LTC3859AL datasheet(12/38 Pages) LINER

12 / 38 page

LTC3899

3899fa

For more information www.linear.com/LTC3899

PIN FUNCTIONS (QFN/TSSOP)

TRACK/SS1, TRACK/SS2, SS3 (Pins 33, 17, 3/Pins 37,

21, 7): External Tracking and Soft-Start Input. For the buck

channels, the LTC3899 regulates the VFB1,2 voltage to the

smaller of 0.8V, or the voltage on the TRACK/SS1,2 pin.

For the boost channel, the LTC3899 regulates the VFB3

voltage to the smaller of 1.2V, or the voltage on the SS3

pin. An internal 10µA pull-up current source is connected

to this pin. A capacitor to ground at this pin sets the ramp

time to final regulated output voltage. Alternatively, a re-

sistor divider on another voltage supply connected to the

TRACK/SS pins of the buck channels allow the LTC3899

buck outputs to track the other supply during start-up.

VPRG3 (Pin 34/Pin 38): Channel 3 Output Control Pin.

This pin sets the boost channel to adjustable output mode

using external feedback resistors or fixed 10V/12V output

mode.Floatingthispinallowstheoutputtobeprogrammed

through the VFB3 pin using external resistors, regulating

VFB3 to the 1.2V reference. Connecting this pin to GND or

INTVCC programs the output to 10V or 12V (respectively),

and VFB3 is used to sense the output voltage.

ITH1, ITH2, ITH3 (Pins 35, 15, 7/Pins 1, 19, 11): Error

AmplifierOutputsandSwitchingRegulatorCompensation

Points. Each associated channel’s current comparator trip

point increases with this control voltage.

VFB1,VFB2(Pins36,14/Pins2,18):Thesepinsreceivethe

remotely sensed feedback voltage for each buck controller

from an external resistive divider across the output.

VFB3(Pins6/Pins10):IfVPRG3isfloating,thispinreceives

the remotely sensed feedback voltage for the boost con-

troller from an external resistive divider across the output.

If VPRG3 is tied to GND or INTVCC, this pin receives the

remotely sensed output voltage of the boost controller.

SENSE1+, SENSE2+, SENSE3+ (Pins 37, 13, 4/Pins 3,

17, 8): The (+) Input to the Differential Current Compara-

tors. The ITH pin voltage and controlled offsets between

the SENSE– and SENSE+ pins in conjunction with RSENSE

set the current trip threshold. For the boost channel, the

SENSE3+ pin supplies current to the current comparator.

SENSE1–, SENSE2–, SENSE3– (Pins 38, 12, 5/Pins 4,

16, 9): The (–) Input to the Differential Current Compara-

tors. When SENSE1,2– for the buck channels is greater

than INTVCC, then SENSE1,2– pin supplies current to the

current comparator.

GND (Exposed Pad Pin 39/Exposed Pad Pin 39): Ground.

The exposed pad must be soldered to the PCB for rated

electrical and thermal performance.

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