LTC3855_15 Datasheet, PDF(10/44 Page) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

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LTC3855_15 Datasheet, PDF (10/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

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LTC3855

Pin Functions (FE38/UJ40)

BOOST1, BOOST2 (Pin 32, Pin 24/Pin 29, Pin 21): Boosted

Floating Driver Supplies. The (+) terminal of the bootstrap

capacitors connect to these pins. These pins swing from a

diode voltage drop below INTVCC up to VIN + INTVCC.

TG1, TG2 (Pin 33, Pin 23/Pin 30, Pin 20): Top Gate

Driver Outputs. These are the outputs of floating drivers

with a voltage swing equal to INTVCC superimposed on

the switch nodes voltages.

SW1, SW2 (Pin 34, Pin 22/Pin 31, Pin 19): Switch Node

Connections to Inductors. Voltage swing at these pins

is from a Schottky diode (external) voltage drop below

ground to VIN.

PHASMD (Pin 36/Pin 33): This pin can be tied to SGND,

tied to INTVCC or left floating. This pin determines the

relative phases between the internal controllers as well

as the phasing of the CLKOUT signal. See Table 1 in the

Operation section.

CLKOUT (Pin 35/Pin 32): Clock output with phase change-

able by PHASMD to enable usage of multiple LTC3855 in

multiphase systems.

MODE/PLLIN (Pin 37/Pin 34): This is a dual purpose pin.

When external frequency synchronization is not used,

this pin selects the operating mode. The pin can be tied

to SGND, tied to INTVCC or left floating. SGND enables

forced continuous mode. INTVCC enables pulse-skipping

mode. Floating enables Burst Mode operation. For external

sync, apply a clock signal to this pin. Both channels will

go into forced continuous mode and the internal PLL will

synchronize the internal oscillator to the clock. The PLL

compensation network is integrated into the IC.

FREQ (Pin 38/Pin 35): There is a precision 10ÂµA current

flowing out of this pin. A resistor to ground sets a voltage

which in turn programs the frequency. Alternatively, this

pin can be driven with a DC voltage to vary the frequency

of the internal oscillator.

SGND (Exposed Pad Pin 39/ Pin 4, Exposed Pad Pin 41):

Signal Ground. All small-signal components and com-

pensation components should connect to this ground,

which in turn connects to PGND at one point. Exposed

pad must be soldered to PCB, providing a local ground

for the control components of the IC, and be tied to the

PGND pin under the IC.

3855f

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