V
OUT
) is approximated by:
∆V
OUT
≈ ∆I
L
ESR+
1
8 • f • C
OUT






where f is the operating frequency, C
OUT
is the output
capacitance and ∆I
L
is the ripple current in the inductor.
The output ripple is highest at maximum input voltage
since ∆I
L
increases with input voltage.
Setting Output Voltage
The LTC3890-3 output voltages are each set by an exter
-
nal feedback resistor divider carefully placed across the
output, as shown in Figure 5. The regulated output voltage
is determined by:
V
OUT
= 0.8V 1+
R
B
R
A






To improve the frequency response, a feedforward ca-
pacitor, C
FF
, may be used. Great care should be taken to
route the V
FB
line away from noise sources, such as the
inductor or the SW line.
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
LTC3890-3 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 6.
An internal 1µA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3890-3 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 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. Dur-
ing 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
1/2 LTC3890-3
TRACK/SS
C
SS
SGND
38903 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
Figure 5. Setting Output Voltage
1/2 LTC3890-3
V
FB
V
OUT
R
B
C
FF
R
A
38903 F05