LTC6906 Micropower, 10kHz to 1MHz Resistor Set Oscillator in SOT-23 U FEATURES DESCRIPTIO ■ 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. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Supply Current: 12µA at 100kHz <0.65% Frequency Accuracy (from 0°C to 70°C) Frequency Range: 10kHz to 1MHz One Resistor Sets the Oscillator Frequency Single Supply: 2.25V to 5.5V –40°C to 125°C Operating Temperature Range No Decoupling Capacitor Needed Start-Up Time Under 200µs at 1MHz First Cycle After Power-Up is Accurate 150Ω CMOS Output Driver Low Profile (1mm) SOT-23 (ThinSOTTM) Package U APPLICATIO S ■ ■ ■ ■ ■ 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: ƒOUT Low Cost Precision Programmable Oscillator Rugged, Compact Micropower Replacement for Crystal and Ceramic Oscillators High Shock and Vibration Environments Portable and Battery-Powered Equipment PDAs and Cellular Phones , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. ⎧10, DIV Pin = V + 1MHz ⎛ 100k ⎞ ⎪ = •⎜ ⎟ , N = ⎨3, DIV Pin = Open N ⎝ RSET ⎠ ⎪1, DIV Pin = GND ⎩ No decoupling capacitor is needed in most cases, yielding an extremely compact solution occupying less than 20mm2. Contact LTC Marketing for a version of the part with a shutdown feature or lower frequency operation. The LTC6906 is available in the 6-lead SOT-23 (ThinSOT) package. U TYPICAL APPLICATIO Typical Supply Current vs Frequency Micropower Clock Generator 2.25V TO 3.6V ÷10 ÷3 ÷1 V+ OUT GND GRD DIV CL = 5pF TA = 25°C 80 10kHz TO 1MHz SET RSET 100k TO 1M 6906 TA01 POWER SUPPLY CURRENT (µA) NO DECOUPLING CAPACITOR NEEDED LTC6906 90 70 V+ = 3.6V 60 50 V+ = 2.25V 40 30 20 10 0 0 200 400 600 800 FREQUENCY (kHz) 1000 1200 6906 G04 6906fa 1 LTC6906 W W W AXI U U U W PACKAGE/ORDER I FOR ATIO U ABSOLUTE RATI GS (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) TOP VIEW 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 OUT 1 6 V+ GND 2 5 GRD DIV 3 4 SET S6 PACKAGE 6-LEAD PLASTIC TSOT-23 TJMAX = 150°C, θJA = 230°C/W ORDER PART NUMBER LTC6906CS6 LTC6906IS6 LTC6906HS6 S6 PART MARKING* LTBJN Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature range. *The temperature grade is identified by a label on the shipping container. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C. V+ = 2.25V to 3.6V, CL = 5pF, Pin 3 = V + unless otherwise noted. All voltages are with respect to GND. SYMBOL PARAMETER CONDITIONS ∆f Frequency Accuracy (Notes 2, 3) V+ = 2.7V to 3.6V 100kHz ≤ f ≤ 1MHz 100kHz ≤ f ≤ 1MHz, LTC6906C 100kHz ≤ f ≤ 1MHz, LTC6906I f = 1MHz, LTC6906H f = 100kHz, LTC6906H ● ● ● ● 100kHz ≤ f ≤ 1MHz 100kHz ≤ f ≤ 1MHz, LTC6906C 100kHz ≤ f ≤ 1MHz, LTC6906I f = 1MHz, LTC6906H f = 100kHz, LTC6906H ● ● ● ● V+ = 2.25V MIN ● RSET Frequency-Setting Resistor Range ∆f/∆T Frequency Drift Over Temp (Note 3) RSET = 316k ∆f/∆V Frequency Drift Over Supply (Note 3) Timing Jitter (Note 4) ≤ 1000k Pin 3 = Open, 100k ≤ RSET ≤ 1000k Pin 3 = 0V, 100k ≤ RSET ≤ 1000k Sf Long-Term Stability of Output Frequency Pin 3 = V+ DC Duty Cycle V+ Operating Supply Range (Note 8) IS Power Supply Current TYP MAX UNITS ±0.25 ±0.5 ±0.65 ±1.3 ±1.3 ±2.2 % % % % % ±0.25 ±0.7 ±0.85 ±1.3 ±1.3 ±2.2 % % % % % 100 1000 kΩ ±0.005 %/°C V+ = 2.25V to 3.6V, 100k ≤ RSET ≤ 1000k 0.06 %/V Pin 3 = V +, 100k ≤ RSET 0.03 0.07 0.15 % % % ● 300 ● 45 ● 2.25 50 ppm/√kHr 55 % 3.6 V RSET = 1000k, Pin 3 = 0V, RL = 10M (DIV = 1, fOUT = 100kHz) V + = 3.6V V + = 2.25V ● ● 12.5 10.0 18 15 µA µA RSET = 100k, Pin 3 = 0V, RL = 10M (DIV = 1, fOUT = 1MHz) V + = 3.6V V + = 2.25V ● ● 78 60 100 80 µA µA 6906fa 2 LTC6906 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C. V+ = 2.25V to 3.6V, CL = 5pF, Pin 3 = V + unless otherwise noted. All voltages are with respect to GND. SYMBOL PARAMETER VIH High Level DIV Input Voltage VIL Low Level DIV Input Voltage IDIV DIV Input Current (Note 5) VOH VOL High Level Output Voltage (Note 5) Low Level Output Voltage (Note 5) CONDITIONS MIN V+ = 3.6V Pin 3 = V + Pin 3 = 0V V+ = 2.25V ● ● V+ = 3.6V V+ = 2.25V ● ● TYP MAX 3.1 2.05 UNITS V V ● ● –2 1 –1 0.5 0.2 V V 2 µA µA V + = 3.6V IOH = – 100µA IOH = – 1mA ● ● 3.40 2.80 3.59 3.30 V V V + = 2.25V IOH = – 100µA IOH = – 1mA ● ● 2.15 1.75 2.2 2.0 V V V + = 3.6V IOL = 100µA IOL = 1mA ● ● 0.02 0.15 0.2 0.8 V V V + = 2.25V IOL = 100µA IOL = 1mA ● ● 0.03 0.30 0.1 0.5 V V tr OUT Rise Time (Note 6) V + = 3.6V V+ = 2.25V 10 25 ns ns tf OUT Fall Time (Note 6) V + = 3.6V V+ = 2.25V 10 25 ns ns VGS GRD Pin Voltage Relative to SET Pin Voltage –10µA ≤ IGRD ≤ 0.3µA Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. 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 5: Current into a pin is given as a positive value. Current out of a pin is given as a negative value. ● –10 10 mV 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 expected 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. 6906fa 3 LTC6906 U W TYPICAL PERFOR A CE CHARACTERISTICS Typical Frequency Error vs Temperature Typical Frequency Error vs Power Supply 0.50 0.40 0.40 1 0.30 0.10 RSET = 1M RSET = 100k –0.10 –0.20 0.30 2.25V 0 0.20 3.6V –1 2.25V 3.6V –2 0.10 V+ = 2.25V 0 V+ = 5V –0.10 RSET = 1M –3 –0.20 –0.30 –0.30 –4 –0.40 –0.50 2.25 3 4 SUPPLY VOLTAGE (V) –0.40 –5 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 5 POWER SUPPLY CURRENT (µA) 180 70 V+ = 3.6V 60 50 V 40 + = 2.25V 30 20 1MHz, 3.6V 100 1200 1MHz, 2.25V 60 40 0.60 0.55 VSET AT V+ = 3.6V VSET AT V+ = 2.25V 0.50 0.45 0.35 100kHz, 2.25V 0 20 10 30 LOAD CAPACITANCE (pF) 40 0.30 –60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 6906 G06 6906 G05 Typical Supply Current vs Temperature, 100kHz Typical Supply Current vs Temperature, 1MHz 90 18 CL = 5pF 85 0.65 0.40 100kHz, 3.6V 6906 G04 ISUPPLY 1200 0.70 80 0 1000 1000 RSET = 100k 0.75 120 0 400 600 800 FREQUENCY (kHz) 600 800 RSET (kΩ) 0.80 140 20 200 400 VSET vs Temperature (VSET is the Voltage Measured at the RSET Pin) TA = 25°C 160 10 0 200 6906 G03 SET PIN VOLTAGE (V) 200 CL = 5pF TA = 25°C 80 0 Typical Supply Current vs Load Capacitance Typical Supply Current vs Frequency 90 –0.50 6906 G02 6906 G01 POWER SUPPLY CURRENT (µA) ERROR (%) 0.20 0 RSET = 100k FREQUENCY ERROR (%) FREQUENCY ERROR (%) Typical Frequency Error vs RSET 0.50 2 17 AT V+ = 3.6V CL = 5pF SUPPLY CURRENT (µA) SUPPLY CURRENT (µA) 16 80 75 70 ISUPPLY AT V+ = 2.25V 65 15 14 ISUPPLY AT V+ = 3.6V 13 12 11 ISUPPLY AT V+ = 2.25V 10 60 9 55 –60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 6906 G07 8 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 6906 G08 6906fa 4 LTC6906 U U U PI FU CTIO S OUT (Pin 1): Oscillator Output. The OUT pin swings from GND to V+ with an output resistance of approximately 150Ω. For micropower operation, the load resistance 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 performance use a precision metal- or thin-film resistor of 0.5% or better tolerance and 50ppm/°C or better temperature coefficient. For lower accuracy 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 voltage 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. W BLOCK DIAGRA DECOUPLING NETWORK 6 2 V+ 20Ω GND V+ FREQUENCY-TO-CURRENT CONVERTERS 5M fOSC THREE-LEVEL INPUT DETECTOR 800pF IFB IFB DIV 3 5M VSET ≅ VGRD ≅ 650mV VSET 4 SET RSET ISET = IFB DIVIDER SELECT VSET BUFFER 5 GRD VSET – + OP AMP VOLTAGE CONTROLLED OSCILLATOR (MASTER OSCILLATOR) 150Ω DRIVER fOSC PROGRAMMABLE DIVIDER (n) (÷1, ÷3, ÷10) OUT 1 fOSC = 1MHz • 100kΩ/RSET 6906 BD 6906fa 5 LTC6906 TEST CIRCUIT LTC6906 SUPPLY VOLTAGE V+ OUT GND GRD DIV SET EQUIVALENT CIRCUIT OF OSCILLOSCOPE OR FREQUENCY COUNTER PROBE CTEST RPROBE 10M CPROBE RSET 0.01% 10ppm/°C 6906 F01 CTEST = 1/(1/5pF – 1/CPROBE) = 7.5pF FOR A 15pF SCOPE PROBE Figure 1. Test Circuit with 5pF Effective Load U U U EQUIVALE T I PUT A D OUTPUT CIRCUITS 6 V+ 6 V+ 20Ω 4 SET 6 1k 5 V+ GRD 200Ω TOTAL OUTPUT RESISTANCE 800pF 2 GND 2 6906 F02 Figure 2. V + Pin 6 GND 2 6906 F03 Figure 3. SET Pin V+ 6 DIV 6906 F04 Figure 4. GRD Pin V+ fOUT 5M 3 GND 1 OUT 300Ω 5M 2 GND 6906 F05 Figure 5. DIV Pin 2 GND 6906 F06 Figure 6. OUT Pin 6906fa 6 LTC6906 U THEORY OF OPERATIO The LTC6906 is a precision, resistor programmable oscillator (see 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 equilibrum, 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: ISET = IFB = VSET • ƒOSC • COSC (1) Where COSC is a precision internal capacitor: COSC = 10pF for the LTC6906 Solving for the oscillator period: tOSC = 1 ƒ OSC = so tOSC = (2) 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: VSET = RSET ISET ƒ OSC = RSET • COSC (4) 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: ƒ OSC N (5) tOUT = N • tOSC (6) ƒ OUT = VSET • COSC ISET 1 and 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. (3) Table 1. Output Frequency Equations PART NUMBER LTC6906 FREQUENCY ƒOUT = 1MHz N ⎛ 100k ⎞ •⎜ ⎟ ⎝ RSET ⎠ PERIOD DIVIDER RATIOS ⎛R ⎞ tOUT = N • 1µs • ⎜ SET ⎟ ⎝ 100k ⎠ ⎧10, DIV Pin = V + ⎪ N = ⎨3, DIV Pin = Open ⎪1, DIV Pin = GND ⎩ 6906fa 7 LTC6906 U W U U APPLICATIO S I FOR ATIO Selecting RSET and the Divider Ratio 10000 ÷10 1000 ÷3 ÷1 RSET (kΩ) The LTC6906 contains a master oscillator followed by a digital divider (see 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. 100 10 1 10 100 1000 OUTPUT FREQUENCY (kHz) 6906 F07 Figure 7. RSET vs Desired Output Frequency (LTC6906) 80 CLOAD = 0 V+ = 3V TA = 25°C 70 Minimizing Power Consumption ƒOUT and CLOAD) ISUPPLY (µA) 60 The supply current of the LTC6906 has four current components: • Constant (Independent V+, 50 40 30 • Proportional to ISET (which is the current in RSET) 20 • Proportional to V+, ƒOUT and CLOAD 10 0 100 • Proportional to V+ and RLOAD 1000 RSET (kΩ) An approximate expression for the total supply current is: I+ ≅ 5µA + 6 • ISET + V + • ƒ OUT • (CLOAD + 5pF ) + ≅ 5µA + 6 • 10000 V+ 2 • RLOAD V+ VSET + V + • ƒ OUT • (CLOAD + 5pF ) + 2 • RLOAD RSET VSET is approximately 650mV at 25°C, but varies with temperature. This behavior is shown in the Typical Performance Characteristics. 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 6906 F09 Figure 8. Unloaded Supply Current vs RSET under typical conditions. Below 100kHz the load current is negligible for the 5pF load shown. 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 6.5µA. Thus, only 6.5nA 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. 6906fa 8 LTC6906 U W U U APPLICATIO S I FOR ATIO 80 Bypassing the Power Supply V+ = 2.7V POWER SUPPLY CURRENT (µA) 70 60 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. ÷1 50 ÷3 40 ÷10 30 20 10 0 0 400 600 200 800 1000 1200 MASTER OSCILLATOR FREQUENCY (kHz) V+ 6906 F09 6 V+ LTC6906-1 Figure 9. Supply Current vs Frequency 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. LTC6906 1 OUT 2 GND 3 DIV V+ NO SOLDER MASK OVER THE GUARD RING 6 GRD 5 SET GUARD RING 4 RSET NO LEAKAGE CURRENT LEAKAGE CURRENT 6906 F10 Figure 10. PC Board Layout with Guard Ring fOUT 1 OUT 20Ω 300Ω CLOAD 800pF 2 GND ESD DIODES DRIVER DECOUPLING NETWORK 6906 F11 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 200µsec, this technique can be used to enable the device only when needed and further reduce power consumption. 6906fa 9 LTC6906 U W U U APPLICATIO S I FOR ATIO LS 330nH 3.3V LTC6906 V+ OUT GND GRD DIV RS 100Ω 1MHz 3.3V CLOAD SET LTC6906 1MHz V+ OUT GND GRD DIV SET RSET 100k RSET 100k 6906 F14 6906 F12 Figure 14. Test Circuit with a Resistive Power Supply 3.5 3.5 3 3 VOUT (V) VOUT (V) Figure 12. Test Circuit with an Inductive Power Supply 2 1 4.75 4.95 4.85 TIME (µs) 2 1 CLOAD = 5pF CLOAD = 10pF CLOAD = 20pF CLOAD = 50pF 0 4.65 5.05 CLOAD 5.15 6906 F13 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: 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. 0 0.4 CLOAD = 5pF CLOAD = 10pF CLOAD = 20pF CLOAD = 50pF 0.5 0.6 0.7 0.8 TIME (µs) 0.9 1.0 1.1 6906 F15 Figure 15. Output Waveforms with a Resistive Supply (See Figure 14) Periodic noise, such as that generated by a switching power supply, can shift the output frequency or increase 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. 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 6906fa 10 LTC6906 U W U U APPLICATIO S I FOR ATIO 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. V+ 3.6V TO 5.5V DC 100Ω OUT GND GRD DIV SET V 1µF RSET 6906 F16 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: tOSC = 1 ƒ OSC V = SET • 10pF ISET If the SET pin is driven with a current source generating ISET, the oscillator output frequency will be: ISET 10pF ƒ OSC ≅ ISET ⎞ ⎛ 25.9mV • n⎜ ⎟ – 2.3mV / °C – 18 ⎝ 82 • 10 A ⎠ LTC6906 + Performance graphs. VSET changes approximately –2.3mV/ °C. At room temperature VSET increases 18mV/octave or 60mV/decade of increase in ISET. 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. LTC6906 V+ (2) V+ OUT GND GRD DIV SET 100kHz TO 1MHz ICTRL 0.65µA TO 6.5µA 6906 F17 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: VSET ≅ VT • Loge ISET IS Figure 17. Current Controlled Oscillator LTC6906 V+ V+ OUT GND GRD DIV SET 6906 F18 ISET ⎛ ⎞ ≅ 25.9mV • Loge ⎜ ⎟ – 2.3mV/ °C ⎝ 82 • 10 –18 A ⎠ 1MHz TO 100kHz RSET 100k VCTRL 0V TO 0.585V Figure 18. Voltage Controlled Oscillator Jitter and Divide Ratio 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 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. 6906fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 11 LTC6906 U TYPICAL APPLICATIO S Setting Frequency to 0.1% Resolution with Standard Resistors 10kHz TO 1MHz LTC6906 2.25V TO 3.6V V ÷10 ÷3 ÷1 + Trimming the Frequency LTC6906 OUT GND GRD DIV SET 2.25V TO 3.6V RA RA < RB/10 1% THIN FILM RB 100k TO 1M 0.1% THIN FILM V+ OUT GND GRD DIV Sine Wave Oscillator 1MHz WITH ±2.5% RANGE 1MHz LTC6906 2.25V TO 3.6V SET RA 97.6k V+ OUT GND GRD DIV SET 0.1µF 1k RSET 100k RB 5k L1 100µH C1 240pF 6906 TA05 6906 TA03 6906 TA04 U PACKAGE DESCRIPTIO S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 2.80 BSC 1.4 MIN 3.85 MAX 2.62 REF 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 0.09 – 0.20 (NOTE 3) 1.90 BSC S6 TSOT-23 0302 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 RELATED PARTS PART NUMBER LTC1799 LTC6900 LTC6902 LTC6903/LTC6904 LTC6905 DESCRIPTION 1kHz to 33MHz ThinSOT Oscillator 1kHz to 20MHz ThinSOT Oscillator Multiphase Oscillator with Spread Spectrum Frequency Modulation 1kHz to 68MHz Serial Port Programmable Oscillator 17MHz to 170MHz ThinSOT Oscillator COMMENTS Single Output, Greater Frequency Range Single Output, Greater Frequency Range 2-, 3- or 4-Phase Outputs Very Wide Frequency Range with Digital Programmability Single Output, Higher Frequency 6906fa 12 Linear Technology Corporation LT/LWI/LT 0705 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005