Datasheet LTC1799 (Analog Devices) - 7

LTC1799
applicaTions inForMaTion SELECTING THE DIVIDER SETTING AND RESISTOR ALTERNATIVE METHODS OF SETTING THE OUTPUT FREqUENCY OF THE LTC1799
The LTC1799’s master oscillator has a frequency range spanning 0.1MHz to 33MHz. However, accuracy may suffer The oscillator may be programmed by any method that if the master oscillator is operated at greater than 10MHz sources a current into the SET pin (Pin 3). The circuit in with a supply voltage lower than 4V. A programmable Figure 3 sets the oscillator frequency using a program- divider extends the frequency range to greater than three mable current source and in the expression for fOSC, the decades. Table 1 describes the recommended frequencies resistor RSET is replaced by the ratio of 1.13V/ICONTROL. for each divider setting. Note that the ranges overlap; at As already explained in the “Theory of Operation,” the some frequencies there are two divider/resistor combina- voltage difference between V+ and SET is approximately tions that result in the desired frequency. 1.13V, therefore, the Figure 3 circuit is less accurate than if a resistor controls the oscillator frequency. In general, any given oscillator frequency (fOSC) should be obtained using the lowest master oscillator frequency. Figure 4 shows the LTC1799 configured as a VCO. A voltage Lower master oscillator frequencies use less power and source is connected in series with an external 10k resis- are more accurate. For instance, fOSC = 100kHz can be tor. The output frequency, fOSC, will vary with VCONTROL, obtained by either RSET = 10k, N = 100, master oscilla- that is the voltage source connected between V+ and the tor = 10MHz or RSET = 100k, N = 10, master oscillator = SET pin. Again, this circuit decouples the relationship 1MHz. The RSET = 100k is preferred for lower power and between the input current and the voltage between V+ better accuracy. and SET; the frequency accuracy will be degraded. The oscillator frequency, however, will monotonically increase
Table 1. Frequency Range vs Divider Setting
with decreasing VCONTROL.
DIVIDER SETTING FREqUENCY RANGE
÷1 ⇒ DIV (Pin 4) = GND >500kHz* 400kHz TO 21MHz ÷10 (APPROXIMATE, SEE TEXT) ⇒ DIV (Pin 4) = Floating 50kHz to 1MHz V+ ÷100 1 5 ⇒ DIV (Pin 4) = V+ <100kHz V+ OUT *At master oscillator frequencies greater than 10MHz (R 0.1µF LTC1799 SET < 10kΩ), the ICONTROL 2 LTC1799 may suffer reduced accuracy with a supply voltage less than 4V. 5µA TO 200µA GND 3 4 N = 1 SET DIV After choosing the proper divider setting, determine the 1799 F03 correct frequency-setting resistor. Because of the linear 10MHz ƒ 10kΩ correspondence between oscillation period and resistance, OSC ≅ N • • ICONTROL 1.13V I a simple equation relates resistance with frequency. CONTROL EXPRESSED IN (A)
Figure 3. Current Controlled Oscillator
⎛ ⎞ 100 ⎧⎪ RSET = 10k • 10MHz ⎜ ⎟, N = 10 ⎨ ⎝N • f V+ OSC ⎠ 1 ⎩⎪ 1 5 V+ OUT (RSETMIN = 3k (5V Supply), 5k (3V Supply), 0.1µF LTC1799 VCONTROL + 2 R 0V TO 1.13V – GND SETMAX = 1M) RSET 10k 3 4 N = 1 Any resistor, R SET DIV SET, tolerance adds to the inaccuracy of the oscillator, f 1799 F04 OSC. 10MHz ƒ 10k V OSC ≅ CONTROL ( ) N • • 1 – RSET 1.13V
Figure 4. Voltage Controlled Oscillator
1799fd For more information www.linear.com/LTC1799 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Theory of Operation Applications Information Typical Application Package Description Revision History Typical Applications