LTC3890-3 Datasheet, PDF (20/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-3
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 +

8
•
f
1
• 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.
V_OUT
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
1/2 LTC3890-3
TRACK/SS
C_SS
SGND
38903 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
1/2 LTC3890-3
V_FB
R_B
C_FF
R_A
38903 F05
Figure 5. Setting Output Voltage
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 =} R_{A •} R_TRACKA + R_TRACKB
V_OUT R_TRACKA
R_A + R_B
20
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38903f