LTC6906CS6#TRPBF

LTC6906 

## Micropower, 10kHz to 1MHz Resistor Set Oscillator in SOT-23 

## **FEATURES** 

- n **Supply Current: 12µA at 100kHz** 

- n **<0.65% Frequency Accuracy (from 0°C to 70°C)** n **Frequency Range: 10kHz to 1MHz** 

- n **One Resistor Sets the Oscillator Frequency** 

- n **Single Supply: 2.25V to 5.5V** 

- n –40°C to 125°C Operating Temperature Range 

- n No Decoupling Capacitor Needed 

- n Start-Up Time Under 200µs at 1MHz 

- n First Cycle After Power-Up is Accurate 

- n 150Ω CMOS Output Driver 

- n Low Profile (1mm) SOT-23 (ThinSOT™) Package 

## **APPLICATIONS** 

- n Low Cost Precision Programmable Oscillator 

- n Rugged, Compact Micropower Replacement for Crystal and Ceramic Oscillators 

- n High Shock and Vibration Environments 

- n Portable and Battery-Powered Equipment 

- n PDAs and Cellular Phones 

## **DESCRIPTION** 

The LTC[®] 6906 is a precision programmable oscillator that is versatile, compact and easy to use. Micropower operation benefits portable and battery-powered equipment. At 100kHz, the LTC6906 consumes 12µA on a 3.3V supply. 

A single resistor programs the oscillator frequency over a 10:1 range with better than 0.5% initial accuracy. The output frequency can be divided by 1, 3 or 10 to span a 100:1 total frequency range, 10kHz to 1MHz. 

The LTC6906 is easily programmed according to this simple formula: 

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No decoupling capacitor is needed in most cases, yielding an extremely compact solution occupying less than 20mm[2] . Contact LTC Marketing for a version of the part with a shutdown feature or lower frequency operation. 

All registered trademarks and trademarks are the property of their respective owners. 

The LTC6906 is available in the 6-lead SOT-23 (ThinSOT) package. 

## **TYPICAL APPLICATION** 

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Typical Supply Current vs Frequency<br>**----- End of picture text -----**<br>


## **Micropower Clock Generator** 

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NO DECOUPLING<br>CAPACITOR<br>NEEDED<br>LTC6906 10kHz TO 1MHz<br>2.25V TO 3.6V V [+] OUT<br>GND GRD<br>10<br>3 DIV SET<br>1 RSET<br>100k TO 1M<br>6906 TA01<br>**----- End of picture text -----**<br>


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90<br>CL = 5pF<br>80 TA = 25°C<br>70<br>V [+]  = 3.6V<br>60<br>50<br>V [+]  = 2.25V<br>40<br>30<br>20<br>10<br>0<br>0 200 400 600 800 1000 1200<br>FREQUENCY (kHz)<br>6906 TA01b<br>POWER SUPPLY CURRENT (µA)<br>**----- End of picture text -----**<br>


Rev. D 

1 

For more information www.analog.com 

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## LTC6906 

## **ABSOLUTE MAXIMUM RATINGS** 

## **PIN CONFIGURATION** 

## **(Note 1)** 

V[+] ................................................................ –0.3V to 6V DIV to GND .....................................–0.3V to (V[+] + 0.3V) SET to GND .....................................–0.3V to (V[+] + 0.3V) GRD to GND ....................................–0.3V to (V[+] + 0.3V) Operating Temperature Range (Note 7) LTC6906C ............................................–40°C to 85°C LTC6906I .............................................–40°C to 85°C LTC6906H .......................................... –40°C to 125°C Specified Temperature Range (Note 7) LTC6906C ................................................ 0°C to 70°C LTC6906I .............................................–40°C to 85°C LTC6906H .......................................... –40°C to 125°C Storage Temperature Range ..................–65°C to 150°C Lead Temperature (Soldering, 10 sec) ...................300°C 

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TOP VIEW<br>OUT 1 6 V [+]<br>GND 2 5 GRD<br>DIV 3 4 SET<br>S6 PACKAGE<br>6-LEAD PLASTIC TSOT-23<br>TJMAX = 150°C, θJA = 230°C/W<br>**----- End of picture text -----**<br>


## **ORDER INFORMATION** 

|**LEAD FREE FINISH**|**TAPE AND REEL**|**PART MARKING***|**PACKAGE DESCRIPTION**|**TEMPERATURE RANGE**|
|---|---|---|---|---|
|LTC6906CS6#PBF|LTC6906CS6#TRPBF|LTBJN|6-Lead Plastic TSOT-23|0°C to 70°C|
|LTC6906IS6#PBF|LTC6906IS6#TRPBF|LTBJN|6-Lead Plastic TSOT-23|–40°C to 85°C|
|LTC6906HS6#PBF|LTC6906HS6#TRPBF|LTBJN|6-Lead Plastic TSOT-23|–40°C to 125°C|
|**LEAD BASED FINISH**|**TAPE AND REEL**|**PART MARKING***|**PACKAGE DESCRIPTION**|**TEMPERATURE RANGE**|
|LTC6906CS6|LTC6906CS6#TR|LTBJN|6-Lead Plastic TSOT-23|0°C to 70°C|
|LTC6906IS6|LTC6906IS6#TR|LTBJN|6-Lead Plastic TSOT-23|–40°C to 85°C|
|LTC6906HS6|LTC6906HS6#TR|LTBJN|6-Lead Plastic TSOT-23|–40°C to 125°C|



Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. 

**ELECTRICAL CHARACTERISTICS The** l **denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.** 

|**SYMBOL**|**PARAMETER**|**CONDITIONS**|**CONDITIONS**|**MIN**<br>**TYP**<br>**MAX**<br>**UNITS**|**MIN**<br>**TYP**<br>**MAX**<br>**UNITS**|
|---|---|---|---|---|---|
|∆f|Frequency Accuracy (Notes 2, 3, 9)|V+= 2.7V to 3.6V<br>100kHz ≤ f ≤ 1MHz<br>100kHz ≤ f ≤ 1MHz, LTC6906C<br>100kHz ≤ f ≤ 1MHz, LTC6906I<br>f = 1MHz, LTC6906H<br>f = 100kHz, LTC6906H|l<br>l<br>l<br>l|±0.25<br>±0.5<br>±0.65<br>±1.3<br>±1.3<br>±2.2|%<br>%<br>%<br>%<br>%|
|||V+= 2.25V<br>100kHz ≤ f ≤ 1MHz<br>100kHz ≤ f ≤ 1MHz, LTC6906C<br>100kHz ≤ f ≤ 1MHz, LTC6906I<br>f = 1MHz, LTC6906H<br>f = 100kHz, LTC6906H|l<br>l<br>l<br>l|±0.25<br>±0.7<br>±0.85<br>±1.3<br>±1.3<br>±2.2|%<br>%<br>%<br>%<br>%|



Rev. D 

2 

For more information www.analog.com 

LTC6906 

## **ELECTRICAL CHARACTERISTICS The** l **denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.** 

|**SYMBOL**|**PARAMETER**|**CONDITIONS**|**CONDITIONS**|**MIN**<br>**TYP**<br>**MAX**<br>**UNITS**|**MIN**<br>**TYP**<br>**MAX**<br>**UNITS**|<br> <br><br><br><br><br> <br><br> <br><br> <br> <br> <br><br> <br> <br> <br> <br> <br><br> <br><br>|
|---|---|---|---|---|---|---|
|RSET|Frequency-Setting Resistor Range||l|100<br>1000|kΩ||
|∆f/∆T|Frequency Drift Over Temp(Note 3)|RSET= 316k|l|±0.005|%/°C||
|∆f/∆V|Frequency Drift Over Supply(Note 3)|V+= 2.25V to 3.6V, 100k ≤ RSET≤ 1000k||0.06|%/V||
||Timing Jitter (Note 4)|Pin 3 = V+, 100k ≤ RSET≤ 1000k<br>Pin 3 = Open, 100k ≤ RSET≤ 1000k<br>Pin 3 = 0V, 100k ≤ RSET≤ 1000k||0.03<br>0.07<br>0.15|%<br>%<br>%||
|Sf|Long-Term Stability of Output Frequency|Pin 3 = V+||300|ppm/√kHr||
|DC|Duty Cycle||l|45<br>50<br>55|%||
|V+|Operating Supply Range(Note 8)||l|2.25<br>3.6|V||
|IS|Power Supply Current|RSET= 1000k, Pin 3 = 0V, RL= 10M<br>V+= 3.6V<br>(DIV = 1, fOUT= 100kHz)<br>V+= 2.25V|l<br>l|12.5<br>10.0<br>18<br>15|µA<br>µA||
|||RSET= 100k, Pin 3 = 0V, RL= 10M<br>V+= 3.6V<br>(DIV = 1, fOUT= 1MHz)<br>V+= 2.25V|l<br>l|78<br>60<br>100<br>80|µA<br>µA||
|VIH|High Level DIV Input Voltage|V+= 3.6V<br>V+= 2.25V|l<br>l|3.1<br>2.05|V<br>V||
|VIL|Low Level DIV Input Voltage|V+= 3.6V<br>V+= 2.25V|l<br>l|0.5<br>0.2|V<br>V||
|IDIV|DIV Input Current (Note 5)|Pin 3 = V+<br>Pin 3 = 0V|l<br>l|–2<br>1<br>–1<br>2|µA<br>µA||
|VOH|High Level Output Voltage (Note 5)|V+= 3.6V<br>IOH= –100µA<br>IOH= –1mA|l<br>l|3.40<br>2.80<br>3.59<br>3.30|V<br>V||
|||V+= 2.25V<br>IOH= –100µA<br>IOH= –1mA|l<br>l|2.15<br>1.75<br>2.2<br>2.0|V<br>V||
|VOL|Low Level Output Voltage (Note 5)|V+= 3.6V<br>IOL= 100µA<br>IOL= 1mA|l<br>l|0.02<br>0.15<br>0.2<br>0.8|V<br>V||
|||V+= 2.25V<br>IOL= 100µA<br>IOL= 1mA|l<br>l|0.03<br>0.30<br>0.1<br>0.5|V<br>V||
|tr|OUT Rise Time (Note 6)|V+= 3.6V<br>V+= 2.25V||10<br>25|ns<br>ns||
|tf|OUT Fall Time (Note 6)|V+= 3.6V<br>V+= 2.25V||10<br>25|ns<br>ns||
|VGS|GRD Pin Voltage Relative to SET Pin<br>Voltage|–10µA ≤ IGRD≤ 0.3µA|l|–10<br>10|mV||



**Note 1:** Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. 

**Note 2:** Some frequencies may be generated using two different values of RSET. For these frequencies, the error is specified assuming that the larger value of RSET is used. 

**Note 3:** Frequency accuracy is defined as the deviation from the fOUT equation. 

**Note 4:** Jitter is the ratio of the peak-to-peak deviation of the period to the mean of the period. This specification is based on characterization and is not 100% tested. 

**Note 6:** Output rise and fall times are measured between the 10% and 90% power supply levels. 

**Note 7:** The LTC6906C is guaranteed to meet specified performance from 0°C to 70°C. The LTC6906C is designed, characterized and ex pected to meet specified performance from –40°C to 85°C but is not tested or QA sampled at these temperatures. The LTC6906I is guaranteed to meet specified performance from –40°C to 85°C. 

**Note 8:** Consult the Applications Information section for operation with supplies higher than 3.6V. 

**Note 9:** Test conditions reflect the master oscillator frequency. The output divider is functionally tested and divided frequency accuracy is guaranteed by design. 

**Note 5:** Current into a pin is given as a positive value. Current out of a pin is given as a negative value. 

Rev. D 

3 

For more information www.analog.com 

LTC6906 

## **TYPICAL PERFORMANCE CHARACTERISTICS** 

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Typical Frequency Error Typical Frequency Error<br>vs Power Supply vs Temperature Typical Frequency Error vs RSET<br>0.50 2 0.50<br>0.40 0.40<br>1<br>0.30 RSET = 100k 0.30<br>2.25V<br>0.20 0 0.20<br>3.6V<br>0.10 0.10<br>RSET = 1M –1 2.25V V [+]  = 2.25V<br>0 3.6V 0<br>RSET = 100k –2 V [+]  = 5V<br>–0.10 –0.10<br>RSET = 1M<br>–0.20 –3 –0.20<br>–0.30 –0.30<br>–4<br>–0.40 –0.40<br>–0.50 –5 –0.50<br>2.25 3 4 5 –50 –30 –10 10 30 50 70 90 110 130 150 0 200 400 600 800 1000 1200<br>SUPPLY VOLTAGE (V) TEMPERATURE (°C) RSET (kΩ)<br>6906 G01 6906 G02 6906 G03<br>Typical Supply Current Typical Supply Current VSET vs Temperature (VSET is the<br>vs Frequency vs Load Capacitance Voltage Measured at the RSET Pin)<br>90 200 0.80<br>CL = 5pF TA = 25°C RSET = 100k<br>80 TA = 25°C 180 0.75<br>1MHz, 3.6V<br>70 160 0.70<br>V [+]  = 3.6V<br>140 0.65<br>60<br>120 0.60 VSET AT<br>50 V [+]  = 2.25V<br>V [+]  = 2.25V 100 0.55 VSET AT<br>40 1MHz, 2.25V V [+]  = 3.6V<br>80 0.50<br>30<br>60 0.45<br>20<br>40 0.40<br>100kHz, 3.6V<br>10 20 0.35<br>100kHz, 2.25V<br>0 0 0.30<br>0 200 400 600 800 1000 1200 0 10 20 30 40 –60 –40 –20 0 20 40 60 80 100 120 140<br>FREQUENCY (kHz) LOAD CAPACITANCE (pF) TEMPERATURE (°C)<br>6906 G04 6906 G05 6906 G06<br>ERROR (%)<br>FREQUENCY ERROR (%) FREQUENCY ERROR (%)<br>SET PIN VOLTAGE (V)<br>POWER SUPPLY CURRENT (µA) POWER SUPPLY CURRENT (µA)<br>**----- End of picture text -----**<br>


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Typical Supply Current Typical Supply Current<br>vs Temperature, 1MHz vs Temperature, 100kHz<br>90 18<br>CL = 5pF CL = 5pF<br>85 ISUPPLY AT V [+]  = 3.6V 17<br>16<br>80 15<br>ISUPPLY AT V [+]  = 3.6V<br>14<br>75<br>13<br>70<br>ISUPPLY AT V [+]  = 2.25V 12 ISUPPLY AT V [+]  = 2.25V<br>65 11<br>10<br>60<br>9<br>55 8<br>–60 –40 –20 0 20 40 60 80 100 120 140 –50 –30 –10 10 30 50 70 90 110 130 150<br>TEMPERATURE (°C) TEMPERATURE (°C)<br>6906 G07 6906 G08<br>SUPPLY CURRENT (µA) SUPPLY CURRENT (µA)<br>**----- End of picture text -----**<br>


Rev. D 

4 

For more information www.analog.com 

LTC6906 

## **PIN FUNCTIONS** 

**OUT (Pin 1):** Oscillator Output. The OUT pin swings from GND to V[+] with an output resis tance of approximately 150Ω. For micropower operation, the load re sistance must be kept as high as possible and the load capacitance as low as possible. 

## **GND (Pin 2):** Ground. 

**DIV (Pin 3):** Divider Setting Input. This three-level input selects one of three internal digital divider settings, determining the value of N in the frequency equation. Tie to GND for ÷1, leave floating for ÷3 and tie to V[+] for ÷10. When left floating, the LTC6906 pulls Pin 3 to mid-supply with a 2.5M resistor. When Pin 3 is floating, care should be taken to reduce coupling from the OUT pin and its trace to Pin 3. Coupling can be reduced by increasing the physical space between traces or by shielding the DIV pin with grounded metal. 

**SET (Pin 4):** Frequency Setting Resistor Input. Connect a resistor, RSET, from this pin to GND to set the oscillator frequency. For best perform ance use a precision metal- or thin-film resistor of 0.5% or better tolerance and 50ppm/°C 

or better temperature coefficient. For lower accu racy applications, an inexpensive 1% thick-film resistor may be used. Limit the capacitance in parallel with RSET to less than 10pF to reduce jitter and to ensure stability. Capacitance greater than 10pF could cause the LTC6906 internal feedback circuits to oscillate. The volt age on the SET pin is approximately 650mV and decreases with temperature by about –2.2mV/°C. 

**GRD (Pin 5):** Guard Signal. This pin can be used to reduce PC board leakage across the frequency setting resistor, RSET. The GRD pin is held within a few millivolts of the SET pin and shunts leakage current away from the SET pin. To control leakage, connect a bare copper trace (a trace with no solder mask) to GRD and loop it around the SET pin and all PC board metal connected to SET. 

**V[+] (Pin 6):** Voltage Supply (2.25V to 3.6V). This supply is internally decoupled with a 20Ω resistor in series with an 800pF capacitor. No external decoupling capacitor is required for OUT pin loads less than 50pF. V[+] should be kept reasonably free of noise and ripple. 

## **BLOCK DIAGRAM** 

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DECOUPLING<br>V [+]<br>NETWORK FREQUENCY-TO-CURRENT<br>V [+] 20 CONVERTERS 5M<br>6 fOSC<br>THREE-LEVEL<br>GND 800pF INPUT DIV 3<br>2 IFB IFB DETECTOR<br>5M<br>DIVIDER<br>VSET ≅ VGRD ≅ 650mV SELECT<br>ISET = IFB<br>VSET 4 SET VSET – VOLTAGE fOSC 150Ω DRIVER<br>PROGRAMMABLE<br>RSET GRD BUFFER VSET OP AMP CONTROLLEDOSCILLATOR DIVIDER (n)(1, 3, 10) OUT 1<br>5 + (MASTER OSCILLATOR)<br>fOSC = 1MHz • 100kΩ/RSET<br>6906 BD<br>**----- End of picture text -----**<br>


Rev. D 

5 

For more information www.analog.com 

LTC6906 

## **TEST CIRCUIT** 

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EQUIVALENT CIRCUIT OF<br>OSCILLOSCOPE OR<br>LTC6906 CTEST FREQUENCY COUNTER PROBE<br>SUPPLY<br>V [+] OUT<br>VOLTAGE<br>GND GRD CPROBE R10MPROBE<br>DIV SET RSET<br>0.01%<br>10ppm/°C<br>6906 F01<br>CTEST = 1/(1/5pF – 1/CPROBE)<br>= 7.5pF FOR A 15pF SCOPE PROBE<br>**----- End of picture text -----**<br>


**Figure 1. Test Circuit with 5pF Effective Load** 

## **EQUIVALENT INPUT AND OUTPUT CIRCUITS** 

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V [+] V [+] V [+]<br>6 6 6<br>200Ω<br>20Ω TOTAL OUTPUT<br>SET 1k GRD RESISTANCE<br>4 5<br>800pF<br>GND GND GND<br>2 2 2<br>6906 F02 6906 F03 6906 F04<br>**----- End of picture text -----**<br>


**Figure 2. V[+] Pin** 

**Figure 3. SET Pin** 

**Figure 4. GRD Pin** 

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V [+] V [+]<br>6 6<br>5M fOUT<br>DIV OUT 300Ω<br>3 1<br>5M<br>GND GND<br>2 2<br>6906 F05 6906 F06<br>**----- End of picture text -----**<br>


**Figure 5. DIV Pin** 

**Figure 6. OUT Pin** 

Rev. D 

6 

For more information www.analog.com 

LTC6906 

## **THEORY OF OPERATION** 

The LTC6906 is a precision, resistor programmable oscillator (see the Block Diagram). It generates a square wave at the OUT pin with a period directly proportional to the value of an external resistor, RSET. A feedback circuit measures and controls the oscillator frequency to achieve the highest possible accuracy. In equilibrium, this circuit ensures that the current in the SET pin, ISET, is balanced by IFB. IFB is proportional to the master oscillator frequency, so we have the relationship: 

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where COSC is a precision internal capacitor: 

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Solving for the oscillator period: 

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This is the fundamental equation for the LTC6906. It holds regardless of how the SET pin is driven. When a resistor, RSET, is connected from the SET pin to ground, we have the relationship: 

The period and frequency are determined exclusively by RSET and the precision internal capacitor. Importantly, the value of VSET is immaterial, and the LTC6906 maintains its accuracy even though VSET is not a precision reference voltage. 

The digital dividers shown in the Block Diagram further divide the master oscillator frequency by 1, 3 or 10 producing: 

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and 

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Table 1 gives specific frequency and period equations for the LTC6906. The Applications Information section gives further detail and discusses alternative ways of setting the LTC6906 output frequency. 

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so 

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**Table 1. Output Frequency Equations** 

|**PART NUMBER**|**FREQUENCY**|**FREQUENCY**|**PERIOD**|**DIVIDER RATIOS**|
|---|---|---|---|---|
|LTC6906|ƒOUT =|1MHz<br>N<br>• 100k<br>RSET<br>⎛<br>⎝⎜<br>⎞<br>⎠~~⎟~~|tOUT =N•1µs • RSET<br>100k<br>⎛<br>⎝⎜<br>⎞<br>⎠~~⎟~~|N=<br>10,<br>3,<br>1,<br>⎧<br>⎨<br>⎪<br>⎩<br>⎪<br>DIV Pin=V +<br>DIV Pin=Open<br>DIV Pin=GND|



Rev. D 

7 

For more information www.analog.com 

LTC6906 

## **APPLICATIONS INFORMATION** 

## **Selecting RSET and the Divider Ratio** 

The LTC6906 contains a master oscillator followed by a digital divider (see the Block Diagram). RSET determines the master oscillator frequency and the DIV pin sets the divider ratio, N. The range of frequencies accessible in each divider ratio overlap, as shown in Figure 7. This figure is derived from the equations in Table 1. _**For any given frequency, power can be minimized by minimizing the master oscillator frequency. This implies maximizing RSET and using the lowest possible divider ratio, N**_ . The relationship between RSET, N and the unloaded power consumption is shown in Figure 8, where we can clearly see that supply current decreases for large values of RSET. For a discussion of jitter and divide ratio, refer to page 11. 

## **Minimizing Power Consumption** 

The supply current of the LTC6906 has four current components: 

- Constant (Independent V[+] , ƒOUT and CLOAD) 

- Proportional to ISET (which is the current in RSET) 

- Proportional to V[+] , ƒOUT and CLOAD 

- Proportional to V[+] and RLOAD 

An approximate expression for the total supply current is: 

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VSET is approximately 650mV at 25°C, but varies with temperature. This behavior is shown in the Typical Performance Characteristics section. 

Power can be minimized by maximizing RSET, minimizing the load on the OUT pin and operating at lower frequencies. Figure 9 shows total supply current vs frequency under typical conditions. Below 100kHz the load current is negligible for the 5pF load shown. 

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10000<br>10 3 1<br>1000<br>100<br>10<br>1 10 100 1000 10000<br>OUTPUT FREQUENCY (kHz)<br>6906 F07<br> (kΩ)<br>SET<br>R<br>**----- End of picture text -----**<br>


**Figure 7. RSET vs Desired Output Frequency (LTC6906)** 

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80<br>CLOAD = 0<br>70 V [+]  = 3V<br>TA = 25°C<br>60<br>50<br>40<br>30<br>20<br>10<br>0<br>100 1000<br>RSET (kΩ)<br>6906 F08<br>Figure 8. Unloaded Supply Current vs RSET<br>80<br>V [+]  = 2.7V<br>70<br>60<br>1<br>50<br>3<br>40<br>10<br>30<br>20<br>10<br>0<br>0 200 400 600 800 1000 1200<br>MASTER OSCILLATOR FREQUENCY (kHz)<br>6906 F09<br> (µA)<br>ISUPPLY<br>POWER SUPPLY CURRENT (µA)<br>**----- End of picture text -----**<br>


**Figure 8. Unloaded Supply Current vs RSET** 

**Figure 9. Supply Current vs Frequency** 

Rev. D 

8 

For more information www.analog.com 

LTC6906 

## **APPLICATIONS INFORMATION** 

## **Guarding Against PC Board Leakage** 

The LTC6906 uses relatively large resistance values for RSET to minimize power consumption. For RSET = 1M, the SET pin current is typically only 0.65µA. Thus, only 0.65nA leaking into the SET pin causes a 0.1% frequency error. Similarly, 1G of leakage resistance across RSET (1000 • RSET) causes the same 0.1% error. 

Achieving the highest accuracy requires controlling potential leakage paths. PC board leakage is aggravated by both dirt and moisture. Effective cleaning is a good first step to minimizing leakage, and some PC board manufacturers offer high impedance or low leakage processing options. 

Another effective method for controlling leakage is to shunt the leakage current away from the sensitive node through a low impedance path. The LTC6906 provides a signal on the GRD pin for this purpose. Figure 10 shows a PC board layout that uses the GRD pin and a “guard ring” to absorb leakage currents. The guard ring surrounds the SET pin and the end of RSET to which it is connected. The guard ring must have no solder mask covering it to be effective. The GRD pin voltage is held within a few millivolts of the SET pin voltage, so any leakage path between the SET pin and the guard ring generates no leakage current. 

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LTC6906<br>1 OUT V [+] 6 NO SOLDER MASK<br>OVER THE GUARD RING<br>GRD<br>2 GND 5<br>GUARD<br>RING<br>3 DIV SET 4<br>RSET<br>NO LEAKAGE<br>CURRENT<br>LEAKAGE<br>CURRENT<br>6906 F10<br>**----- End of picture text -----**<br>


**Figure 10. PC Board Layout with Guard Ring** 

## **Bypassing the Power Supply** 

The LTC6906 has on-chip power supply decoupling that eliminates the need for an external decoupling capacitor in most cases. Figure 11 shows a simplified equivalent circuit of the output driver and on-chip decoupling network. When the output driver switches from low to high, the 800pF capacitor delivers the current needed to charge the off-chip capacitive load. Within nanoseconds the system power supply recharges the 800pF capacitor. 

**==> picture [186 x 122] intentionally omitted <==**

**----- Start of picture text -----**<br>
LTC6906-1<br>V [+]<br>V [+] 6<br>fOUT<br>20Ω<br>OUT 300Ω<br>1<br>CLOAD 800pF<br>GND<br>2<br>ESD DIODES DRIVER DECOUPLING<br>NETWORK<br>6906 F11<br>**----- End of picture text -----**<br>


**Figure 11. Simplified Equivalent of the Output Driver and On-Chip Decoupling Circuit** 

Figure 12 shows a test circuit for evaluating performance of the LTC6906 with a highly inductive, 330nH power supply. Figure 13 shows the effectiveness of the on-chip decoupling network. For CLOAD = 5pF to 50pF, the output waveforms remain well formed. 

The extremely low supply current of the LTC6906 allows operation with substantial resistance in the power supply. Figure 14 shows a test circuit for evaluating performance of the LTC6906 with a highly resistive, 100Ω power supply. Figure 15 shows the effectiveness of the on-chip decoupling network. For CLOAD = 5pF to 50pF, the output waveforms remain well formed. With a 50pF load, a very small (2.5%) slow tail can be seen on the rising edge. The output waveform is still well formed even in this case. The ability of the LTC6906 to operate with a resistive supply permits supplying power via a CMOS logic gate or microcontroller pin. Since the LTC6906 has a turn-on time of less than 200µs, this technique can be used to enable the device only when needed and further reduce power consumption. 

Rev. D 

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For more information www.analog.com 

LTC6906 

## **APPLICATIONS INFORMATION** 

**==> picture [182 x 287] intentionally omitted <==**

**----- Start of picture text -----**<br>
330nHLS LTC6906 1MHz<br>3.3V V [+] OUT<br>GND GRD CLOAD<br>DIV SET<br>RSET<br>100k<br>6906 F12<br>Figure 12. Test Circuit with an Inductive Power Supply<br>3.5<br>3<br>2<br>1<br>CLOAD = 5pF<br>CLOAD = 10pF<br>CLOAD = 20pF<br>CLOAD = 50pF<br>0<br>4.65 4.75 4.85 4.95 5.05 5.15<br>TIME (µs)<br>6906 F13<br> (V)<br>OUT<br>V<br>**----- End of picture text -----**<br>


**Figure 12. Test Circuit with an Inductive Power Supply** 

**Figure 13. Output Waveforms with an Inductive Supply (See Figure 12)** 

## **Start-Up Time** 

When the LTC6906 is powered up, it holds the OUT pin low. After the master oscillator has settled, the OUT pin is enabled and the first output cycle is guaranteed to be within specification. The time from power-up to the first output transition is given approximately by: 

**==> picture [169 x 287] intentionally omitted <==**

**----- Start of picture text -----**<br>
100ΩRS LTC6906 1MHz<br>3.3V V [+] OUT<br>GND GRD CLOAD<br>DIV SET<br>RSET<br>100k<br>6906 F14<br>Figure 14. Test Circuit with a Resistive Power Supply<br>3.5<br>3<br>2<br>1<br>CLOAD = 5pF<br>CLOAD = 10pF<br>CLOAD = 20pF<br>CLOAD = 50pF<br>0<br>0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1<br>TIME (µs)<br>6906 F15<br> (V)<br>OUT<br>V<br>**----- End of picture text -----**<br>


**Figure 14. Test Circuit with a Resistive Power Supply** 

**Figure 15. Output Waveforms with a Resistive Supply (See Figure 14)** 

jitter. The risk increases when the fundamental frequency or harmonics of the noise fall near the master oscillator frequency. It is relatively easy to filter the LTC6906 power supply because of the very low supply current. For example, an RC filter with R = 160Ω and C = 10µF provides a 100Hz lowpass filter while dropping the supply voltage only about 10mV. 

tSTART ≅ 64 • tOSC + 100µs 

The digital divider ratio, N, does not affect the start-up time. 

## **Power Supply Rejection** 

The LTC6906 has a very low supply voltage coefficient, meaning that the output frequency is nearly insensitive to the DC power supply voltage. In most cases, this error term can be neglected. 

High frequency noise on the power supply (V[+] ) pin has the potential to interfere with the LTC6906’s master oscillator. Periodic noise, such as that generated by a switching power supply, can shift the output frequency or increase 

## **Operating the LTC6906 with Supplies Higher Than 3.6V** 

The LTC6906 may also be used with supply voltages between 3.6V and 5.5V under very specific conditions. To ensure proper functioning above 3.6V, a filter circuit must be attached to the power supply and located within 1cm of the device. A simple RC filter consisting of a 100Ω resistor and 1µF capacitor (Figure 16) will ensure that supply resonance at higher supply voltages does not induce unpredictable oscillator behavior. Accuracy under higher supplies may be estimated from the typical Frequency vs Supply Voltage curves in the Typical Performance Characteristics section of this data sheet. 

Rev. D 

10 

For more information www.analog.com 

LTC6906 

## **APPLICATIONS INFORMATION** 

**==> picture [138 x 100] intentionally omitted <==**

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V [+]<br>3.6V TO 5.5V DC<br>100Ω LTC6906<br>V [+] OUT<br>1µF GND GRD<br>DIV SET<br>RSET<br>6906 F16<br>**----- End of picture text -----**<br>


**Figure 16. Using the LTC6906 at Higher Supply Voltages** 

## **Alternative Methods for Setting the Output Frequency** 

Any means of sinking current from the SET pin will control the output frequency of the LTC6906. Equation 2 (repeated below) gives the fundamental relationship between frequency and the SET pin voltage and current: 

**==> picture [239 x 30] intentionally omitted <==**

This equation shows that the LTC6906 converts conductance (ISET/VSET) to frequency or, equivalently, converts resistance (RSET = VSET/ISET) to period. 

VSET is the voltage across an internal diode, and as such it is given approximately by: 

**==> picture [227 x 67] intentionally omitted <==**

where 

VT = kT/q = 25.9mV at T = 300°K (27°C) 

IS ≅ 82 • 10[–18] Amps 

(IS is also temperature dependent) 

VSET varies with temperature and the SET pin current. The response of VSET to temperature is shown in the Typical Performance graphs. VSET changes approximately –2.3mV/°C. At room temperature VSET increases 18mV/ octave or 60mV/decade of increase in ISET. 

If the SET pin is driven with a current source generating ISET, the oscillator output frequency will be: 

**==> picture [221 x 61] intentionally omitted <==**

Figure 17 and Figure 18 show a current controlled oscillator and a voltage controlled oscillator. These circuits are not highly accurate if used alone, but can be very useful if they are enclosed in an overall feedback circuit such as a phase-locked loop. 

**==> picture [133 x 76] intentionally omitted <==**

**----- Start of picture text -----**<br>
LTC6906 100kHz TO 1MHz<br>V [+] V [+] OUT<br>GND GRD<br>DIV SET<br>ICTRL<br>0.65µA TO 6.5µA<br>6906 F17<br>**----- End of picture text -----**<br>


**Figure 17. Current Controlled Oscillator** 

**==> picture [136 x 65] intentionally omitted <==**

**----- Start of picture text -----**<br>
LTC6906 1MHz TO 100kHz<br>V [+] V [+] OUT<br>GND GRD<br>DIV SET VCTRL<br>0V TO 0.585V<br>6906 F18 R100kSET<br>**----- End of picture text -----**<br>


**Figure 18. Voltage Controlled Oscillator** 

## **Jitter and Divide Ratio** 

At a given output frequency, a higher master oscillator frequency and a higher divide ratio will result in lower jitter and higher power supply dissipation. Indeterminate jitter percentage will decrease by a factor of slightly less than the square root of the divider ratio, while determinate jitter will not be similarly attenuated. Please consult the specification tables for typical jitter at various divider ratios. 

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LTC6906 

## **TYPICAL APPLICATIONS** 

## **Setting Frequency to 0.1% Resolution with Standard Resistors** 

## **Trimming the Frequency** 

## **Sine Wave Oscillator** 

**==> picture [524 x 108] intentionally omitted <==**

**----- Start of picture text -----**<br>
LTC6906 10kHz TO 1MHz 1MHz WITH<br>LTC6906 2.5% RANGE 1MHz<br>2.25V TO 3.6V V [+] OUT 2.25V TO 3.6V V [+] OUT LTC6906 0.1µF 1k<br>10 GND GRD GND GRD 2.25V TO 3.6V V [+] OUT<br>31 DIV SET RR1% THIN FILMAA < RB/10 DIV SET R97.6kA GNDDIV GRDSET RSET L1100µH C1240pF<br>RB 100k<br>100k TO 1M RB 6906 TA05<br>0.1% THIN FILM 5k<br>6906 TA03<br>6906 TA04<br>**----- End of picture text -----**<br>


## **PACKAGE DESCRIPTION** 

## **S6 Package 6-Lead Plastic TSOT-23** (Reference LTC DWG # 05-08-1636) 

**==> picture [450 x 292] intentionally omitted <==**

**----- Start of picture text -----**<br>
2.90 BSC<br>0.62 0.95<br>MAX REF (NOTE 4)<br>1.22 REF<br>1.50 – 1.75<br>fu 2.62 REF 1.4 MIN 2.80 BSC (NOTE 4)<br>PIN ONE ID<br>‘ pagag | fare<br>RECOMMENDED SOLDER PAD LAYOUT 0.30 – 0.45<br>— PER IPC CALCULATOR a 0.95 BSC 6 PLCS (NOTE 3)<br>0.80 – 0.90<br>0.20 BSC<br>0.01 – 0.10<br>1.00 MAX<br>DATUM ‘A’<br>0.30 – 0.50 REF<br>1.90 BSC<br>0.09 – 0.20<br>(NOTE 3) S6 TSOT-23 0302<br>NOTE:<br>**----- End of picture text -----**<br>


**==> picture [166 x 181] intentionally omitted <==**

**----- Start of picture text -----**<br>
0.62 0.95<br>MAX REF<br>1.22 REF<br>3.85 MAX fu 2.62 REF 1.4 MIN<br>‘ pagag<br>RECOMMENDED SOLDER PAD LAYOUT<br>— PER IPC CALCULATOR<br>**----- End of picture text -----**<br>


1. DIMENSIONS ARE IN MILLIMETERS 

2. DRAWING NOT TO SCALE 

3. DIMENSIONS ARE INCLUSIVE OF PLATING 

4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 

5. MOLD FLASH SHALL NOT EXCEED 0.254mm 

6. JEDEC PACKAGE REFERENCE IS MO-193 

Rev. D 

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For more information www.analog.com 

LTC6906 

## **REVISION HISTORY (Revision history begins at Rev B)** 

|**RE**|**ISIO**|**N HISTORY**<br>**(Revision history begins at Rev B)**||
|---|---|---|---|
|**REV**|**DATE**|**DESCRIPTION**|**PAGE NUMBER**|
|B|12/10|Added Note 9.<br>Revised values in the Guarding Against PC Board Leakage section.|2-3<br>9|
|C|11/11|Reinsert symbols missing from Rev B.|1, 4, 5, 6, 8, 9,<br>10, 11, 12|
|D|02/21|Reinsert missing symbols in plots.|1, 4, 5, 10|



Rev. D 

13 

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implicatiFor more informati **on** or otherwise under any patent or patent rights of Analog Devices.www.analog.com 

LTC6906 

## **RELATED PARTS** 

|**PART NUMBER**|**DESCRIPTION**|**COMMENTS**|
|---|---|---|
|LTC1799|1kHz to 33MHz ThinSOT Oscillator|Single Output, Greater Frequency Range|
|LTC6900|1kHz to 20MHz ThinSOT Oscillator|Single Output, Greater Frequency Range|
|LTC6902|Multiphase Oscillator with Spread Spectrum Frequency Modulation|2-, 3- or 4-Phase Outputs|
|LTC6903/LTC6904|1kHz to 68MHz Serial Port Programmable Oscillator|Very Wide Frequency Range with Digital Programmability|
|LTC6905|17MHz to 170MHz ThinSOT Oscillator|Single Output, Higher Frequency|



Rev. D 

02/21 

14 

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 ANALOG DEVICES, INC. 2005-2021 

For more information www.analog.com