LT1310 datasheet(7/12 Pages) LINER

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LT1310

1310f

CT Selection for Operating Frequency

To synchronize to an external input signal, the timing

capacitor and PLL filter components must be chosen

properly. This is a simple process and can be done using

the graph in Figure 2a.

In Figure 2a, operating frequency is plotted versus timing

capacitor (CT) with the upper and lower lines correspond-

ing to the minimum and maximum lock frequency given a

specific CT value. To choose the right timing capacitor,

find the intersection of the desired operating frequency

and the dashed line. Then move to the corresponding CT

value.

Alternately, use the following equations as a starting point:

for fLOCK ≥ 2MHz:

LOCK









075

250 10

40 10

•

–•

–

for fLOCK ≤ 2MHz:

LOCK









075

310 10

60 10

•

–•

–

Figure 2a. CT vs Operating Frequency

Because the lock range for the PLL is nearly 2:1, the

nearest standard value NP0 capacitor can be used. For

the application shown in Figure 1, a 1.6MHz switching

frequency corresponds to an 100pF timing capacitor.

Since the switching frequency affects inductor ripple

current, the inductor must also be scaled. Table 1 shows

recommended component values for various switching

frequencies.

Table 1. Recommended Component Values for Various Switching

Frequencies (RLP = 3.01k)

SWITCHING

FREQUENCY

CLP

600kHz

330pF

1500pF

2700pF

10k

µH

1MHz

180pF

1000pF

2200pF

10k

6.2

µH

1.6MHz

100pF

820pF

1500pF

15k

5.6

µH

2MHz

68pF

820pF

1500pF

15k

4.7

µH

2.5MHz

47pF

330pF

1500pF

20k

3.3

µH

3MHz

33pF

330pF

1000pF

20k

2.7

µH

FREQUENCY (Hz)

10M

1310 F02a

10k

100k

100

10k

MINIMUM

LOCK

FREQUECY

MAXIMUM

LOCK

FREQUECY

OPERATIO

VIN

SHDN

SHUTDOWN

SYNC IN

SYNC

PLL-LPF

RLP

VIN

178k

20.5k

1310 F02a

4.7

µF

CERAMIC

4.7

µF

CERAMIC

VOUT

12V

CLP

LT1310

GND

Figure 2b. Circuit Used for CT Selection

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