LTC6908-1/LTC6908-2 Resistor Set SOT-23 Oscillator with Spread Spectrum Modulation U FEATURES DESCRIPTIO ■ The LTC®6908 is an easy-to-use precision oscillator that provides 2-outputs, shifted by either 180° or 90°. The oscillator frequency is programmed by a single external resistor (RSET) and spread spectrum frequency modulation (SSFM) can be activated for improved electromagnetic compatibility (EMC) performance. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ LTC6908-1: Complementary Outputs (0°/180°) LTC6908-2: Quadrature Outputs (0°/90°) 50kHz to 10MHz Frequency Range One External Resistor Sets the Frequency Optional Spread Spectrum Frequency Modulation for Improved EMC Performance ±10% Frequency Spreading 400µA Supply Current Typical (V+ = 5V, 50kHz) Frequency Error ≤1.5% Max (TA = 25°C, V+ = 3V) ±40ppm/°C Temperature Stability Fast Start-Up Time: 260µs Typical (1MHz) Outputs Muted Until Stable Operates from a Single 2.7V to 5.5V Supply Available in Low Profile (1mm) ThinSOT and DFN (2mm × 3mm) Packages The LTC6908 operates with a single 2.7V to 5.5V supply and provides rail-to-rail, 50% duty cycle square wave outputs. A single resistor from 10k to 2M is used to select an oscillator frequency from 50kHz to 10MHz (5V supply). The oscillator can be easily programmed using the simple formula outlined below: fOUT =10MHz • 10k/RSET The LTC6908’s SSFM capability modulates the output frequency by a pseudorandom noise (PRN) signal to decrease the peak electromagnetic radiation level and improve EMC performance. The amount of frequency spreading is fixed at ±10% of the center frequency. When SSFM is enabled, the rate of modulation is selected by the user. The three possible modulation rates are fOUT/16, fOUT/32 and fOUT/64. U APPLICATIO S ■ ■ ■ ■ Switching Power Supply Clock Reference Portable and Battery-Powered Equipment Precision Programmable Oscillator Charge Pump Driver , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. U TYPICAL APPLICATIO 150kHz to 30MHz Output Frequency Spectrum (9kHz Res BW) 2.25MHz, 2.5V/8A Step-Down Regulator 0.1µF SVIN TRACK PVIN V+ LTC6908-1 GND 2.2M RUN/SS ITH 1000pF SET MOD 690812 TA01a fOUT = 10MHz • 10k/RSET LTC3418 41.2k OUT2 44.2k SW RT OUT1 4.99k 820pF PGOOD PGND SGND SYNC/MODE VFB 4.32k 2k VOUT 2.5V 8A SSFM DISABLED –10 –20 –30 –40 COUT 100µF ×2 0 OUTPUT (dBc) CBYP CIN 100µF 0.2µH OUTPUT (dBc) 0 VIN 2.8V TO 5.5V SSFM ENABLED –10 –20 –30 –40 150kHz 30MHz FREQUENCY (FUNDAMENTAL AND HARMONICS SHOWN) 690812 TA01b 690812fa 1 LTC6908-1/LTC6908-2 U W W W ABSOLUTE AXI U RATI GS (Note 1) Total Supply Voltage (V+ to GND) ...............................6V Maximum Voltage on any Pin (GND – 0.3V) ≤ VPIN ≤ (V+ + 0.3V) Output Short Circuit Duration .......................... Indefinite Operating Temperature Range (Note 2) LTC6908CS6-1/LTC6908CS6-2 ............ –40°C to 85°C LTC6908IS6-1/LTC6908IS6-2 .............. –40°C to 85°C LTC6908HS6-1/LTC6908HS6-2 ......... –40°C to 125°C LTC6908CDCB-1/LTC6908CDCB-2 ...... –40°C to 85°C LTC6908IDCB-1/LTC6908IDCB-2 ......... –40°C to 85°C Specified Temperature Range (Note 3) LTC6908CS6-1/LTC6908CS6-2 ................ 0°C to 70°C LTC6908IS6-1/LTC6908IS6-2 .............. –40°C to 85°C LTC6908HS6-1/LTC6908HS6-2 ......... –40°C to 125°C LTC6908CDCB-1/LTC6908CDCB-2 .......... 0°C to 70°C LTC6908IDCB-1/LTC6908IDCB-2 ......... –40°C to 85°C Storage Temperature Range (S6) ........... –65°C to 150°C Storage Temperature Range (DCB) ........ –65°C to 125°C Lead Temperature (Soldering, 10sec) ................... 300°C U W U PACKAGE/ORDER I FOR ATIO MOD OUT2 OUT1 TOP VIEW 6 5 4 TOP VIEW 7 V 6 OUT1 5 OUT2 SET 3 4 MOD S6 PACKAGE 6-LEAD PLASTIC TSOT-23 TJMAX = 150°C, θJA = 230°C/W 3 GND 2 + SET 1 V+ 1 GND 2 DCB PACKAGE 6-LEAD (2mm × 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 64°C/W EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB ORDER PART NUMBER DCB PART MARKING* ORDER PART NUMBER S6 PART MARKING* LTC6908CDCB-1 LTC6908IDCB-1 LTC6908CDCB-2 LTC6908IDCB-2 LBXZ LBXZ LBYB LBYB LTC6908CS6-1 LTC6908IS6-1 LTC6908HS6-1 LTC6908CS6-2 LTC6908IS6-2 LTC6908HS6-2 LTBYC LTBYC LTBYC LTBYD LTBYD LTBYD 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 ranges. *The temperature grade is identified by a label on the shipping container. 690812fa 2 LTC6908-1/LTC6908-2 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Test conditions are V+ = 2.7V to 5.5V, RL = 5k, CL = 5pF unless otherwise noted. The modulation is turned off (MOD is connected to OUT2) unless otherwise specified. RSET is defined as the resistor connected from the SET pin to the V+ pin. SYMBOL ΔfOUT RSET PARAMETER CONDITIONS Frequency Accuracy (Note 4) V+ = 2.7V 250kHz ≤ fOUT ≤ 5MHz 250kHz ≤ fOUT ≤ 5MHz 50kHz ≤ fOUT < 250kHz V+ = 5V Frequency Setting Resistor Range V+ = 2.7V V+ = 5V ΔfOUT/ΔT ΔfOUT/ΔV+ MIN TYP MAX UNITS ● ● ±0.5 ±2 ±2.5 ±1.5 ±2.5 ±3.5 % % % 250kHz ≤ fOUT ≤ 5MHz 250kHz ≤ fOUT ≤ 5MHz 50kHz ≤ fOUT < 250kHz 5MHz < fOUT ≤ 10MHz ● ● ● ±1 ±2.5 ±3 ±3.5 ±2 ±3 ±4 ±4.5 % % % % | ΔfOUT | ≤ 1.5% | ΔfOUT | ≤ 2.5% | ΔfOUT | ≤ 3.5% ● ● 20 20 400 400 400 2000 k k k | ΔfOUT | ≤ 2% | ΔfOUT | ≤ 3% | ΔfOUT | ≤ 4% | ΔfOUT | ≤ 4.5% ● ● ● 20 20 400 10 400 400 2000 20 k k k k Frequency Drift Over Temperature RSET = 100k ● ±0.004 Frequency Drift Over Supply (Note 4) V+ = 2.7V to 3.6V, RSET = 100k V+ = 4.5V to 5.5V, RSET = 100k RSET = 100k, MOD Pin = V+, GND or OPEN ● ● 0.04 0.4 0.25 0.9 %/V %/V ±10 ±12.5 % Period Variation (Frequency Spreading) ● ±7.5 Long-Term Stability of Output Frequency (Note 8) Duty Cycle (Note 5) V+ IS 300 No Modulation, 250kHz ≤ fOUT ≤ 1MHz Operating Supply Range Power Supply Current RSET = 2000k, RL = ∞, fOUT V+ = 5V V+ = 2.7V High Level MOD Input Voltage VIL_MOD Low Level MOD Input Voltage IMOD VOH VOL MOD Pin Input Current (Note 6) High Level Output Voltage (Note 6) (OUT1, OUT2) Low Level Output Voltage (Note 6) ● 45 ● 2.7 50 ppm/√kHr 55 % 5.5 V = 50kHz, MOD Pin = V+ RSET = 20k, RL = ∞, fOUT = 5MHz, MOD Pin = GND V+ = 5V V+ = 2.7V VIH_MOD %/°C ● ● 0.4 0.4 0.65 0.6 mA mA ● ● 1.25 0.9 1.7 1.3 mA mA ● V+ – 0.4 V ● 0.4 V 4 µA µA MOD Pin = V+, V+ = 5V MOD Pin = GND, V+ = 5V ● ● –4 2 –2 V+ = 5V IOH = –0.3mA IOH = –1.2mA ● ● 4.75 4.4 4.9 4.7 V V V+ = 2.7V IOH = –0.3mA IOH = –0.8mA ● ● 2.35 1.85 2.6 2.2 V V V+ = 5V IOL = 0.3mA IOL = 1.2mA ● ● 0.05 0.2 0.15 0.5 V V V+ = 2.7V IOL = 0.3mA IOL = 0.8mA ● ● 0.1 0.4 0.3 0.7 V V tr Output Rise Time (Note 7) V+ = 5V V+ = 2.7V 6 11 ns ns tf Output Fall Time (Note 7) V+ = 5V V+ = 2.7V 5 9 ns ns 690812fa 3 LTC6908-1/LTC6908-2 ELECTRICAL CHARACTERISTICS Note 6: To conform to the Logic IC Standard, current out of a pin is arbitrarily given a negative value. Note 7: Output rise and fall times are measured between the 10% and the 90% power supply levels with no output loading. These specifications are based on characterization. Note 8: Long term drift on silicon oscillators is primarily due to the movement of ions and impurities within the silicon and is tested at 30°C under otherwise nominal operating conditions. Long term drift is specified as ppm/√kHr due to the typically non-linear nature of the drift. To calculate drift for a set time period, translate that time into thousands of hours, take the square root and multiply by the typical drift number. For instance, a year is 8.77kHr and would yield a drift of 888ppm at 300ppm/√kHr. Ten years is 87.7kHr and would yield a drift of 2,809 ppm at 300 ppm/√kHr. Drift without power applied to the device may be approximated as 1/10th of the drift with power, or 30ppm/√kHr for a 300ppm/√kHr device. 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: LTC6908C and LTC6908I are guaranteed functional over the operating temperature range of –40°C to 85°C. Note 3: LTC6908C is guaranteed to meet specified performance from 0°C to 70°C. The LTC6908C 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 LTC6908I is guaranteed to meet the specified performance limits from –40°C to 85°C. The LTC6908H is guaranteed to meet the specified performance limits from –40°C to 125°C. Note 4: Frequency accuracy is defined as the deviation from the fOUT equation. Note 5: Guaranteed by 5V test U W TYPICAL PERFOR A CE CHARACTERISTICS Frequency Error vs RSET, V+ = 5V 5 FREQUENCY ERROR (%) 3 TA = 25°C TYPICAL MAX 1 0 –1 TYPICAL MIN –2 –3 –4 GUARANTEED MIN OVER TEMPERATURE –5 10k 100k 0.75 3 2 Frequency Error vs Temperature 1.00 TA = 25°C 4 GUARANTEED MAX OVER TEMPERATURE FREQUENCY ERROR (%) 4 5 GUARANTEED MAX OVER TEMPERATURE 2 FREQUENCY ERROR (%) Frequency Error vs RSET, V+ = 3V 1 TYPICAL MAX 0 –1 GUARANTEED MIN OVER TEMPERATURE –2 TYPICAL MIN –3 –5 10k 10M RSET (Ω) 100k 1M TYPICAL MIN –20 0 40 20 TEMPERATURE (°C) 60 80 690812 G03 Supply Current vs Output Frequency Supply Current vs Temperature 800 CL 5pF ON BOTH OUTPUTS 750 FREQUENCY = 1MHz SSFM DISABLED 2.0 0.9 5V 0.6 3V 0.4 0.3 0.2 SUPPLY CURRENT (µA) SUPPLY CURRENT (mA) 0.8 JITTER (% P-P) –0.50 –1.00 –40 10M 1.0 1.5 1.0 5V SSFM ENABLED 3V SSFM ENABLED 0.5 3V SSFM DISABLED 0.1 0 10k –0.25 690812 G02 Peak to Peak Jitter vs Output Frequency 0.5 TYPICAL MAX 0 RSET (Ω) 690812 G01 0.7 0.25 –0.75 –4 1M 0.50 700 V+ = 5V 650 600 550 V+ = 3V 500 450 5V SSFM DISABLED 100k 1M FREQUENCY (Hz) 10M 690812 G04 0 10k 100k 1M FREQUENCY (Hz) 10M 690812 G05 400 –40 –20 0 40 20 TEMPERATURE (°C) 60 80 690812 G06 690812fa 4 LTC6908-1/LTC6908-2 U W TYPICAL PERFOR A CE CHARACTERISTICS 500 OUTPUT SOURCING CURRENT 350 300 250 VOUT (1V/DIV) 400 VOUT (1V/DIV) TA = 25°C 450 200 150 OUTPUT SINKING CURRENT 690812 G08 80ns/DIV 40ns/DIV 100 690812 G09 50 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 6.0 690812 G07 Output Frequency Spectrum with SSFM Enabled and Disabled 20dBm RES BW = 220Hz Output Frequency Spectrum with SSFM Enabled and Disabled 20dBm SSFM ENABLED (N = 16) RES BW = 9kHz SSFM ENABLED (N = 16) 10dB/DIV 0 2.5 10dB/DIV OUTPUT RESISTANCE (Ω) Output Operating at 10MHz, V+ = 5V Output Operating at 5MHz, V+ = 3V Output Resistance vs Supply Voltage SSFM DISABLED –80dBm 150kHz FREQUENCY (7.5kHz/DIV) SSFM DISABLED 690812 G10 –80dBm 5MHz 690812 G11 FREQUENCY (250kHz/DIV) 690812fa 5 LTC6908-1/LTC6908-2 U U U PI FU CTIO S (DCB Package/S6 Package) MOD (Pin 6/Pin 4): Modulation-Setting Input. This threestate input selects among four modulation rate settings. The MOD pin should be tied to ground for the fOUT/16 modulation rate. Floating the MOD pin selects the fOUT/32 modulation rate. The MOD pin should be tied to V+ for the fOUT/64 modulation rate. Tying one of the outputs to the MOD pin turns the modulation off. To detect a floating MOD pin, the LTC6908 attempts to pull the pin toward midsupply. This is realized with two internal current sources, one tied to V+ and MOD and the other one tied to ground and MOD. Therefore, driving the MOD pin high requires sourcing approximately 2µA. Likewise, driving the MOD pin low requires sinking 2µA. When the MOD pin is floated, it must be bypassed by a 1nF capacitor to ground. Any AC signal coupling to the MOD pin could potentially be detected and stop the frequency modulation. SET (Pin 1/Pin 3): Frequency-Setting Resistor Input. The value of the resistor connected between this pin and V+ determines the oscillator frequency. The voltage on this pin is held by the LTC6908 to approximately 1.1V below the V+ voltage. For best performance, use a precision metal film resistor with a value between 20k and 400k and limit the capacitance on this pin to less than 10pF. V+ (Pin 2/Pin 1): Voltage Supply (2.7V ≤ V+ ≤ 5.5V). This supply must be kept free from noise and ripple. It should be bypassed directly to a ground plane with a 0.1µF capacitor. GND (Pin 3/Pin 2): Ground. Should be tied to a ground plane for best performance. OUT1 (Pin 4/Pin 6), OUT2 (Pin 5/Pin 5): Oscillator Outputs. These pins can drive 5k and/or 10pF loads. Larger loads may cause inaccuracies due to supply bounce at high frequencies. W BLOCK DIAGRA Exposed Pad (Pin 7/NA): Ground. The Exposed Pad must be soldered to PCB. (S6 Package Pin Numbers) I fMASTER = 20MHz • 10k • + MASTER = 20MHz • 10k/RSET V – V(SET) V + + 1 GAIN = 1 RSET SET MASTER OSCILLATOR V+ – V(SET) ≈ 1.13V OUT ISET = VBIAS 0 V – 3 V+ – V(SET) RSET fOUT = fMASTER/2 1-POLE LPF COMPLEMENTARY OR QUADRATURE OUTPUTS IMASTER 6 OUT1 90/180 5 OUT2 MUTE OUTPUT UNTIL STABLE IREF 2 GND MDAC POR V+ + – 2µA PSEUDO RANDOM CODE GENERATOR MOD 4 3-STATE INPUT DECODER + – CLK DIVIDE BY 16/32/64 DIVIDER SELECT 2µA GND DETECT CLOCK INPUT WHEN A CLOCK SIGNAL IS PRESENT AT THE MOD INPUT, DISABLE THE MODULATION. 690812 BD 690812fa 6 LTC6908-1/LTC6908-2 U OPERATIO The voltage on the SET pin is forced to approximately 1.1V below V+ by the PMOS transistor and its gate bias voltage. This voltage is accurate to ±5% at a particular input current and supply voltage (see Figure 1). The LTC6908 is optimized for use with resistors between 20k and 400k, corresponding to output frequencies between 250kHz and 5MHz. Accurate frequencies up to 10MHz (RSET = 10k) are attainable if the supply voltage is greater than 4V. The RSET resistor, connected between the V+ and SET pins, locks together the (V+ – VSET) voltage and the current ISET. This allows the parts to attain excellent frequency accuracy regardless of the precision of the SET pin voltage. The master oscillation frequency is: output frequency, fOUT, below (see Figure 2): fOUT = 10MHz • 10k/RSET When the spread spectrum frequency modulation (SSFM) is disabled, the frequency fOUT is the final output frequency. When SSFM is enabled, 0.9 • fOUT is the minimum output frequency and 1.1 • fOUT is the maximum output frequency. Both outputs are nominally 50% duty cycle. There are 2 possible output configurations for the LTC6908, shown in Figure 3. Output Configurations The only difference between the two versions of the LTC6908 is the phase relationship between the two outputs. The LTC6908-1 outputs are 180 degrees out of phase and the LTC6908-2 outputs are 90 degrees out of phase. These convenient output options are useful in synchronizing the clocking of multiple phase switching regulator designs. In very high current applications, a significant improvement 10M 1M RSET (Ω) As shown in the Block Diagram, the LTC6908’s master oscillator is controlled by the ratio of the voltage between the V+ and SET pins and the current entering the SET pin (IMASTER). When the spread spectrum frequency modulation (SSFM) is disabled, IMASTER is strictly determined by the (V+ – VSET) voltage and the RSET resistor. When SSFM is enabled, IMASTER is modulated by a filtered pseudorandom noise (PRN) signal. Here the IMASTER current is a random value uniformly distributed between (ISET – 10%) and (ISET + 10%). In this way the frequency of the master oscillator is modulated to produce an approximately flat frequency spectrum that is centered at the frequency set by the ISET current, with a bandwidth equal to approximately 20% of the center frequency. 100k fMASTER = 20MHz • 10k/RSET The master oscillator signal is divided by 2 before driving the output pins, resulting in the simple formula for the 1.4 100k 1M DESIRED OUTPUT FREQUENCY (Hz) TA = 25°C 10M 690812 F02 Figure 2. RSET vs Desired Output Frequency 1.3 VRES = V+ – VSET 10k 10k 1.2 LTC6908-1 (COMPLEMENTARY) V+ = 5V OUT1 1.1 V+ = 3V OUT2 1.0 LTC6908-2 (QUADRATURE) 0.9 OUT1 0.8 0.1 1 10 IRES (µA) 100 1000 690812 F01 Figure 1. V+ – VSET Variation with IRES OUT2 690812 F03 Figure 3. Output Waveforms for LTC6908-1, LTC6908-2 690812fa 7 LTC6908-1/LTC6908-2 U OPERATIO To disable the SSFM, connect one of the outputs to the MOD pin. An AC detector circuit shuts down the modulation circuitry if a frequency in the vicinity of the output frequency is detected at the MOD pin. in conducted EMI results due to the reduced levels of input and output ripple currents. The LTC6908-1 is ideal for use with two single output switching regulators. The quadrature outputs of the LTC6908-2, together with two dual output switching regulators, provide the 0°, 90°, 180° and 270° phased shifted clocks for four-phase control. As stated previously, the modulating waveform is a pseudorandom noise-like waveform. The pseudorandom signal is generated by a linear feedback shift register that is 15 bits long. The pseudorandom sequence will repeat every (215 – 1) • N clock cycles. This guarantees a repetition rate below 20Hz for output frequencies up to 10MHz. Seven bits of the shift register are sent in parallel to the MDAC which produces the modulating current waveform. Being a digitally generated signal, the output of the MDAC is not a perfectly smooth waveform, but consists of (27) discrete steps that change every shift register clock cycle. Note that the shift register clock is the output frequency, fOUT, divided by N, where N is the modulation rate divider setting, which is determined by the state of the MOD pin. The MOD pin should be tied to ground for the N = 16 setting. Floating the MOD pin selects N = 32. The MOD pin should be tied to V+ for the N = 64 setting. The rise and fall times are typically 6ns with a 5V supply and 11ns with a 3V supply. An internal counter mutes the outputs for the first 64 clock cycles after power-up, ensuring that the first clock cycle is close to the desired operating frequency. Spread Spectrum Frequency Modulation The LTC6908 provides the additional feature of spread spectrum frequency modulation (SSFM). The oscillator’s frequency is modulated by a pseudorandom noise (PRN) signal to spread the oscillator’s energy over a wide frequency band. This spreading decreases the peak electromagnetic radiation levels and improves electromagnetic compatibility (EMC) performance. The amount of frequency spreading is fixed at 20% (±10%), where frequency spreading is defined as: The output of the MDAC is then filtered by a lowpass filter with a corner frequency set to the modulation rate (fOUT/N). This limits the frequency change rate and softens corners of the waveform, but allows the waveform to fully settle at each frequency step. The rise and fall times of this single pole filter are approximately 0.35/fCORNER. This is beneficial when the LTC6908 is used to clock switching regulators as will be discussed in the Applications Information section. Figure 4 illustrates how the output frequency varies over time. Frequency Spreading (in %) = 100 • ( fMAX – fMIN)/fOUT The IMASTER current is a dynamic signal generated by a multiplying digital to analog converter (MDAC) referenced to ISET and lowpass filtered. IMASTER varies in a pseudorandom noise-like manner between 0.9 • ISET and 1.1 • ISET. This causes the output frequency to vary in a pseudorandom noise-like manner between 0.9 • fOUT and 1.1 • fOUT. FREQUENCY fOUT + 10% 128 STEPS fOUT – 10% tSTEP = N/fOUT tSTEP tREPEAT TIME tREPEAT = ((215 – 1) • N)/fOUT 690812 F04 Figure 4 690812fa 8 LTC6908-1/LTC6908-2 U W U U APPLICATIO S I FOR ATIO SELECTING THE FREQUENCY-SETTING RESISTOR The LTC6908 has an output frequency range spanning 50kHz to 10MHz. However, accuracy may suffer if the oscillator is operated at a frequency greater than 5MHz with a supply voltage lower than 4V. With a linear relationship correspondence between oscillation period and resistance, a simple equation relates resistance with frequency. R SETMIN = 10k (5V supply), 20k (3V supply), RSETMAX = 2M Any resistor, RSET, tolerance will shift the output frequency, fOUT. ALTERNATIVE METHODS OF SETTING THE OUTPUT FREQUENCY OF THE LTC6908 The oscillator may be programmed by any method that sources a current into the SET pin. The circuit in Figure 5 sets the oscillator frequency using a programmable current source and in the expression for fOUT, the resistor RSET is replaced by the ratio of 1.1V/ICONTROL. As already explained in the Operation section, the voltage difference between V+ and SET is approximately 1.1V ±5%, therefore, the Figure 5 circuit is less accurate than if a resistor controls the output frequency. ICONTROL CBYP V+ OUT1 GND OUT2 OUT1 V+ SET VCONTROL + – V+ OUT1 GND OUT2 OUT1 V+ SET RSET MOD OUT2 FLOAT 690812 F06 fOUT = 10k • 10MHz/RSET(1 – VCONTROL/1.13V) Figure 6. Voltage Controlled Oscillator RSET =10k • 10MHz/fOUT V+ CBYP V+ MOD OUT2 FLOAT 690812 F05 fOUT = 10k • (10MHz/1.13V) • ICONTROL(A) Figure 5. Current Controlled Oscillator Figure 6 shows the LTC6908 configured as a VCO. A voltage source is connected in series with an external 10k resistor. The output frequency, fOUT, will vary with VCONTROL, that is the voltage source connected between V+ and the SET pin. Again, this circuit decouples the relationship between the input current and the voltage between V+ and SET; the frequency accuracy will be degraded. The oscillator frequency, however, will increase monotonically with decreasing VCONTROL. SETTING THE MODULATION RATE OF THE LTC6908 The modulation rate of the LTC6908 is equal to fOUT/N, where N is the modulation rate divider setting, which is determined by the state of the MOD pin. The MOD pin should be tied to ground for the N = 16 setting. Floating the MOD pin selects N = 32. The MOD pin should be tied to V+ for the N = 64 setting. To disable the SSFM, connect one of the outputs to the MOD pin. An AC detector circuit shuts down the modulation circuitry if a frequency that is close to the output frequency is detected at the MOD pin. DRIVING LOGIC CIRCUITS The outputs of the LTC6908 are suitable for driving general digital logic circuits. However, the form of frequency spreading used in the LTC6908 may not be suitable for many logic designs. Many logic designs have fairly tight timing and cycle-to-cycle jitter requirements. These systems often benefit from a spread spectrum clocking system where the frequency is slowly and linearly modulated by a triangular waveform, not a pseudorandom waveform. This type of frequency spreading maintains a minimal difference in the timing from one clock edge to the next adjacent clock edge (cycle-to-cycle jitter). The LTC6908 uses a pseudorandom modulating signal where the frequency transitions have been slowed and the corners rounded by a first order lowpass filter with a corner frequency set to the modulation rate (fOUT/N), where N is the modulation rate divider setting, which is determined by the state 690812fa 9 LTC6908-1/LTC6908-2 U W U U APPLICATIO S I FOR ATIO of the MOD pin. This filtered modulating signal may be acceptable for many logic systems but the cycle-to-cycle jitter issues must be considered carefully. DRIVING SWITCHING REGULATORS The LTC6908 is designed primarily to provide an accurate and stable clock for switching regulator systems. The complementary (LTC6908-1) or quadrature (LTC6908-2) CMOS logic outputs are suitable for directly driving most switching regulators and switching controllers. Linear Technology has a broad line of fully integrated switching regulators and switching regulator controllers designed for synchronization to an external clock. All of these parts have one pin assigned for external clock input. The nomenclature varies depending on the part’s family history. SYNC, PLLIN, SYNC/MODE, SHDN, EXTCLK, FCB and S/S (shorthand for SYNC/SHDN) are examples of clock input pin names used with Linear Technology ICs. to ground for the N = 16 setting. Floating the MOD pin selects N = 32. The MOD pin should be tied to V+ for the N = 64 setting. This is an important feature when driving a switching regulator. The switching regulator is itself a servo loop with a bandwidth typically on the order of 1/10, but can vary from 1/50 to 1/2 of the operating frequency. When the clock frequency’s transition is within the bandwidth of the switching regulator, the regulator’s output stays in regulation. If the transition is too sharp, beyond the bandwidth of the switching regulator, the regulator’s output will experience a sharp jump and then settle back into regulation. If the bandwidth of the regulator is sufficiently high, beyond fOUT/N, then there will not be any regulation issues. For the best EMC performance, the LTC6908 should be run with the MOD pin tied to ground (SSFM enabled, modulation rate set to fOUT/16). Regulatory testing is done with strictly specified bandwidths and conditions. Modulating faster than the test bandwidth or as close to the bandwidth as possible gives the lowest readings. The optimal modulating rate is not as straightforward when the goal is to lower radiated signal levels interfering with other circuitry in the system. The modulation rate will have to be evaluated with the specific system conditions to determine the optimal rate. Depending on the specific frequency synchronization method a switching regulator employs, the modulation rate must be within the synchronization capability of the regulator. Many regulators use a phase-locked loop (PLL) for synchronization. For these parts, the PLL loop filter should be designed to have sufficient capture range and bandwidth. One aspect of the output voltage that will change is the output ripple voltage. Every switching regulator has some output ripple at the clock frequency. For most switching regulator designs with fixed MOSFET’s, fixed inductor, fixed capacitors, the amount of ripple will vary some with the regulators operating frequency (the main exception being hysteretic architecture regulators). An increase in frequency results in lower ripple and a frequency decrease gives more ripple. This is true for static frequencies or dynamic frequency modulated systems. If the modulating signal was a triangle wave, the regulator’s output would have a ripple that is amplitude modulated by the triangle wave. This repetitive signal on the power supply could cause system problems by mixing with other desired signals creating distortion. Depending on the inductor design and triangle wave frequency, it may even result in an audible noise. The LTC6908 uses a pseudorandom noise-like signal. On an oscilloscope, it looks essentially noise-like of even amplitude. The signal is broadband and any mixing issues are eliminated. Additionally, the pseudorandom signal repeats at such a low rate that it is well below the audible range. The frequency hopping transitions of the LTC6908 are slowed by a lowpass filter. The corner frequency of this filter is set to the modulation rate (fOUT/N), where N is the modulation rate divider setting, which is determined by the state of the MOD pin. The MOD pin should be tied The LTC6908 directly drives many switching regulators. The LTC6908 with the spread spectrum frequency modulation results in improved EMC performance. If the bandwidth of the switching regulator is sufficient, not a difficult requirement in most cases, the regulator’s regulation, efficiency 690812fa 10 LTC6908-1/LTC6908-2 U W U U APPLICATIO S I FOR ATIO and load response are maintained while peak electromagnetic radiation (or conduction) is reduced. Output ripple may be somewhat increased, but its behavior is very much like noise and its system impact is benign. Figure 7 shows start-up times for various RSET resistors. An internal counter mutes the outputs for the first 64 clock cycles after power-up, ensuring that the first clock cycle is close to the desired operating frequency. HIGH FREQUENCY REJECTION JITTER Using the LTC6908 in spread spectrum mode naturally eliminates any concerns for output frequency accuracy and stability as it is continually hopping to new settings. In fixed frequency applications however, some attention to V+ supply voltage ripple is required to minimize additional output frequency error. Ripple frequency components on the supply line near the programmed output frequency of the LTC6908 in excess of 30mVP-P could create an additional 0.2% of frequency error. In applications where a fixed frequency LTC6908 output clock is used to synchronize the same switching regulator that provides the V+ supply to the oscillator, noticeable jitter of the clock may occur if the ripple exceeds 30mVP-P. The Peak-to-Peak Jitter vs Output Frequency graph, in the Typical Performance Characteristics section, shows the typical clock jitter as a function of oscillator frequency and power supply voltage. These specifications assume that the capacitance on SET is limited to less than 10pF, as suggested in the Pin Functions description. If this requirement is not met, the jitter will increase. The start-up time and settling time to within 1% of the final value can be estimated by tSTART ≈ RSET • (2.5µs/k) + 10µs. For instance, with RSET = 100k, the LTC6908 will settle to within 1% of its 1MHz final value in approximately 260µs. STARTUP DELAY (µs) START-UP TIME 10000 TA = 25°C V+ = 3V 1000 100 10 1k 10k 100k RSET (Ω) 1M 10M 690812 F07 Figure 7. Start-Up Time 690812fa 11 LTC6908-1/LTC6908-2 U TYPICAL APPLICATIO S 3 VIN 3.3V CIN1 100µF ×4 4 9 10 22 RSS1 2.2M RPG1 100k 23 28 29 24 CSVIN1 1µF X7R RSVIN1 100Ω C1A 47pF X7R VIN 2.8V TO 5.5V 0.1µF V+ SW PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW SVIN VFB OUT1 2 11 12 OUT2 SET MOD C2 1000pF X7R 20 R1 2.55k 21 30 31 25 5 TRACK PGND PGOOD PGND RUN/SS PGND 26 I ROSC1 69.8k 6 TH RT 8 SGND 13 PGND 14 PGND 15 PGND 27 SYNC/MODE PGND PGND PGND PGND PGND PGND VREF 38 37 36 COUT 100µF ×4 34 33 VOUT 1.8V 16A R2 2k 32 19 18 17 16 LTC6908-1 GND L1 0.2µH LTC3418 35 CSS1 1000pF X7R 7 CBYP PVIN 1 CREF1 2.2µF X7R 90.9k RITH 2k fOUT = 10MHz • 10k/RSET 3 CIN2 100µF ×4 4 9 10 22 RPG2 100k 23 28 29 24 RSVIN2 100Ω CSVIN2 1µF X7R CITH 2200pF X7R PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW PVIN SW SVIN VFB 1 L2 0.2µH 2 11 12 20 21 30 31 25 LTC3418 35 5 7 C1B 47pF X7R TRACK PGND PGOOD PGND RUN/SS 26 I ROSC2 69.8k 6 TH RT 8 SGND 13 PGND 14 PGND 15 PGND 27 SYNC/MODE PGND PGND PGND PGND PGND PGND PGND VREF 38 37 36 34 33 32 19 18 17 16 CREF2 2.2µF X7R CIN1, CIN2, COUT: TDK C3225X5R0J107M L1, L2: VISHAY DALE IHLP-2525CZ-01 690812 TA02 Figure 8a. 1.1MHz, 1.8V/16A Step-Down Regulator 690812fa 12 LTC6908-1/LTC6908-2 U TYPICAL APPLICATIO S 10dB/DIV RES BW = 9kHz 30MHz 150kHz FREQUENCY (3MHz/DIV) 690812 TA03 Figure 8b. Output Frequency Spectrum of Two-Phase Regulator, Figure 8a, with SSFM Disabled 10dB/DIV RES BW = 9kHz 30MHz 150kHz FREQUENCY (3MHz/DIV) 690812 TA04 Figure 8c. Output Frequency Spectrum of Two-Phase Regulator, Figure 8a, with SSFM Enabled 690812fa 13 LTC6908-1/LTC6908-2 U PACKAGE DESCRIPTIO DCB Package 6-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1715) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 (2 SIDES) 2.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 1.35 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP R = 0.05 TYP 2.00 ±0.10 (2 SIDES) 3.00 ±0.10 (2 SIDES) 0.40 ± 0.10 4 6 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R0.20 OR 0.25 × 45° CHAMFER PIN 1 BAR TOP MARK (SEE NOTE 6) 3 0.200 REF 0.75 ±0.05 1 (DCB6) DFN 0405 0.25 ± 0.05 0.50 BSC 1.35 ±0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (TBD) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 690812fa 14 LTC6908-1/LTC6908-2 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 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 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 690812fa 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. 15 LTC6908-1/LTC6908-2 U TYPICAL APPLICATIO Quick Evaluation Circuit for Effects Of Frequency Spreading Modulation Rate. (DFN Package Demo Board DC814D-J/K) V+ V+ OUT1 OUT1 Doubling the Output Frequency V+ V+ LTC6908-X 0.1µF RSET GND OUT2 SET OUT2 MOD RSET GND SET 690812 TA05a V+ OUT OUT2 NC7SZ86 MOD 690812 TA05b N = 64 N = 32 OUT1 LTC6908-2 0.1µF 1nF N = 16 JUMPER BLOCK RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1799 1kHz to 33MHz ThinSOT Oscillator, Resistor Set Wide Frequency Range LTC6900 1kHz to 20MHz ThinSOT Oscillator, Resistor Set Low Power, Wide Frequency Range LTC6902 Multiphase Oscillator with Spread Spectrum Modulation 2-, 3-, or 4-Phase Outputs LTC6903/LTC6904 1kHz to 68MHz Serial Port Programmable Oscillator 0.1% Frequency Resolution, I2C or SPI Interface LTC6905 17MHz to 170MHz ThinSOT Oscillator, Resistor Set High Frequency, 100µs Startup, 7ps RMS Jitter LTC6905-XXX Fixed Frequency ThinSOT Oscillators, Up to 133MHz No Trim Components Required LTC6906/LTC6907 Micropower ThinSOT Oscillator, Resistor Set 10kHz to 1MHz or 40kHz to 4MHz, 36µA at 400kHz 690812fa 16 Linear Technology Corporation LT 0506 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 2006