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LTC6908-1/LTC6908-2 Resistor Set SOT-23 Oscillator with Spread Spectrum Modulation FEATURES

DESCRIPTIO
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 400A Supply Current Typical (V+ = 5V, 50kHz) Frequency Error 1.5% Max (TA = 25C, V+ = 3V) 40ppm/C Temperature Stability Fast Start-Up Time: 260s 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 x 3mm) Packages
The LTC(R)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. 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.
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APPLICATIO S

Switching Power Supply Clock Reference Portable and Battery-Powered Equipment Precision Programmable Oscillator Charge Pump Driver
TYPICAL APPLICATIO
VIN 2.8V TO 5.5V CBYP 0.1F
2.25MHz, 2.5V/8A Step-Down Regulator
0 OUTPUT (dBc) SVIN TRACK PVIN V+ OUT1 2.2M 41.2k RT LTC3418 RUN/SS ITH 4.99k 820pF PGOOD PGND SGND SW CIN 100F 0.2H -10 -20 -30 -40 0 OUTPUT (dBc)
VOUT 2.5V 8A COUT 100F x2
LTC6908-1 GND OUT2
44.2k SET fOUT = 10MHz * 10k/RSET MOD
690812 TA01a
1000pF
SYNC/MODE VFB 4.32k 2k
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150kHz to 30MHz Output Frequency Spectrum (9kHz Res BW)
SSFM DISABLED SSFM ENABLED -10 -20 -30 -40 150kHz FREQUENCY (FUNDAMENTAL AND HARMONICS SHOWN)
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30MHz
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LTC6908-1/LTC6908-2 ABSOLUTE
(Note 1)
AXI U RATI GS
Specified Temperature Range (Note 3) LTC6908CS6-1/LTC6908CS6-2 ................ 0C to 70C LTC6908IS6-1/LTC6908IS6-2 .............. -40C to 85C LTC6908HS6-1/LTC6908HS6-2 ......... -40C to 125C LTC6908CDCB-1/LTC6908CDCB-2 .......... 0C to 70C LTC6908IDCB-1/LTC6908IDCB-2 ......... -40C to 85C Storage Temperature Range (S6) ........... -65C to 150C Storage Temperature Range (DCB) ........ -65C to 125C Lead Temperature (Soldering, 10sec) ................... 300C
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 ............ -40C to 85C LTC6908IS6-1/LTC6908IS6-2 .............. -40C to 85C LTC6908HS6-1/LTC6908HS6-2 ......... -40C to 125C LTC6908CDCB-1/LTC6908CDCB-2 ...... -40C to 85C LTC6908IDCB-1/LTC6908IDCB-2 ......... -40C to 85C
PACKAGE/ORDER I FOR ATIO
TOP VIEW OUT2 OUT1 MOD
6
5
4 TOP VIEW V+ 1 6 OUT1 5 OUT2 4 MOD
7
1 SET
2 V
+
3 GND
DCB PACKAGE 6-LEAD (2mm x 3mm) PLASTIC DFN TJMAX = 125C, JA = 64C/W EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER LTC6908CDCB-1 LTC6908IDCB-1 LTC6908CDCB-2 LTC6908IDCB-2
DCB PART MARKING* LBXZ LBXZ LBYB LBYB
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.
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GND 2 SET 3
S6 PACKAGE 6-LEAD PLASTIC TSOT-23 TJMAX = 150C, JA = 230C/W
ORDER PART NUMBER LTC6908CS6-1 LTC6908IS6-1 LTC6908HS6-1 LTC6908CS6-2 LTC6908IS6-2 LTC6908HS6-2
S6 PART MARKING* LTBYC LTBYC LTBYC LTBYD LTBYD LTBYD
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LTC6908-1/LTC6908-2
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. 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 fOUT PARAMETER Frequency Accuracy (Note 4) CONDITIONS V+ = 2.7V 250kHz fOUT 5MHz 250kHz fOUT 5MHz 50kHz fOUT < 250kHz 250kHz fOUT 5MHz 250kHz fOUT 5MHz 50kHz fOUT < 250kHz 5MHz < fOUT 10MHz | fOUT | 1.5% | fOUT | 2.5% | fOUT | 3.5% | fOUT | 2% | fOUT | 3% | fOUT | 4% | fOUT | 4.5%

ELECTRICAL CHARACTERISTICS
MIN
TYP 0.5 2 2.5 1 2.5 3 3.5
MAX 1.5 2.5 3.5 2 3 4 4.5 400 400 2000 400 400 2000 20
UNITS % % % % % % % k k k k k k k %/C %/V %/V % ppm/kHr
V+ = 5V
RSET
Frequency Setting Resistor Range
V+ = 2.7V
20 20 400 20 20 400 10 0.004 0.04 0.4 7.5 10 300
V+ = 5V
fOUT/T fOUT/V+
Frequency Drift Over Temperature Frequency Drift Over Supply (Note 4) Period Variation (Frequency Spreading) Long-Term Stability of Output Frequency (Note 8) Duty Cycle (Note 5)
RSET = 100k V+ = 2.7V to 3.6V, RSET = 100k V+ = 4.5V to 5.5V, RSET = 100k RSET = 100k, MOD Pin = V+, GND or OPEN
0.25 0.9 12.5
No Modulation, 250kHz fOUT 1MHz RSET = 2000k, RL = , fOUT V+ = 5V V+ = 2.7V = 50kHz, MOD Pin = V+

45 2.7
50
55 5.5
% V mA mA mA mA V V A A V V V V
V+ IS
Operating Supply Range Power Supply Current
0.4 0.4 1.25 0.9 V+ - 0.4
0.65 0.6 1.7 1.3 0.4
RSET = 20k, RL = , fOUT = 5MHz, MOD Pin = GND V+ = 5V V+ = 2.7V VIH_MOD VIL_MOD IMOD VOH High Level MOD Input Voltage Low Level MOD Input Voltage MOD Pin Input Current (Note 6) High Level Output Voltage (Note 6) (OUT1, OUT2) MOD Pin = V+, V+ = 5V MOD Pin = GND, V+ = 5V V+ = 5V V+ = 2.7V VOL Low Level Output Voltage (Note 6) V+ = 5V V+ = 2.7V tr tf Output Rise Time (Note 7) Output Fall Time (Note 7) V+ = 5V V+ = 2.7V V+ = 5V V+ = 2.7V IOH = -0.3mA IOH = -1.2mA IOH = -0.3mA IOH = -0.8mA IOL = 0.3mA IOL = 1.2mA IOL = 0.3mA IOL = 0.8mA

-4 4.75 4.4 2.35 1.85
2 -2 4.9 4.7 2.6 2.2 0.05 0.2 0.1 0.4 6 11 5 9
4
0.15 0.5 0.3 0.7
V V V V ns ns ns ns
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LTC6908-1/LTC6908-2 ELECTRICAL CHARACTERISTICS
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 -40C to 85C. Note 3: LTC6908C is guaranteed to meet specified performance from 0C to 70C. The LTC6908C is designed, characterized and expected to meet specified performance from -40C to 85C but is not tested or QA sampled at these temperatures. The LTC6908I is guaranteed to meet the specified performance limits from -40C to 85C. The LTC6908H is guaranteed to meet the specified performance limits from -40C to 125C. Note 4: Frequency accuracy is defined as the deviation from the fOUT equation. Note 5: Guaranteed by 5V test 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 30C 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.
TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Error vs RSET, V+ = 3V
5 4 3 FREQUENCY ERROR (%) 2 1 0 -1 -2 -3 -4 GUARANTEED MIN OVER TEMPERATURE 100k RSET ()
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GUARANTEED MAX OVER TEMPERATURE FREQUENCY ERROR (%)
FREQUENCY ERROR (%)
TYPICAL MAX
TYPICAL MIN
-5 10k
1M
Peak to Peak Jitter vs Output Frequency
1.0 0.9 0.8 SUPPLY CURRENT (mA) 0.7 JITTER (% P-P) 0.6 0.5 0.4 0.3 0.2 0.1 0 10k 100k 1M FREQUENCY (Hz) 10M
690812 G04
5V
1.5 5V SSFM ENABLED 3V SSFM ENABLED 0.5 3V SSFM DISABLED 5V SSFM DISABLED 0 10k 100k 1M FREQUENCY (Hz) 10M
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SUPPLY CURRENT (A)
3V
4
UW
TA = 25C
Frequency Error vs RSET, V+ = 5V
5 4 3 2 1 TYPICAL MAX 0 -1 -2 TYPICAL MIN -3 -4 10M -5 10k 100k RSET ()
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Frequency Error vs Temperature
TA = 25C 1.00 0.75
GUARANTEED MAX OVER TEMPERATURE
0.50 0.25 0 -0.25 -0.50 -0.75 TYPICAL MIN TYPICAL MAX
GUARANTEED MIN OVER TEMPERATURE
1M
10M
-1.00 -40
-20
0 40 20 TEMPERATURE (C)
60
80
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Supply Current vs Output Frequency
800 2.0
Supply Current vs Temperature
CL 5pF ON BOTH OUTPUTS 750 FREQUENCY = 1MHz SSFM DISABLED
700 650 600 550 500 450 400 -40 -20 0 40 20 TEMPERATURE (C) 60 80
690812 G06
V+ = 5V
1.0
V+ = 3V
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LTC6908-1/LTC6908-2 TYPICAL PERFOR A CE CHARACTERISTICS
Output Resistance vs Supply Voltage
500 450 OUTPUT RESISTANCE () 400 350 300 250 200 150 100 50 0 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 6.0 OUTPUT SINKING CURRENT 80ns/DIV
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VOUT (1V/DIV)
OUTPUT SOURCING CURRENT
VOUT (1V/DIV)
Output Frequency Spectrum with SSFM Enabled and Disabled
20dBm SSFM ENABLED (N = 16) 10dB/DIV 10dB/DIV RES BW = 220Hz 20dBm
-80dBm 150kHz FREQUENCY (7.5kHz/DIV)
UW
TA = 25C
Output Operating at 5MHz, V+ = 3V
Output Operating at 10MHz, V+ = 5V
40ns/DIV
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Output Frequency Spectrum with SSFM Enabled and Disabled
RES BW = 9kHz SSFM ENABLED (N = 16)
SSFM DISABLED
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SSFM DISABLED -80dBm 5MHz FREQUENCY (250kHz/DIV)
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LTC6908-1/LTC6908-2 PI FU CTIO S
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 0.1F 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.
BLOCK DIAGRA
V RSET SET 3 VBIAS
+
1
+
GAIN = 1
-
ISET = V+ - V(SET) RSET 1-POLE LPF IMASTER
V+
+ -
2A PSEUDO RANDOM CODE GENERATOR CLK DIVIDE BY 16/32/64
MOD 4
+ -
GND
2A DETECT CLOCK INPUT WHEN A CLOCK SIGNAL IS PRESENT AT THE MOD INPUT, DISABLE THE MODULATION.
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(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 2A. Likewise, driving the MOD pin low requires sinking 2A. 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. 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) MASTER OSCILLATOR V OUT COMPLEMENTARY OR QUADRATURE OUTPUTS
fOUT = fMASTER/2 0 6 OUT1 90/180 5 OUT2
V+ - V(SET) 1.13V
IREF MDAC
MUTE OUTPUT UNTIL STABLE 2 GND POR
3-STATE INPUT DECODER
DIVIDER SELECT
690812 BD
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LTC6908-1/LTC6908-2 OPERATIO
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. 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: 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 1.3 VRES = V+ - VSET 1.2 V+ = 5V 1.1 V+ = 3V 1.0 0.9 0.8 0.1
LTC6908-2 (QUADRATURE) OUT1 OUT2
RSET () 100k 10k 10k
TA = 25C
Figure 1. V+ - VSET Variation with IRES
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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 100k 1M DESIRED OUTPUT FREQUENCY (Hz) 10M
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Figure 2. RSET vs Desired Output Frequency
LTC6908-1 (COMPLEMENTARY) OUT1
1
10 IRES (A)
100
1000
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OUT2
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Figure 3. Output Waveforms for LTC6908-1, LTC6908-2
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LTC6908-1/LTC6908-2 OPERATIO
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. 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: 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
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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. 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 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.
fOUT + 10% 128 STEPS fOUT - 10% tSTEP = N/fOUT tSTEP tREPEAT TIME tREPEAT = ((215 - 1) * N)/fOUT
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Figure 4
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LTC6908-1/LTC6908-2 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. RSET =10k * 10MHz/fOUT 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.
V+ CBYP V+ OUT1 OUT1
ICONTROL
GND
OUT2 V+
SET
MOD
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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+
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V+ CBYP V+ OUT1 OUT1 VCONTROL
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+ -
RSET
GND
OUT2 V+
OUT2
SET
MOD
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FLOAT
fOUT = 10k * 10MHz/RSET(1 - VCONTROL/1.13V)
Figure 6. Voltage Controlled Oscillator
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
OUT2
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LTC6908-1/LTC6908-2 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. 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. 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
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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. 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 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
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LTC6908-1/LTC6908-2 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. HIGH FREQUENCY REJECTION 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. START-UP TIME The start-up time and settling time to within 1% of the final value can be estimated by tSTART RSET * (2.5s/k) + 10s. For instance, with RSET = 100k, the LTC6908 will settle to within 1% of its 1MHz final value in approximately 260s.
STARTUP DELAY (s)
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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. JITTER 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.
10000 TA = 25C V+ = 3V 1000 100 10 1k 10k 100k RSET () 1M 10M
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Figure 7. Start-Up Time
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LTC6908-1/LTC6908-2 TYPICAL APPLICATIO S
VIN 3.3V 3 CIN1 100F x4 4 9 10 22 RSS1 2.2M RPG1 100k 23 28 29 24 RSVIN1 100 CSVIN1 1F X7R PVIN PVIN PVIN PVIN PVIN PVIN PVIN PVIN SVIN LTC3418 CSS1 1000pF X7R 35 5 7 C1A 47pF X7R TRACK PGOOD RUN/SS PGND PGND PGND PGND PGND PGND PGND PGND PGND VREF 38 37 36 34 33 32 19 18 17 16 CREF1 2.2F X7R R2 2k COUT 100F x4 VOUT 1.8V 16A SW SW SW SW SW SW SW SW VFB 1 2 11 12 20 21 30 31 25 C2 1000pF X7R R1 2.55k L1 0.2H
VIN 2.8V TO 5.5V CBYP 0.1F V+ OUT1
LTC6908-1 GND OUT2 RITH 2k
90.9k SET fOUT = 10MHz * 10k/RSET CIN2 100F x4 MOD CITH 2200pF X7R SW SW SW SW SW SW SW SW VFB LTC3418 35 5 7 TRACK PGOOD PGND PGND PGND PGND PGND PGND PGND PGND PGND VREF 38 37 36 34 33 32 19 18 17 16 CREF2 2.2F X7R 1 2 11 12 20 21 30 31 25
RSVIN2 100
CSVIN2 1F X7R
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26 I ROSC1 69.8k 6 TH RT 8 SGND 13 PGND 14 PGND 15 PGND 27 SYNC/MODE
3 4 9 10 22 RPG2 100k 23 28 29 24
L2 0.2H
PVIN PVIN PVIN PVIN PVIN PVIN PVIN PVIN SVIN
C1B 47pF X7R
RUN/SS 26 I ROSC2 69.8k 6 TH RT 8 SGND 13 PGND 14 PGND 15 PGND 27 SYNC/MODE
CIN1, CIN2, COUT: TDK C3225X5R0J107M L1, L2: VISHAY DALE IHLP-2525CZ-01
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Figure 8a. 1.1MHz, 1.8V/16A Step-Down Regulator
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LTC6908-1/LTC6908-2 TYPICAL APPLICATIO S
RES BW = 9kHz
10dB/DIV
10dB/DIV
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150kHz FREQUENCY (3MHz/DIV)
30MHz
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Figure 8b. Output Frequency Spectrum of Two-Phase Regulator, Figure 8a, with SSFM Disabled
RES BW = 9kHz
150kHz FREQUENCY (3MHz/DIV)
30MHz
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Figure 8c. Output Frequency Spectrum of Two-Phase Regulator, Figure 8a, with SSFM Enabled
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LTC6908-1/LTC6908-2 PACKAGE DESCRIPTIO U
DCB Package 6-Lead Plastic DFN (2mm x 3mm)
(Reference LTC DWG # 05-08-1715)
0.70 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 1.35 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 2.00 0.10 (2 SIDES) R = 0.115 TYP R = 0.05 TYP 0.40 0.10 4 6 3.00 0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 3 0.200 REF 0.75 0.05 1 1.65 0.10 (2 SIDES) PIN 1 NOTCH R0.20 OR 0.25 x 45 CHAMFER
(DCB6) DFN 0405
3.55 0.05
1.65 0.05 (2 SIDES)
2.15 0.05
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
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LTC6908-1/LTC6908-2 PACKAGE DESCRIPTIO U
S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
0.62 MAX 0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE ID 0.95 BSC 0.30 - 0.45 6 PLCS (NOTE 3) 0.80 - 0.90 0.20 BSC 1.00 MAX DATUM `A' 0.01 - 0.10 0.09 - 0.20 (NOTE 3) 1.90 BSC
S6 TSOT-23 0302
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.30 - 0.50 REF
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
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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.
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LTC6908-1/LTC6908-2 TYPICAL APPLICATIO
V+ 0.1F RSET V+ OUT1
Quick Evaluation Circuit for Effects Of Frequency Spreading Modulation Rate. (DFN Package Demo Board DC814D-J/K)
OUT1
LTC6908-X GND OUT2 OUT2
SET
MOD
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RELATED PARTS
PART NUMBER LTC1799 LTC6900 LTC6902 LTC6903/LTC6904 LTC6905 LTC6905-XXX LTC6906/LTC6907 DESCRIPTION 1kHz to 33MHz ThinSOT Oscillator, Resistor Set 1kHz to 20MHz ThinSOT Oscillator, Resistor Set Multiphase Oscillator with Spread Spectrum Modulation 1kHz to 68MHz Serial Port Programmable Oscillator 17MHz to 170MHz ThinSOT Oscillator, Resistor Set Fixed Frequency ThinSOT Oscillators, Up to 133MHz Micropower ThinSOT Oscillator, Resistor Set COMMENTS Wide Frequency Range Low Power, Wide Frequency Range 2-, 3-, or 4-Phase Outputs 0.1% Frequency Resolution, I2C or SPI Interface High Frequency, 100s Startup, 7ps RMS Jitter No Trim Components Required 10kHz to 1MHz or 40kHz to 4MHz, 36A at 400kHz
16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006
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Doubling the Output Frequency
V+ 0.1F RSET V+ OUT1 OUT NC7SZ86 LTC6908-2 GND OUT2 SET
V+ N = 64 N = 32 N = 16 JUMPER BLOCK 1nF
MOD
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