LTC6900 查看數據表（PDF） - Linear Technology

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LTC6900

Low Power, 1kHz to 20MHz Resistor Set SOT-23 Oscillator

Linear Technology 

LTC6900

APPLICATIONS INFORMATION

Once RIN/RSET is known, calculate RSET from:

RSET

=

10MHz

N

•

20k

fOSC(MAX )

•

( ) ⎡

⎢

⎢

VIN(MAX) − V+

+

VRES

⎛

⎝⎜

1+

RIN

RSET

⎞

⎠⎟

⎤

⎥

⎥

⎢

⎢

⎣⎢

VRES

⎛

⎝⎜

RIN

RSET

⎞

⎠⎟

⎥

⎥

⎦⎥

(3)

Example 1:

In this example, the oscillator output frequency has small

excursions. This is useful where the frequency of a system

should be tuned around some nominal value.

Let V+ = 3V, fOSC(MAX) = 2MHz for VIN(MAX) = 3V and

fOSC(MIN) = 1.5MHz for VIN = 0V. Solve for RIN/RSET by

Equation (2), yielding RIN/RSET = 9.9/1. RSET = 110.1k by

Equation (4). RIN = 9.9RSET = 1.089M. For standard resis-

tor values, use RSET = 110k (1%) and RIN = 1.1M (1%).

Figure 10 shows the measured fOSC vs VIN. The 1.5MHz

to 2MHz frequency excursion is quite limited, so the curve

of fOSC vs VIN is linear.

Example 2:

Vary the oscillator frequency by one octave per volt. As-

sume fOSC(MIN) = 1MHz and fOSC(MAX) = 2MHz, when the

input voltage varies by 1V. The minimum input voltage is

half supply, that is VIN(MIN) = 1.5V, VIN(MAX) = 2.5V and

V+ = 3V.

Equation (2) yields RIN/RSET = 1.273 and Equation (3) yields

RSET = 142.8k. RIN = 1.273RSET = 181.8k. For standard

resistor values, use RSET = 143k (1%) and RIN = 182k

(1%). Figure 11 shows the measured fOSC vs VIN. For VIN

higher than 1.5V, the VCO is quite linear; nonlinearities

occur when VIN becomes smaller than 1V, although the

VCO remains monotonic.

Maximum VCO Modulation Bandwidth

The maximum VCO modulation bandwidth is 25kHz; that

is, the LTC6900 will respond to changes in VIN at a rate

up to 25kHz. In lower frequency applications however, the

modulation frequency may need to be limited to a lower

rate to prevent an increase in output jitter. This lower limit

is the master oscillator frequency divided by 20, (fOSC/20).

In general, for minimum output jitter the modulation fre-

quency should be limited to fOSC/20 or 25kHz, whichever

is less. For best performance at all frequencies, the value

for fOSC should be the master oscillator frequency (N = 1)

when VIN is at the lowest level.

2.00

RIN = 1.1M

1.95

1.90

RV+SE=T3=V110k

N=1

1.85

1.80

1.75

1.70

1.65

1.60

1.55

1.50

0 0.5 1 1.5 2

VIN (V)

2.5 3

6900 F10

Figure 10. Output Frequency vs Input Voltage

3000

2500

2000

RIN = 182k

RV+SE=T3=V143k

N=1

1500

1000

500

0

0 0.5 1 1.5 2 2.5 3

VIN (V)

6900 F11

Figure 11. Output Frequency vs Input Voltage

6900fa

10

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