ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 21

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Setting Output Voltage
The LTC3858-2 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:
To improve the frequency response, a feedforward ca-
pacitor, C
route the V
inductor or the SW line.
V
OUT
= 0.8V 1+
FF
, may be used. Great care should be taken to
FB
line away from noise sources, such as the
Figure 5. Setting Output Voltage
1/2 LTC3858-2
⎛
⎜
⎝
R
R
V
FB
B
A
⎞
⎟
⎠
V
OUT
R
R
38582 F05
B
A
C
FF
Soft-Start (SS Pins)
The start-up of each V
the respective SS pin. When the voltage on the SS pin
is less than the internal 0.8V reference, the LTC3858-2
regulates the V
pin instead of 0.8V. The SS pin can be used to program
an external soft-start function or to allow V
another supply during start-up.
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 6. An internal
1μA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3858-2 will
regulate the V
voltage on the SS pin, allowing V
0V to its final regulated value. The total soft-start time will
be approximately:
t
SS
= C
Figure 6. Using the SS Pin to Program Soft-Start
SS
•
FB
0.8V
FB
1μA
pin (and hence V
pin voltage to the voltage on the SS
OUT
C
SS
is controlled by the voltage on
SS
SGND
1/2 LTC3858-2
OUT
38582 F06
OUT
LTC3858-2
to rise smoothly from
) according to the
OUT
to track
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