LTC3736_15 Datasheet, PDF(20/28 Page) Linear Technology – Dual 2-Phase, No RSENSE, Synchronous Controller with Output Tracking

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LTC3736_15 Datasheet, PDF (20/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE, Synchronous Controller with Output Tracking

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LTC3736

APPLICATIO S I FOR ATIO

EXTERNAL

OSCILLATOR

SYNC/

FCB

DIGITAL

PHASE/

FREQUENCY

DETECTOR

2.4V

RLP

CLP

PLLLPF

OSCILLATOR

3736 F09

Figure 9. Phase-Locked Loop Block Diagram

If the external clock frequency is greater than the internal

oscillatorâs frequency, fOSC, then current is sourced con-

tinuously from the phase detector output, pulling up the

PLLLPF pin. When the external clock frequency is less than

fOSC, current is sunk continuously, pulling down the PLLLPF

pin. If the external and internal frequencies are the same

but exhibit a phase difference, the current sources turn on

for an amount of time corresponding to the phase differ-

ence. The voltage on the PLLLPF pin is adjusted until the

phase and frequency of the internal and external oscilla-

tors are identical. At the stable operating point, the phase

detector output is high impedance and the filter capacitor

CLP holds the voltage.

The loop filter components, CLP and RLP, smooth out the

current pulses from the phase detector and provide a

stable input to the voltage-controlled oscillator. The filter

components CLP and RLP determine how fast the loop

acquires lock. Typically RLP = 10k and CLP is 2200pF to

0.01ÂµF.

Typically, the external clock (on SYNC/FCB pin) input high

level is 1.6V, while the input low level is 1.2V.

Table 2 summarizes the different states in which the

PLLLPF pin can be used.

Table 2

PLLLPF PIN

Floating

VIN

RC Loop Filter

SYNC/FCB PIN

DC Voltage

Clock Signal

FREQUENCY

300kHz

550kHz

750kHz

Phase-Locked to External Clock

Auxiliary Winding Control Using SYNC/FCB Pin

The SYNC/FCB can be used as an auxiliary feedback to

provide a means of regulating a flyback winding output.

When this pin drops below its ground-referenced 0.6V

threshold, continuous mode operation is forced.

During continuous mode, current flows continuously in

the transformer primary. The auxiliary winding draws

current only when the bottom, synchronous N-channel

MOSFET is on. When primary load currents are low and/or

the VIN/VOUT ratio is close to unity, the synchronous

MOSFET may not be on for a sufficient amount of time to

transfer power from the output capacitor to the auxiliary

load. Forced continuous operation will support an auxil-

iary winding as long as there is a sufficient synchronous

MOSFET duty factor. The FCB input pin removes the

requirement that power must be drawn from the trans-

former primary in order to extract power from the auxiliary

winding. With the loop in continuous mode, the auxiliary

output may nominally be loaded without regard to the

primary output load.

The auxiliary output voltage VAUX is normally set as shown

in Figure 10 by the turns ratio N of the transformer:

VAUX â (N + 1) VOUT

However, if the controller goes into Burst Mode operation

and halts switching due to a light primary load current,

then VAUX will droop. An external resistor divider from

VAUX to the FCB sets a minimum voltage VAUX(MIN):

VAUX(MIN)

0.6Vââ1+

R6â

R5â â

3736fa

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