Datasheet LTC6900 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Low Power, 1kHz to 20MHz Resistor Set SOT-23 Oscillator |
| Pages / Page | 12 / 7 — APPLICATIONS INFORMATION. SELECTING THE DIVIDER SETTING AND RESISTOR |
| File Format / Size | PDF / 164 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. SELECTING THE DIVIDER SETTING AND RESISTOR
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LTC6900
APPLICATIONS INFORMATION SELECTING THE DIVIDER SETTING AND RESISTOR ALTERNATIVE METHODS OF SETTING THE OUTPUT FREQUENCY OF THE LTC6900
The LTC6900’s master oscillator has a frequency range spanning 0.1MHz to 20MHz. 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 programmable divider extends the frequency range to greater than three current source and in the expression for fOSC, the resistor decades. Table 1 describes the recommended frequencies RSET is replaced by the ratio of 1.1V/ICONTROL. As already for each divider setting. Note that the ranges overlap; at explained in the Operation section, the voltage difference some frequencies there are two divider/resistor combina- between V + and SET is approximately 1.1V, therefore, the tions that result in the desired frequency. 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 LTC6900 confi gured as a VCO. A voltage Lower master oscillator frequencies use less power and source is connected in series with an external 20k resis- are more accurate. For instance, fOSC = 100kHz can be tor. The output frequency, fOSC, will vary with VCONTROL, obtained by either RSET = 20k, N = 100, master oscillator = that is the voltage source connected between V+ and the 10MHz or RSET = 200k, N = 10, master oscillator = 1MHz. SET pin. Again, this circuit decouples the relationship The RSET = 200k approach is preferred for lower power between the input current and the voltage between V+ and 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* 182kHz TO 18MHz (TYPICALLY ±8%) ÷10 ⇒ DIV (Pin 4) = Floating 50kHz to 1MHz V+ 1 5 V+ OUT ÷100 ⇒ DIV (Pin 4) = V+ < 100kHz 0.1μF LTC6900 * ICONTROL 2 At master oscillator frequencies greater than 10MHz (RSET < 20kΩ), 1μA TO 100μA GND the LTC6900 may experience reduced accuracy with a supply voltage 3 4 N = 1 less than 4V. SET DIV 6900 F03 10MHz 20kΩ After choosing the proper divider setting, determine the ƒOSC • • I N CONTROL 1.1V correct frequency-setting resistor. Because of the linear ICONTROL EXPRESSED IN (A) correspondence between oscillation period and resistance,
Figure 3. Current Controlled Oscillator
a simple equation relates resistance with frequency. ⎛ 10MHz ⎞ 100 ⎧⎪ R V+ SET = 20k • 10 ⎨ ⎝⎜ N • f ⎠⎟ , N = 1 5 1 V+ OUT OSC ⎩⎪ 0.1μF LTC6900 VCONTROL 2 + (R 0V TO 1.1V GND – SETMIN = 10k, RSETMAX = 2M) RSET 20k 3 4 N = 1 Any resistor, R SET DIV SET, tolerance adds to the inaccuracy of the oscillator, f 6900 F04 OSC. 10MHz ƒ 20k VCONTROL OSC • • 1 – ( ) N RSET 1.1V TYPICAL fOSC ACCURACY ±0.5%, VCONTROL = 0V ±8%, VCONTROL = 0.5V
Figure 4. Voltage Controlled Oscillator
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APPLICATIONS INFORMATION SELECTING THE DIVIDER SETTING AND RESISTOR ALTERNATIVE METHODS OF SETTING THE OUTPUT FREQUENCY OF THE LTC6900
The LTC6900’s master oscillator has a frequency range spanning 0.1MHz to 20MHz. 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 programmable divider extends the frequency range to greater than three current source and in the expression for fOSC, the resistor decades. Table 1 describes the recommended frequencies RSET is replaced by the ratio of 1.1V/ICONTROL. As already for each divider setting. Note that the ranges overlap; at explained in the Operation section, the voltage difference some frequencies there are two divider/resistor combina- between V + and SET is approximately 1.1V, therefore, the tions that result in the desired frequency. 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 LTC6900 confi gured as a VCO. A voltage Lower master oscillator frequencies use less power and source is connected in series with an external 20k resis- are more accurate. For instance, fOSC = 100kHz can be tor. The output frequency, fOSC, will vary with VCONTROL, obtained by either RSET = 20k, N = 100, master oscillator = that is the voltage source connected between V+ and the 10MHz or RSET = 200k, N = 10, master oscillator = 1MHz. SET pin. Again, this circuit decouples the relationship The RSET = 200k approach is preferred for lower power between the input current and the voltage between V+ and 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* 182kHz TO 18MHz (TYPICALLY ±8%) ÷10 ⇒ DIV (Pin 4) = Floating 50kHz to 1MHz V+ 1 5 V+ OUT ÷100 ⇒ DIV (Pin 4) = V+ < 100kHz 0.1μF LTC6900 * ICONTROL 2 At master oscillator frequencies greater than 10MHz (RSET < 20kΩ), 1μA TO 100μA GND the LTC6900 may experience reduced accuracy with a supply voltage 3 4 N = 1 less than 4V. SET DIV 6900 F03 10MHz 20kΩ After choosing the proper divider setting, determine the ƒOSC • • I N CONTROL 1.1V correct frequency-setting resistor. Because of the linear ICONTROL EXPRESSED IN (A) correspondence between oscillation period and resistance,
Figure 3. Current Controlled Oscillator
a simple equation relates resistance with frequency. ⎛ 10MHz ⎞ 100 ⎧⎪ R V+ SET = 20k • 10 ⎨ ⎝⎜ N • f ⎠⎟ , N = 1 5 1 V+ OUT OSC ⎩⎪ 0.1μF LTC6900 VCONTROL 2 + (R 0V TO 1.1V GND – SETMIN = 10k, RSETMAX = 2M) RSET 20k 3 4 N = 1 Any resistor, R SET DIV SET, tolerance adds to the inaccuracy of the oscillator, f 6900 F04 OSC. 10MHz ƒ 20k VCONTROL OSC • • 1 – ( ) N RSET 1.1V TYPICAL fOSC ACCURACY ±0.5%, VCONTROL = 0V ±8%, VCONTROL = 0.5V
Figure 4. Voltage Controlled Oscillator
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