LTC6900CS5 datasheet(7/12 Pages) LINER

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LTC6900

6900f

APPLICATIO S I FOR ATIO

Figure 3. Current Controlled Oscillator

V+

0.1

µF

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

Figure 4. Voltage Controlled Oscillator

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 program-

mable 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 “Theory of Operation,” 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

resistor. 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 mono-

tonically increase with decreasing VCONTROL.

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 program-

mable divider extends the frequency range to greater than

three decades. Table 1 describes the recommended fre-

quencies for each divider setting. Note that the ranges

overlap; at some frequencies there are two divider/resistor

combinations 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 resis-

tance, a simple equation relates resistance with frequency.

MHz

SET

OSC

















100

•

, N =

(RSETMIN = 10k, RSETMAX = 2M)

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

oscillator, fOSC.

V+

182kHz TO 18MHz (TYPICALLY

±8%)

V+

0.1

µF

ICONTROL

µA TO 100µA

6900 F03

GND

N = 1

LTC6900

SET

OUT

DIV

10MHz

ƒOSC ≅

•• ICONTROL

ICONTROL EXPRESSED IN (A)

20k

Ω

1.1V

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