LTC3728 datasheet(21/32 Pages) LINER

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LTC3728

3728fb

The phase detector used is an edge sensitive digital type

which provides zero degrees phase shift between the

external and internal oscillators. This type of phase detec-

tor will not lock up on input frequencies close to the

harmonics of the VCO center frequency. The PLL hold-in

range,

∆fH, is equal to the capture range, ∆fC:

∆fH = ∆fC = ±0.5 fO (250kHz-550kHz)

The output of the phase detector is a complementary pair

of current sources charging or discharging the external

filter network on the PLLFLTR pin.

If the external frequency (fPLLIN) is greater than the oscil-

lator frequency f0SC, current is sourced continuously,

pulling up the PLLFLTR pin. When the external frequency

is less than f0SC, current is sunk continuously, pulling

down the PLLFLTR pin. If the external and internal fre-

quencies are the same but exhibit a phase difference, the

current sources turn on for an amount of time correspond-

ing to the phase difference. Thus the voltage on the

PLLFLTR pin is adjusted until the phase and frequency of

the external and internal oscillators are identical. At this

stable operating point the phase comparator output is

open and the filter capacitor CLP holds the voltage. The

LTC3728 PLLIN pin must be driven from a low impedance

source such as a logic gate located close to the pin. When

using multiple LTC3728’s (or LTC3729’s, as shown in

Figure 14) for a phase-locked system, the PLLFLTR pin of

the master oscillator should be biased at a voltage that will

guarantee the slave oscillator(s) ability to lock onto the

master’s frequency. A DC voltage of 0.7V to 1.7V applied

to the master oscillator’s PLLFLTR pin is recommended in

order to meet this requirement. The resultant operating

frequency can range from 300kHz to 470kHz.

The loop filter components (CLP, 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 =10kΩ and CLP is 0.01µF to

0.1

µF.

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the LTC3728 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the top MOSFET. Low duty cycle

applications may approach this minimum on-time limit

and care should be taken to ensure that

ON MIN

OUT

()

If the duty cycle falls below what can be accommodated by

the minimum on-time, the LTC3728 will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTC3728 is approximately

100ns. However, as the peak sense voltage decreases the

minimum on-time gradually increases up to about 150ns.

This is of particular concern in forced continuous applica-

tions with low ripple current at light loads. If the duty cycle

drops below the minimum on-time limit in this situation,

a significant amount of cycle skipping can occur with

correspondingly larger current and voltage ripple.

FCB Pin Operation

The FCB pin can be used to regulate a secondary winding

or as a logic level input. Continuous operation is forced on

both controllers when the FCB pin drops below 0.8V.

During continuous mode, current flows continuously in

the transformer primary. The secondary winding(s) draw

current only when the bottom, synchronous switch is on.

When primary load currents are low and/or the VIN/VOUT

ratio is low, the synchronous switch may not be on for a

sufficient amount of time to transfer power from the

output capacitor to the secondary load. Forced continuous

operation will support secondary windings providing there

is sufficient synchronous switch duty factor. Thus, the

FCB input pin removes the requirement that power must

be drawn from the inductor primary in order to extract

APPLICATIO S I FOR ATIO

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