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LTC3858-2

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+

⎛

⎝

⎜

⎞

⎠

⎟

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

CSS

SGND

38582 F06

Figure 5. Setting Output Voltage

1/2 LTC3858-2

VFB

VOUT

CFF

38582 F05

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