LTC6906 Micropower, 10kHz to 1MHz Resistor Set Oscillator in SOT-23 DESCRIPTION FEATURES n n n n n n n n n n n 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 (ThinSOT™) Package APPLICATIONS n n n n n 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: 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 L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. DIV Pin = V + ⎧ 10, ⎪ 1MHz ⎛ 100k ⎞ = •⎜ ⎟, N = ⎨ 3, N ⎝ RSET ⎠ ⎪ ⎩ 1, DIV Pin = Open 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. TYPICAL APPLICATION Micropower Clock Generator Typical Supply Current vs Frequency 90 2.25V TO 3.6V 10 3 1 V+ OUT GND GRD DIV SET CL = 5pF TA = 25°C 80 10kHz TO 1MHz RSET 100k TO 1M 6906 TA01 POWER SUPPLY CURRENT (A) NO DECOUPLING CAPACITOR NEEDED LTC6906 70 V+ = 3.6V 60 50 V+ = 2.25V 40 30 20 10 0 0 200 400 600 800 FREQUENCY (kHz) 1000 1200 6906 TA01b 6906fc 1 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) 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 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 Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 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 Δf Frequency Accuracy (Notes 2, 3, 9) V+ = 2.7V to 3.6V 100kHz ≤ f ≤ 1MHz 100kHz ≤ f ≤ 1MHz, LTC6906C 100kHz ≤ f ≤ 1MHz, LTC6906I f = 1MHz, LTC6906H f = 100kHz, LTC6906H MIN l l l l V+ = 2.25V 100kHz ≤ f ≤ 1MHz 100kHz ≤ f ≤ 1MHz, LTC6906C 100kHz ≤ f ≤ 1MHz, LTC6906I f = 1MHz, LTC6906H f = 100kHz, LTC6906H l l l l 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 % % % % % 6906fc 2 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 RSET Frequency-Setting Resistor Range Δf/ΔT Frequency Drift Over Temp (Note 3) RSET = 316k Δf/ΔV Frequency Drift Over Supply (Note 3) Timing Jitter (Note 4) Sf MIN l 100 l 0.06 %/V Pin 3 = V+, 100k ≤ RSET ≤ 1000k Pin 3 = Open, 100k ≤ RSET ≤ 1000k Pin 3 = 0V, 100k ≤ RSET ≤ 1000k 0.03 0.07 0.15 % % % 300 ppm/√kHr Duty Cycle l 45 Operating Supply Range (Note 8) l 2.25 % V l l 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 l l 78 60 100 80 μA μA V+ = 3.6V V+ = 2.25V l l VIL Low Level DIV Input Voltage V+ = 3.6V V+ = 2.25V l l IDIV DIV Input Current (Note 5) Low Level Output Voltage (Note 5) 55 3.6 RSET = 1000k, Pin 3 = 0V, RL = 10M (DIV = 1, fOUT = 100kHz) High Level DIV Input Voltage VOL 50 V+ = 3.6V V+ = 2.25V VIH High Level Output Voltage (Note 5) kΩ V+ = 2.25V to 3.6V, 100k ≤ RSET ≤ 1000k DC VOH 1000 UNITS %/°C Long-Term Stability of Output Frequency Pin 3 = V+ Power Supply Current MAX ±0.005 V+ IS TYP Pin 3 = V+ Pin 3 = 0V 3.1 2.05 V V l l –2 1 –1 0.5 0.2 V V 2 μA μA V+ = 3.6V IOH = –100μA IOH = –1mA l l 3.40 2.80 3.59 3.30 V V V+ = 2.25V IOH = –100μA IOH = –1mA l l 2.15 1.75 2.2 2.0 V V V+ = 3.6V IOL = 100μA IOL = 1mA l l 0.02 0.15 0.2 0.8 V V V+ = 2.25V IOL = 100μA IOL = 1mA l l 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: 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 5: Current into a pin is given as a positive value. Current out of a pin is given as a negative value. l –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. Note 9: Test conditions reflect the master oscillator frequency. The output divider is functionally tested and divided frequency accuracy is guaranteed by design. 6906fc 3 LTC6906 TYPICAL PERFORMANCE CHARACTERISTICS Typical Frequency Error vs Temperature Typical Frequency Error vs Power Supply 0.50 0.40 0.40 1 0.20 0.10 RSET = 1M 0 RSET = 100k –0.10 –0.20 RSET = 100k 0.30 2.25V 0 0.20 3.6V –1 ERROR (%) FREQUENCY ERROR (%) 0.30 FREQUENCY ERROR (%) Typical Frequency Error vs RSET 0.50 2 2.25V 3.6V –2 V+ = 2.25V 0 V+ = 5V –0.10 RSET = 1M –3 –0.20 –0.30 –0.30 –4 –0.40 3 4 SUPPLY VOLTAGE (V) –0.40 –5 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 5 6906 G01 200 POWER SUPPLY CURRENT (A) 180 70 V+ = 3.6V 60 50 V+ = 2.25V 40 30 20 100 1200 1MHz, 2.25V 60 40 85 0 40 20 10 30 LOAD CAPACITANCE (pF) 0.55 VSET AT V+ = 3.6V VSET AT V+ = 2.25V 0.50 0.45 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 18 ISUPPLY 0.60 0.35 100kHz, 2.25V Typical Supply Current vs Temperature, 1MHz CL = 5pF 0.65 0.40 100kHz, 3.6V 6906 G04 90 1200 0.70 80 0 1000 1000 RSET = 100k 0.75 1MHz, 3.6V 120 0 400 600 800 FREQUENCY (kHz) 600 800 RSET (kΩ) 0.80 TA = 25C 160 20 200 400 VSET vs Temperature (VSET is the Voltage Measured at the RSET Pin) 140 10 0 200 6906 G03 Typical Supply Current vs Load Capacitance CL = 5pF TA = 25°C 80 0 6906 G02 Typical Supply Current vs Frequency 90 –0.50 SET PIN VOLTAGE (V) –0.50 2.25 POWER SUPPLY CURRENT (A) 0.10 17 AT V+ = 3.6V CL = 5pF SUPPLY CURRENT (A) SUPPLY CURRENT (A) 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 6906fc 4 LTC6906 PIN FUNCTIONS 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. BLOCK DIAGRAM DECOUPLING NETWORK 6 2 V+ GND VSET VGRD 650mV VSET 4 SET 20 5 GRD 5M fOSC THREE-LEVEL INPUT DETECTOR 800pF IFB ISET = IFB IFB DIV 3 5M DIVIDER SELECT VSET – BUFFER RSET V+ FREQUENCY-TO-CURRENT CONVERTERS VSET + OP AMP VOLTAGE CONTROLLED OSCILLATOR (MASTER OSCILLATOR) fOSC 150Ω DRIVER PROGRAMMABLE DIVIDER (n) (1, 3, 10) OUT 1 fOSC.)[tLĀ3SET 6906 BD 6906fc 5 LTC6906 TEST CIRCUIT LTC6906 SUPPLY VOLTAGE V+ OUT GND GRD DIV SET CTEST EQUIVALENT CIRCUIT OF OSCILLOSCOPE OR FREQUENCY COUNTER PROBE CPROBE RPROBE 10M 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 EQUIVALENT INPUT AND 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 6906fc 6 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: ISET = IFB = VSET • ƒOSC • COSC (1) where COSC is a precision internal capacitor: (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 (3) 1 ƒOSC ƒOUT = ƒOSC N (5) tOUT = N • tOSC Solving for the oscillator period: V 1 tOSC = = SET • COSC ƒOSC ISET tOSC = The digital dividers shown in the Block Diagram further divide the master oscillator frequency by 1, 3 or 10 producing: and COSC = 10pF for the LTC6906 so 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. (6) 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. = RSET • COSC (4) Table 1. Output Frequency Equations PART NUMBER FREQUENCY PERIOD LTC6906 1MHz ⎛100k ⎞ ƒOUT = • ⎜⎜ ⎟⎟ N ⎝RSET ⎠ tOUT ⎛R ⎞ = N • 1μs • ⎜ SET ⎟ ⎝100k ⎠ DIVIDER RATIOS ⎧ 10, ⎪ N = ⎨ 3, ⎪ ⎩ 1, DIV Pin = V + DIV Pin = Open DIV Pin = GND 6906fc 7 LTC6906 APPLICATIONS INFORMATION 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 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. 100 10 1 100 1000 10 OUTPUT FREQUENCY (kHz) 6906 F07 Figure 7. RSET vs Desired Output Frequency (LTC6906) 80 Minimizing Power Consumption CLOAD = 0 V+ = 3V TA = 25°C 70 The supply current of the LTC6906 has four current components: OUT and CLOAD) • Proportional to ISET (which is the current in RSET) 60 ISUPPLY (A) • Constant (Independent V+, ƒ 50 40 30 • Proportional to V+, ƒOUT and CLOAD 20 • Proportional to V+ and RLOAD 10 0 100 An approximate expression for the total supply current is: I+ ≅ 5μA + 6 • ISET + V+ • ƒOUT • (CLOAD + 5pF ) + V+ 2 • RLOAD 6906 F08 Figure 8. Unloaded Supply Current vs RSET VSET V+ + V+ • ƒOUT • (CLOAD + 5pF ) + RSET 2 • RLOAD 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. 1000 RSET (kΩ) 80 V+ = 2.7V 70 POWER SUPPLY CURRENT (A) ≅ 5μA + 6 • 10000 60 1 50 3 40 10 30 20 10 0 0 400 200 600 800 1000 1200 MASTER OSCILLATOR FREQUENCY (kHz) 6906 F09 Figure 9. Supply Current vs Frequency 6906fc 8 LTC6906 APPLICATIONS INFORMATION Guarding Against PC Board Leakage Bypassing the Power Supply 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. 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. 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 V+ NO SOLDER MASK OVER THE GUARD RING 6 GRD 3 DIV 5 SET GUARD RING 4 RSET NO LEAKAGE CURRENT 6906 F10 LEAKAGE CURRENT Figure 10. PC Board Layout with Guard Ring V+ 6 V+ LTC6906-1 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 200μs, this technique can be used to enable the device only when needed and further reduce power consumption. 6906fc 9 LTC6906 APPLICATIONS INFORMATION LS 330nH 3.3V LTC6906 V+ OUT GND GRD DIV SET RS 100Ω 1MHz 3.3V CLOAD RSET 100k LTC6906 V+ OUT GND GRD DIV SET 1MHz CLOAD 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 1 CLOAD = 5pF CLOAD = 10pF CLOAD = 20pF CLOAD = 50pF 0 4.65 4.75 4.95 4.85 TIME (s) 2 5.05 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: CLOAD = 5pF CLOAD = 10pF CLOAD = 20pF CLOAD = 50pF 0 0.4 0.5 0.6 0.7 0.8 TIME (s) 0.9 1.0 1.1 6906 F15 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. 6906fc 10 LTC6906 APPLICATIONS INFORMATION If the SET pin is driven with a current source generating ISET, the oscillator output frequency will be: V+ 3.6V TO 5.5V DC 100Ω 1μF LTC6906 V+ OUT GND GRD DIV SET ƒOSC ≅ 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: V 1 tOSC = = SET • 10pF ƒOSC ISET (2) ISET 10pF ISET ⎛ ⎞ 25.9mV • C n ⎜ ⎟ – 2.3mV/°C –18 ⎝ 82 • 10 A ⎠ 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+ 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 I ≅ VT • Loge SET IS ⎛ ⎞ ISET ≅ 25.9mV • Loge ⎜ ⎟ – 2.3mV/ °C ⎝ 82 • 10 –18 A ⎠ 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. Figure 17. Current Controlled Oscillator LTC6906 V+ V+ OUT GND GRD DIV 1MHz TO 100kHz SET 6906 F18 RSET 100k VCTRL 0V TO 0.585V 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. 6906fc 11 LTC6906 TYPICAL APPLICATIONS Setting Frequency to 0.1% Resolution with Standard Resistors LTC6906 V+ 2.25V TO 3.6V GND 10 3 1 DIV Trimming the Frequency 10kHz TO 1MHz 2.25V TO 3.6V GRD SET 1MHz WITH 2.5% RANGE LTC6906 OUT RA RA < RB/10 1% THIN FILM RB 100k TO 1M 0.1% THIN FILM Sine Wave Oscillator V+ OUT GND GRD DIV SET 1MHz LTC6906 2.25V TO 3.6V RA 97.6k V+ OUT GND GRD DIV SET 0.1μF 1k RSET 100k L1 100μH RB 5k C1 240pF 6906 TA05 6906 TA03 6906 TA04 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636 Rev B) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 3.85 MAX 2.62 REF 1.4 MIN 2.80 BSC 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 0.09 – 0.20 (NOTE 3) 1.90 BSC S6 TSOT-23 0302 REV B NOTE: 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 6906fc 12 LTC6906 REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 12/10 Added Note 9. 2-3 C 11/11 Reinsert symbols missing from Rev B Revised values in the Guarding Against PC Board Leakage section. 9 1, 4, 5, 6, 8, 9, 10, 11, 12 6906fc 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. 13 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 6906fc 14 Linear Technology Corporation LT 1111 REV C • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005