参数资料
型号: LTC3858EUH-2#TRPBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: DUAL SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PQCC32
封装: 5 X 5 MM, LEAD FREE, PLASTIC, MO-220WHHD, QFN-32
文件页数: 14/40页
文件大小: 432K
代理商: LTC3858EUH-2#TRPBF
LTC3858-2
21
38582f
APPLICATIONS INFORMATION
Figure 6. Using the SS Pin to Program Soft-Start
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:
VOUT = 0.8V 1+
RB
RA
To improve the frequency response, a feedforward ca-
pacitor, CFF, may be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor or the SW line.
Soft-Start (SS Pins)
The start-up of each VOUT is controlled by the voltage on
the respective SS pin. When the voltage on the SS pin
is less than the internal 0.8V reference, the LTC3858-2
regulates the VFB pin voltage to the voltage on the SS
pin instead of 0.8V. The SS pin can be used to program
an external soft-start function or to allow VOUT to track
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 VFB pin (and hence VOUT) according to the
voltage on the SS pin, allowing VOUT to rise smoothly from
0V to its final regulated value. The total soft-start time will
be approximately:
tSS = CSS
0.8V
1μA
1/2 LTC3858-2
SS
CSS
SGND
38582 F06
Figure 5. Setting Output Voltage
1/2 LTC3858-2
VFB
VOUT
RB
CFF
RA
38582 F05
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