LTC7803 datasheet(11/42 Pages) AD

11 / 42 page

LTC7811

Rev. 0

For more information www.analog.com

PIN FUNCTIONS

MODE (Pin 1): Mode Select Input. This input, which acts

on all three channels, determines how the LTC7811 oper-

ates at light loads. Pulling this pin to ground selects Burst

Mode operation. An internal 100k resistor to ground also

invokes Burst Mode operation when the pin is floating.

Tying this pin to INTVCC forces continuous inductor cur-

rent operation. Tying this pin to INTVCC through a 100k

resistor selects pulse-skipping operation.

TRACK/SS3, TRACK/SS2, SS1 (Pins 2, 15, 35): External

Tracking and Soft-Start Input. For the buck channels, the

LTC7811 regulates the VFB1,2 voltage to the lesser of 0.8V

or the voltage on the TRACK/SS1,2 pin. For the boost

channel, the LTC7811 regulates VFB3 to the lesser of 1.2V

or the voltage on the SS3 pin. Internal 12.5µA pull-up

current sources are connected to these pins. A capacitor

to ground sets the startup ramp time to the final regu-

lated output voltage. The ramp time is equal to 0.65ms

for every 10nF of capacitance for the buck channels and

1ms for every 10nF for the boost channel. Alternatively,

a resistor divider on another voltage supply connected

to the TRACK/SS pins of the buck channels allows the

LTC7811 output to track the other supply during startup.

SENSE3+, SENSE2+, SENSE1+ (Pins 3, 12, 38): The

Positive (+) Input to the Differential Current Comparators.

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

when it is greater than INTVCC.

SENSE3–, SENSE2–, SENSE1– (Pins 4, 11, 39): The

Negative (–) Input to the Differential Current Comparators.

When SENSE1– is 3.2V or greater, it supplies the major-

ity of the sleep mode quiescent current instead of VBIAS,

further reducing the input-referred quiescent current. For

the buck channels, the SENSE– pins supply current to the

current comparators when they are greater than INTVCC.

VFB3 (Pin 5): Boost Controller Feedback Input. When

VPRG3 is floating, connect an external resistor divider

between the boost output voltage and the VFB3 pin to

set the regulated voltage. When VPRG3 is connected

to ground or INTVCC, tie VFB3 directly to the boost

converter output.

ITH3, ITH2, ITH1 (Pins 6, 14, 36): Error Amplifier Outputs

and Regulator Compensation Points. Each associated

channel’s current comparator trip point increases with

this control voltage. Place compensation components

between the ITH pins and ground.

IMON3 (Pin 7): Boost Channel Current Monitor. This pin

generates a voltage between 0.4V and 1.4V that corre-

sponds to the boost channel inductor current between

zero current and full-load. Optionally place a capacitor

from this pin to ground to average the current reading.

RUN1, RUN2, RUN3 (Pins 8, 9, 10): Run Control Inputs

for Each Controller. Forcing any of these pins below

1.1V disables switching of the corresponding controller.

Forcing all of these pins below 0.7V shuts down the entire

LTC7811, reducing the quiescent current to approximately

1.5µA. These pins can be tied to VIN or VBIAS for always-

on operation.

VFB2, VFB1 (Pins 13, 37): Buck Controller Feedback

Inputs. Connect an external resistor divider between the

output voltage and the VFB pin to set the regulated output

voltage. Tie VFB2 to INTVCC to configure the bucks for a

two-phase single output application, in which both buck

channels share VFB1, ITH1, and TRACK/SS1.

VPRG3 (Pin 16): Boost Output Voltage Programming Pin.

This pin sets the boost channel to adjustable output volt-

age or to a fixed output voltage. Floating this pin allows

the boost channel output to be programmed through the

VFB3 pin using external resistors, regulating VFB3 to the

1.2V reference. Connecting this pin to GND or INTVCC

programs the boost channel output to 8V or 9.5V (respec-

tively), with VFB3 directly connected to the output.

BATTGOOD (Pin 17): Battery Monitor Open-Drain Logic

Output. BATTGOOD is pulled to ground when the voltage

on BATTSNS is below 9.5V, and becomes high impedance

when BATTSNS exceeds 9.75V. Connect to ground if the

battery monitor feature is not used.

TG2, TG1 (Pins 18, 32): High Current Gate Drives for Top

N-Channel MOSFETs. These are the outputs of floating

drivers with a voltage swing of INTVCC superimposed on

the switch node voltage SW.

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