Datasheet LTC6900 (Analog Devices) - 9
| 制造商 | Analog Devices |
| 描述 | Low Power, 1kHz to 20MHz Resistor Set SOT-23 Oscillator |
| 页数 / 页 | 12 / 9 — APPLICATIONS INFORMATION. Figure 7. A Ground Referenced Voltage … |
| 文件格式/大小 | PDF / 164 Kb |
| 文件语言 | 英语 |
APPLICATIONS INFORMATION. Figure 7. A Ground Referenced Voltage Controlled Oscillator
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LTC6900
APPLICATIONS INFORMATION
When V 3V OR 5V IN = V+, the output frequency of the LTC6900 as- 1 5 20k V+ OUT fOSC = 10MHz • (N • R ) SET1 sumes the highest value and it is set by the parallel com- S1 LTC6900 OR 2 bination of R 20k IN and RSET. Also note, the output frequency, RSET1 V+ GND fOSC = 10MHz • (N • RSET1//RSET2) ÷100 f R OSC, is independent of the value of VRES = (V+ – VSET) so SET2 3 4 SET DIV ÷10 the accuracy of fOSC is within the data sheet limits. 6900 F07 ÷1 When VIN is less than V+, and expecially when VIN ap- proaches the ground potential, the oscillator frequency,
Figure 7
fOSC, assumes its lowest value and its accuracy is affected by the change of V
A Ground Referenced Voltage Controlled Oscillator
RES = (V+ – VSET). At 25°C VRES varies by ±8%, assuming the variation of V+ is ±5%. The tem- The LTC6900 output frequency can also be programmed by perature coeffi cient of VRES is 0.02%/°C. steering current in or out of the SET pin, as conceptually By manipulating the algebraic relation for f shown in Figure 8. This technique can degrade accuracy OSC above, a simple algorithm can be derived to set the values of external as the ratio of (V+ – VSET) / IRES is no longer uniquely resistors R dependent of the value of R SET and RIN, as shown in Figure 9. SET, as shown in the LTC6900 Block Diagram. This loss of accuracy will become noticeable 1. Choose the desired value of the maximum oscillator when the magnitude of I frequency, f PROG is comparable to IRES. The OSC(MAX), occurring at maximum input frequency variation of the LTC6900 is still monotonic. voltage VIN(MAX) ≤ V+. Figure 9 shows how to implement the concept shown in 2. Set the desired value of the minimum oscillator fre- Figure 8 by connecting a second resistor, R quency, f IN, between the OSC(MIN), occurring at minimum input voltage SET pin and a ground referenced voltage source, V V IN. IN(MIN) ≥ 0. For a given power supply voltage in Figure 9, the output 3. Choose VRES = 1.1 and calculate the ratio of RIN/RSET frequency of the LTC6900 is a function of V from the following: IN, RIN, RSET and (V+ – VSET) = VRES: RIN = 20k f RSET OSC = 10MHz • • N RIN RSET ⎛ f ⎞ OSC(MAX) ⎡ ⎛ ⎞ ⎤ VIN(MAX) − V+ ( )−⎜ f ⎟ VIN(MIN) − V+ ( ) ⎢ ⎝ ⎠ V ⎥ OSC(MIN) IN − V+ ( ) ⎜ 1 ⎟ − 1 ⎢1+ • ⎜ ⎟ ⎥ ⎡ f( ) ⎤ ⎢ V OSC(MAX) RES ⎜ 1+ RIN ⎟ ⎥ V ⎢ − 1⎥ ⎢ RES ⎝⎜ R ⎠⎟ ⎥ ⎢ fOSC(MIN) ⎥ SET ⎣ ⎦ (1) ⎣ ⎦ (2) 1 5 1 5 V+ V+ OUT V+ V+ OUT fOSC 0.1μF LTC6900 + 0.1μF LTC6900 2 2 RSET 5V GND V R RES SET 5V GND ÷100 ÷100 – 3 R 4 IN 3 ÷10 4 ÷10 SET DIV OPEN SET DIV OPEN + I 6900 F08 RES ÷1 6900 F09 ÷1 IPR VIN –
Figure 8. Concept for Programming via Current Steering Figure 9. Implementation of Concept Shown in Figure 8
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APPLICATIONS INFORMATION
When V 3V OR 5V IN = V+, the output frequency of the LTC6900 as- 1 5 20k V+ OUT fOSC = 10MHz • (N • R ) SET1 sumes the highest value and it is set by the parallel com- S1 LTC6900 OR 2 bination of R 20k IN and RSET. Also note, the output frequency, RSET1 V+ GND fOSC = 10MHz • (N • RSET1//RSET2) ÷100 f R OSC, is independent of the value of VRES = (V+ – VSET) so SET2 3 4 SET DIV ÷10 the accuracy of fOSC is within the data sheet limits. 6900 F07 ÷1 When VIN is less than V+, and expecially when VIN ap- proaches the ground potential, the oscillator frequency,
Figure 7
fOSC, assumes its lowest value and its accuracy is affected by the change of V
A Ground Referenced Voltage Controlled Oscillator
RES = (V+ – VSET). At 25°C VRES varies by ±8%, assuming the variation of V+ is ±5%. The tem- The LTC6900 output frequency can also be programmed by perature coeffi cient of VRES is 0.02%/°C. steering current in or out of the SET pin, as conceptually By manipulating the algebraic relation for f shown in Figure 8. This technique can degrade accuracy OSC above, a simple algorithm can be derived to set the values of external as the ratio of (V+ – VSET) / IRES is no longer uniquely resistors R dependent of the value of R SET and RIN, as shown in Figure 9. SET, as shown in the LTC6900 Block Diagram. This loss of accuracy will become noticeable 1. Choose the desired value of the maximum oscillator when the magnitude of I frequency, f PROG is comparable to IRES. The OSC(MAX), occurring at maximum input frequency variation of the LTC6900 is still monotonic. voltage VIN(MAX) ≤ V+. Figure 9 shows how to implement the concept shown in 2. Set the desired value of the minimum oscillator fre- Figure 8 by connecting a second resistor, R quency, f IN, between the OSC(MIN), occurring at minimum input voltage SET pin and a ground referenced voltage source, V V IN. IN(MIN) ≥ 0. For a given power supply voltage in Figure 9, the output 3. Choose VRES = 1.1 and calculate the ratio of RIN/RSET frequency of the LTC6900 is a function of V from the following: IN, RIN, RSET and (V+ – VSET) = VRES: RIN = 20k f RSET OSC = 10MHz • • N RIN RSET ⎛ f ⎞ OSC(MAX) ⎡ ⎛ ⎞ ⎤ VIN(MAX) − V+ ( )−⎜ f ⎟ VIN(MIN) − V+ ( ) ⎢ ⎝ ⎠ V ⎥ OSC(MIN) IN − V+ ( ) ⎜ 1 ⎟ − 1 ⎢1+ • ⎜ ⎟ ⎥ ⎡ f( ) ⎤ ⎢ V OSC(MAX) RES ⎜ 1+ RIN ⎟ ⎥ V ⎢ − 1⎥ ⎢ RES ⎝⎜ R ⎠⎟ ⎥ ⎢ fOSC(MIN) ⎥ SET ⎣ ⎦ (1) ⎣ ⎦ (2) 1 5 1 5 V+ V+ OUT V+ V+ OUT fOSC 0.1μF LTC6900 + 0.1μF LTC6900 2 2 RSET 5V GND V R RES SET 5V GND ÷100 ÷100 – 3 R 4 IN 3 ÷10 4 ÷10 SET DIV OPEN SET DIV OPEN + I 6900 F08 RES ÷1 6900 F09 ÷1 IPR VIN –
Figure 8. Concept for Programming via Current Steering Figure 9. Implementation of Concept Shown in Figure 8
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