LT8686SJVPBF datasheet(14/26 Pages) AD

14 / 26 page

LT8686S

Rev. 0

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3MHz. See the Applications Information section for fur-

ther frequency selection information.

The oscillator generates four-clock phases. When operat-

ing independently, the relative phases of the channels are

0° for CH1, 180° for CH2, 270° for CH3 and 90° for CH4.

Multiphase operation reduces input current ripple ampli-

tude while increasing the ripple frequency resulting in

lower required input capacitance. When combining chan-

nels, the master channel phase determines the combined

channel’s clock phase.

MODE SELECTION AND SYNCHRONIZATION

The LT8686S offers two primary operating modes, Burst

Mode operation and pulse-skipping mode, plus the

option to select frequency spread-spectrum for each. In

pulse-skipping mode, all switching cycles remain aligned

to the internal clock. Further, full switching frequency is

maintained to lower load currents than Burst Mode oper-

ation. Conversely, Burst Mode operation reduces input

current at low load currents, thereby achieving higher

low load efficiency than pulse-skipping mode. Selecting

spread-spectrum results in frequency modulation of

the programmed clock frequency to reduce EMI/EMC

emissions. When selected, the clock frequency will vary

between the programmed frequency and the programmed

frequency plus approximately 20%.

The SYNC/MODE pin is used to select the desired oper-

ating mode. To select low ripple, high efficiency Burst

Mode operation, connect the SYNC/Mode pin to ground.

Alternatively, connect a 26k (5% tolerance or better)

resistor to enable low ripple high efficiency Burst Mode

operation with spread spectrum. To select pulse-skipping

mode, float the SYNC/MODE pin, or to enable pulse skip-

ping with spread spectrum connect the SYNC/MODE pin

to INTVCC.

Finally, the LT8686S may be synchronized to an external

clock source by driving the SYNC/MODE pin with a clock

signal that has a minimum high voltage of 1.5V and max-

imum low voltage of 0.4V. The minimum required pulse

width is 100ns for a high pulse and 100ns for a low pulse.

When syncing, the RT resistor should be chosen to set the

LT8686S switching frequency close to the synchroniza-

tion frequency. The LT8686S will operate in pulse-skip-

ping mode while synchronized to an external clock.

INTVCC REGULATOR

The INTVCC regulator supplies internal LT8686S circuitry.

The current draw will depend on the operating frequency,

the higher the switching frequency, the greater the current

drawn from INTVCC. The regulator is supplied by VIN1 at

start-up, but it will draw its supply current from bias if

the bias voltage is above 4.5V and VVIN1 is greater than

VBIAS + 1V. If the BIAS pin is connected to a switching

regulator channel ≥ 4.5V it will improve efficiency, reduce

on-chip power dissipation, and lower the required cur-

rent. The INTVCC regulator may be used to configure other

input pins and output pull-ups related to the LT8686S. It

should not be used to connect to any components not

related to the LT8686S to avoid unexpected interactions.

OPERATION

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