LT3781 datasheet(13/20 Pages) LINER

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LT3781

3781f

Oscillator Synchronization

Synchronization of the LT3781 system clock is accom-

plished by driving a TTL level logic pulse train at the

desired system switching frequency into the SYNC pin. In

order to assure proper synchronization, each phase of the

synchronization signal must be less than an oscillator

free-run cycle.

The SYNC input pulse controls the phasing as well as the

frequency of controller switching. The SYNC circuit func-

tions by forcing the phase of the oscillator output flip-flop

to match the phase of the SYNC pulse and prematurely

ending the oscillator charge cycle on each transition edge.

At the SYNC logic low-to-high transition, the LT3781

starts a switch-on cycle and the minimum switch-off

period is forced during the SYNC logic low period. Be-

cause the SYNC logic low period corresponds directly to

the minimum off time, the converter maximum duty cycle

can be forced using the SYNC input. For example, a 30%

duty cycle SYNC pulse forces 30% maximum duty cycle

operation for the converter. Because the logic-low pulse

width exceeds the logic-high pulse width in < 50% duty

cycle operation, the oscillator free-run cycle time must be

programmed to exceed the logic-low duration.

Figure 4. Oscillator/SYNC Waveforms

It is also possible to run the LT3781 in a SYNC-only mode

by disabling the oscillator completely. Connecting a resis-

tor divider from the 5VREF pin to the FSET pin forcing a

voltage within the charge range of 1.5V-2.5V will allow the

oscillator to follow the SYNC input exclusively with no

provision for free-run. Setting values to force a voltage as

close to 2V as possible is recommended.

1.5V

FSET

SYNC

3781 F04

SYSTEM

CLOCK

(INTERNAL)

2.5V

APPLICATIO S I FOR ATIO

5VREF

FSET

75k

51k

100pF

3781 F05

LT3781

Figure 5. Oscillator Connection for SYNC-Only Mode Operation

Bootstrap Start

It is inefficient as well as impractical to power a controller

IC from a high-voltage input supply. Using a linear

preregulation scheme to provide the required VCC voltage

for the LT3781 would waste significant power, reducing

converter efficiencies and creating additional thermal con-

cerns. Self-biased power schemes take advantage of

inherent converter efficiencies to significantly reduce losses

associated with powering the controller. Bootstrapped

power can be derived using auxiliary windings on the

power transformer or inductor, rectified taps on switching

nodes, or the converter output directly.

Start-up circuitry built into the LT3781 allows VCC to

increase from 0V to 14.5V before the converter is enabled.

During this time, start-up current is less than 1mA. The

trickle current required for charging the VCC supply is

typically generated with a resistor from the converter high

voltage input. When combined with the VCC bypass ca-

pacitor, the current through the start-up resistor creates a

voltage ramp on VCC whose slope governs the turn-on

time of the converter. The low quiescent current of the

LT3781 allows the input voltage to be trickled up with

minimal power dissipation in the start-up resistor. At

VCC = 14.5V, the LT3781 internal circuitry is enabled and

switching begins. If enough bootstrap power is fed back

into VCC to keep that supply voltage above 8.4V, then

switching continues and a bootstrap start is accom-

plished. If the input voltage drops below 8.4V, the LT3781

is disabled and the switching regulator returns to the

start-up low current state.

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