Datasheet LTC6900 (Analog Devices) - 7
| 制造商 | Analog Devices |
| 描述 | Low Power, 1kHz to 20MHz Resistor Set SOT-23 Oscillator |
| 页数 / 页 | 12 / 7 — APPLICATIONS INFORMATION. SELECTING THE DIVIDER SETTING AND RESISTOR |
| 文件格式/大小 | PDF / 164 Kb |
| 文件语言 | 英语 |
APPLICATIONS INFORMATION. SELECTING THE DIVIDER SETTING AND RESISTOR
该数据表的模型线
文件文字版本
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
6900fa 7
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
6900fa 7