LTC3864 datasheet(20/34 Pages) LINER

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LTC3807

3807fa

For more information www.linear.com/LTC3807

TherisingandfallingUVLOthresholdsarecalculatedusing

the RUN pin thresholds:

VUVLO(RISING) =1.21V 1+







 – 7µA •RB

VUVLO(FALLING) =1.16V 1+







 – 7µA •RB

The resistor values should be carefully chosen such that

the absolute maximum ratings of the RUN pin do not get

violated over the entire VIN voltage range.

Tracking and Soft-Start (TRACK/SS Pin)

The start-up of VOUT is controlled by the voltage on the

TRACK/SS pin. When the voltage on the TRACK/SS pin is

lessthantheinternal0.8Vreference,theLTC3807regulates

the VFB 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 VOUT 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 5.

An internal 10μA current source charges the capacitor,

providing a linear ramping voltage at the TRACK/SS pin.

The LTC3807 will regulate the VFB pin (and hence VOUT)

according to the voltage on the TRACK/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

10µA

APPLICATIONS INFORMATION

pin of the slave supply (VOUT), as shown in Figure 7.

During start-up VOUT will track VX according to the ratio

set by the resistor divider:

VOUT

RTRACKA

•

RTRACKA +RTRACKB

RA +RB

For coincident tracking (VOUT = VX during start-up):

RA = RTRACKA

RB = RTRACKB

Figure 5. Using the TRACK/SS Pin to Program Soft-Start

LTC3807

TRACK/SS

CSS

SGND

3807 F05

Alternatively, the TRACK/SS pin can be used to track

another supply during start-up, as shown qualitatively in

Figures 6a and 6b. To do this, a resistor divider should be

connected from the master supply (VX) to the TRACK/SS

TIME

VX(MASTER)

VOUT(SLAVE)

3807 F06a

TIME

3807 F06b

VX(MASTER)

VOUT(SLAVE)

(6a) Coincident Tracking

(6b) Ratiometric Tracking

Figure 6. Two Different Modes of Output Voltage Tracking

LTC3807

VOUT

VFB

TRACK/SS

3807 F07

RTRACKA

RTRACKB

Figure 7. Using the TRACK/SS Pin for Tracking

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