LT3579 datasheet(21/40 Pages) LINER

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LT3579/LT3579-1

35791f

3.3μH

10μF

200k

VOUT

18V

VOUT

12V

1.7A

VIN

3579 F12

SW1 SW2

GATE

GND

SYNC

CLKOUT

VIN

SHDN

FAULT

LT3579

SLAVE

10μF

100k

110k

3.3nF

0.1μF

68pF

47pF

4.7μF

86.6k

2.2μH

10k

SW1 SW2

GATE

CLKOUT

GND

SYNC

VIN

FAULT

SHDN

LT3579

MASTER

2.2nF

130k

0.1μF

Figure 12. Synchronize Multiple LT3579s. The External PMOS

Disconnects the Input from Both Power Paths During FAULT Events

Also, the FAULT pins can be tied together so that a fault

condition from one LT3579 causes all of the LT3579s to

enter fault, until the fault condition disappears.

2-Phase Converters using LT3579-1

The CLKOUT pin on the LT3579-1 is ~180° out of phase

with the internal oscillator, which allows two LT3579-1s to

operate in parallel for a high current, high power output.

The advantage of multiphase converters is that the ripple

current ﬂowing into the output node is divided by the

number of phases or ICs used to generate the output

voltage. The VIN, SHDN, FAULT, FB, and VC pins of all the

LT3579-1s should be connected together. Figure 13 shows

a typical application of a 2-phase 12V to 24V boost with

output disconnect.

APPLICATIONS INFORMATION

Clock Synchronization

An external source can set the operating frequency of the

LT3579 by providing a digital clock signal into the SYNC

pin (RT resistor still required). The LT3579 will operate at

the SYNC clock frequency. The LT3579 will revert to its

internal free-running oscillator clock when the SYNC pin is

driven below 0.4V for a few free-running clock periods.

Driving SYNC high for an extended period of time

effectively stops the operating clock and prevents latch

SR1 from becoming set (see Block Diagram). As a result,

the switching operation of the LT3579 will stop and the

CLKOUT pin will be held at ground.

The duty cycle of the SYNC signal must be between 20%

and 80% for proper operation. Also, the frequency of the

SYNC signal must meet the following two criteria:

SYNC may not toggle outside the frequency

range of 200kHz-2.5MHz unless it is stopped

below 0.4V to enable the free-running oscillator.

The SYNC frequency can always be higher than

the free-running oscillator frequency (as set by the

RT resistor), fOSC, but should not be less than 25%

below fOSC.

CLOCK SYNCHRONIZATION OF ADDITIONAL

REGULATORS

The CLKOUT pin of the LT3579 can synchronize additional

switching regulators and/or additional LT3579s as shown

in Figure 12.

The frequency of the master LT3579 is set by the external

RT resistor. The SYNC pin of the slave LT3579 is driven

by the CLKOUT pin of the master LT3579. Note that the

RT pin of the slave LT3579 must have a resistor tied to

ground. It takes a few clock cycles for the CLKOUT signal

to begin oscillating, and it’s preferable for all LT3579s to

have the same internal free-running frequency. Therefore,

in general, use the same value RT resistor for all of the

synchronized LT3579s.

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