LTC6900 datasheet(7/12 Pages) LINER

7 / 12 page

LTC6900

6900fa

APPLICATIONS INFORMATION

SELECTING THE DIVIDER SETTING AND RESISTOR

The LTC6900’s master oscillator has a frequency range

spanning 0.1MHz to 20MHz. However, accuracy may suffer

if the master oscillator is operated at greater than 10MHz

with a supply voltage lower than 4V. A programmable

divider extends the frequency range to greater than three

decades. Table 1 describes the recommended frequencies

for each divider setting. Note that the ranges overlap; at

some frequencies there are two divider/resistor combina-

tions that result in the desired frequency.

In general, any given oscillator frequency (fOSC) should

be obtained using the lowest master oscillator frequency.

Lower master oscillator frequencies use less power and

are more accurate. For instance, fOSC = 100kHz can be

obtained by either RSET = 20k, N = 100, master oscillator =

10MHz or RSET = 200k, N = 10, master oscillator = 1MHz.

The RSET = 200k approach is preferred for lower power

and better accuracy.

Table 1. Frequency Range vs Divider Setting

DIVIDER SETTING

FREQUENCY RANGE

÷1

⇒ DIV (Pin 4) = GND

> 500kHz*

÷10

⇒ DIV (Pin 4) = Floating

50kHz to 1MHz

÷100

⇒ DIV (Pin 4) = V+

< 100kHz

*At master oscillator frequencies greater than 10MHz (RSET < 20kΩ),

the LTC6900 may experience reduced accuracy with a supply voltage

less than 4V.

After choosing the proper divider setting, determine the

correct frequency-setting resistor. Because of the linear

correspondence between oscillation period and resistance,

a simple equation relates resistance with frequency.

RSET = 20k •

10MHz

N• fOSC

⎛

⎝⎜

⎞

⎠⎟

, N =

100

⎧

⎨

⎪

⎩⎪

(RSETMIN = 10k, RSETMAX = 2M)

Any resistor, RSET, tolerance adds to the inaccuracy of the

oscillator, fOSC.

ALTERNATIVE METHODS OF SETTING THE OUTPUT

FREQUENCY OF THE LTC6900

The oscillator may be programmed by any method that

sources a current into the SET pin (Pin 3). The circuit in

Figure 3 sets the oscillator frequency using a programmable

current source and in the expression for fOSC, the resistor

RSET is replaced by the ratio of 1.1V/ICONTROL. As already

explained in the Operation section, the voltage difference

between V + and SET is approximately 1.1V, therefore, the

Figure 3 circuit is less accurate than if a resistor controls

the oscillator frequency.

Figure 4 shows the LTC6900 conﬁgured as a VCO. A voltage

source is connected in series with an external 20k resis-

tor. The output frequency, fOSC, will vary with VCONTROL,

that is the voltage source connected between V+ and the

SET pin. Again, this circuit decouples the relationship

between the input current and the voltage between V+

and SET; the frequency accuracy will be degraded. The

oscillator frequency, however, will monotonically increase

with decreasing VCONTROL.

Figure 3. Current Controlled Oscillator

Figure 4. Voltage Controlled Oscillator

V+

182kHz TO 18MHz (TYPICALLY ±8%)

V+

0.1μF

ICONTROL

1μA TO 100μA

6900 F03

GND

N = 1

LTC6900

SET

OUT

DIV

10MHz

ƒOSC

•• ICONTROL

ICONTROL EXPRESSED IN (A)

20kΩ

1.1V

V+

0.1μF

RSET

20k

VCONTROL

0V TO 1.1V

6900 F04

GND

N = 1

LTC6900

SET

OUT

DIV

–

10MHz

ƒOSC

•

• 1 –

VCONTROL

1.1V

20k

RSET

()

TYPICAL fOSC ACCURACY

±0.5%, VCONTROL = 0V

±8%, VCONTROL = 0.5V

LTC6900 Datasheet(PDF) 7 Page - Linear Technology

LTC6900 Datasheet(HTML) 7 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12

Datasheet Download

Link URL