LTC7813 datasheet(27/38 Pages) LINER

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LTC3895

3895fa

For more information www.linear.com/LTC3895

applicaTions inForMaTion

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 difference.

The voltage at the VCO input is adjusted until the phase

and frequency of the internal and external oscillators are

identical. At the stable operating point, the phase detector

output is high impedance and the internal filter capacitor,

CLP, holds the voltage at the VCO input.

Note that the LTC3895 can only be synchronized to an

external clock whose frequency is within range of the

LTC3895’sinternalVCO,whichisnominally55kHzto1MHz.

This is guaranteed to be between 75kHz and 850kHz. The

LTC3895 is guaranteed to synchronize to an external clock

that swings up to at least 2.8V and down to 0.5V or less.

Rapid phase-locking can be achieved by using the FREQ

pin to set a free-running frequency near the desired

synchronization frequency. The VCO’s input voltage is

prebiased at a frequency corresponding to the frequency

set by the FREQ pin. Once prebiased, the PLL only needs

to adjust the frequency slightly to achieve phase lock and

synchronization. Although it is not required that the free-

running frequency be near the external clock frequency,

doingsowillpreventtheoperatingfrequencyfrompassing

through a large range of frequencies as the PLL locks.

Table 2 summarizes the different states in which the FREQ

pin can be used. When synchronized to an external clock,

the LTC3895 operates in forced continuous mode at light

loads if the MODE pin is set to Burst Mode operation or

forced continuous operation. If the MODE pin is set to

pulse-skipping operation, the LTC3895 maintains pulse-

skipping operation when synchronized.

Table 2.

FREQ PIN

PLLIN PIN

FREQUENCY

DC Voltage

350kHz

INTVCC

DC Voltage

535kHz

Resistor to GND

DC Voltage

50kHz to 900kHz

Any of the Above

External Clock

75kHz to 850kHz

Phase Locked to

External Clock

Minimum On-Time Considerations

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

that the LTC3895 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:

tON(MIN) <

VOUT

VIN(f)

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller 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 LTC3895 is approximately

80ns. However, as the peak sense voltage decreases

the minimum on-time gradually increases up to about

130ns. This is of particular concern in forced continuous

applications 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.

FREQ PIN RESISTOR (k)

600

800

1000

35 45 55

3895 F12

400

200

500

700

900

300

100

65 75 85 95 105 115 125

Figure 12. Relationship Between Oscillator Frequency

and Resistor Value at the FREQ Pin

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