LTC3811_15 Datasheet, PDF(11/48 Page) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

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LTC3811_15 Datasheet, PDF (11/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

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LTC3811

PIN FUNCTIONS

BG1, BG2: High Current Gate Driver Outputs for the

N-Channel Lower Power MOSFETs.

BOOST1, BOOST2: Bootstrapped Supply Inputs to the

Topside Floating Drivers. A low ESR (X5R or better) ceramic

bypass capacitor should be connected between the BOOST

pin and the SW pin as close as possible to the IC.

CLKOUT: A Digital Output Used for Daisy-Chaining Multiple

LTC3811 ICs in Multiphase Systems. The PHASEMODE

pin voltage controls the phase relationship between the

channel 1 TG signal and CLKOUT.

COMP1, COMP2: Error Ampliï¬er Output Voltages. The

error ampliï¬ers in the LTC3811 are high bandwidth, low

offset true operational ampliï¬ers that have low output

impedance. As a result, the outputs of two active error

ampliï¬ers cannot be directly connected together! For

multiphase operation, connecting the FB pin of a slave

error ampliï¬er to INTVCC will disable the output of that

ampliï¬er. Multiphase operation can then be achieved by

connecting all of the COMP pins together and using one

channel as the master and all of the others as slaves. The

FB and COMP pins are also used for compensating the

control loop of the converter.

CSOUT (QFN Only): Output of the Voltage Positioning

gm Ampliï¬er. This pin allows the user to program the

amount of voltage droop in the output voltage at high

load current. The output of the voltage positioning gm

ampliï¬er is a bi-directional current proportional to the

(SENSE+ â SENSEâ) voltages for both channels. The

gm is internally ï¬xed to 5mS. Forcing the gm ampliï¬er

output current through a low value external resistor will

program the amount of voltage droop seen at the output.

See Applications Information for more details regarding

voltage positioning.

DIFF/IN+: Remote Sense Differential Ampliï¬er Positive

Input. A low offset, high bandwidth operational ampliï¬er

is conï¬gured with four precision 80k resistors for a non-

inverting gain of one. This pin is normally connected to the

positive terminal of the decoupling capacitor at the load.

DIFF/INâ: Remote Sense Differential Ampliï¬er Negative

Input. This pin is normally connected to the negative

terminal of the decoupling capacitor at the remote load.

The DIFF/IN+ and DIFF/INâ PCB traces should be routed

as close as possible and parallel to each other from the

IC to the output capacitor.

DIFF/OUT: Remote Sense Differential Ampliï¬er Output

Voltage, Conï¬gured for a Noninverting Gain of One. The

voltage at the DIFF/OUT pin is normally connected through

an external resistor divider to the FB pin of one channel.

The bottom of the divider should be connected to the

SGND pin of the IC.

DRVCC: Output of the Internal 6V Low Dropout Regulator

(LDO), Supply Pin for the Bottom Gate Drivers and Output

of the PMOS EXTVCC Switch. A low ESR (X5R or better)

4.7Î¼F ceramic bypass capacitor should be connected

between the DRVCC pin and the PGND pin, as close as

possible to the IC.

Exposed Pad (QFN Only): The Exposed Pad of the QFN

Leadframe is PGND.

EXTVCC: External Power Supply Input to an Internal PMOS

Power Switch Connected Between EXTVCC (Drain) and

DRVCC (Source). This pin allows an external supply to be

used for the high current gate drivers, thereby reducing

power dissipation in the LDO and increasing efï¬ciency.

When EXTVCC exceeds 4.5V (rising), the high current

PMOS switch turns on and shorts EXTVCC to DRVCC,

bypassing the internal LDO. See Applications Information

for more details.

3811f

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