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LTC6902

6902f

■

2-, 3- or 4-Phase Outputs

■

Optional Spread Spectrum Frequency Modulation

for Improved EMC Performance

■

5kHz to 20MHz Frequency Range

■

One External Resistor Sets the Frequency

■

One External Resistor Sets Percent Frequency

Spreading

■

400µA Typical Supply Current, V

= 3V, 1MHz

■

Frequency Error ≤1.5% Max, 5kHz to 10MHz

= 25°C)

■

Frequency Error ≤2% Max, 5kHz to 10MHz

= 0°C to 70°C)

■

±40ppm/°C Temperature Stability

■

Fast Start-Up Time: 50µs to 1.5ms

■

100Ω CMOS Output Driver

■

Operates from a Single 2.7V to 5.5V Supply

■

Available in 10-Lead MS Package

Multiphase Oscillator

with Spread Spectrum

Frequency Modulation

■

Switching Power Supply Clock Reference

■

Portable and Battery-Powered Equipment

■

PDAs

■

Cell Phones

■

Clocking Switched Capacitor Filters

The LTC

6902 is a precision, low power and easy-to-use

oscillator that provides multiphase outputs in a small

package. The oscillator frequency is set by a single exter-

nal resistor (R

SET

). The LTC6902 also provides an optional

spread spectrum frequency modulation (SSFM) capability

that can be activated and controlled by an additional

external resistor (R

MOD

The LTC6902’s master oscillator is controlled by the R

SET

resistor and has a range of 100kHz and 20MHz. In order

to accommodate a wider output frequency range, a pro-

grammable divider (divide by 1, 10 or 100) is included.

The integrated programmable multiphase circuit pro-

vides either 2-, 3- or 4-phase waveforms.

The LTC6902’s SSFM capability modulates the oscillator’s

frequency by a pseudorandom noise (PRN) signal to

spread the oscillator’s energy over a wide frequency band.

This spreading decreases the peak electromagnetic radia-

tion level and improves electromagnetic compatibility

(EMC) performance. The amount of frequency spreading

is programmable by a single additional external resistor

MOD

) and is disabled by grounding the MOD pin.

, LTC and LT are registered trademarks of Linear Technology Corporation.

DIV

OUT1

OUT2

OUT4

OUT3

OUT2

OUT1

SET

MOD

GND

OUT4

OUT3

LTC6902

SET

10k

MOD

10k

OPEN

6902 TA01

0.1µF

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

500kHz, 4-Phase Clock with 20% Frequency Spreading

FREQUENCY (kHz)

RELATIVE AMPLITUDE (dBm)

–40

–20

6902 TA02

–60

–80

–100

400

450

550 600

500

OUTPUT

SPECTRUM

WITH 20%

SPREADING

OUTPUT

SPECTRUM

WITH SSFM

DISABLED

Output Frequency Spectrum With and Without SSFM

Summary of content (16 pages)

PAGE 1
LTC6902 Multiphase Oscillator with Spread Spectrum Frequency Modulation U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 2-, 3- or 4-Phase Outputs Optional Spread Spectrum Frequency Modulation for Improved EMC Performance 5kHz to 20MHz Frequency Range One External Resistor Sets the Frequency One External Resistor Sets Percent Frequency Spreading 400µA Typical Supply Current, VS = 3V, 1MHz Frequency Error ≤1.
PAGE 2
LTC6902 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Supply Voltage (V +) to GND ........................– 0.3V to 6V Voltage On Any Pin (Referred to GND) ......................... – 0.3V to (V + + 0.3V) Operating Temperature Range (Note 9) LTC6902C .......................................... – 40°C to 85°C LTC6902I ............................................ – 40°C to 85°C Specified Temperature Range (Note 10) LTC6902C ..........................................
PAGE 3
LTC6902 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. V+ = 2.7V to 5.5V, RL= 5k, CL = 5pF, Pin 3 (PH) = 0V (2-phase, M = 1) unless otherwise specified. Pin 9 (MOD) is at 0V unless otherwise specified. RSET is defined as a resistor connected from the SET pin to the V+ pin. RMOD is defined as a resistor connected from the MOD pin to the V+ pin.
PAGE 4
LTC6902 ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: Frequencies near 100kHz and 1MHz may be generated using two different values of RSET (see Applications Information). For these frequencies, the error is specified under the following assumption: 20k < RSET ≤ 400k for 5kHz ≤ fOUT ≤ 10MHz. Note 3: Frequency accuracy is defined as the deviation from the fOUT equation.
PAGE 5
LTC6902 U W TYPICAL PERFOR A CE CHARACTERISTICS Output Operating at 10MHz, VS = 3V Output Operating at 20MHz, VS = 5V V + = 5V, RSET = 10k, CL = 10pF V + = 3V, RSET = 20k, CL = 10pF 1V/DIV 1V/DIV 0V 0V 69012 G06 12.5ns/DIV 25ns/DIV 69012 G07 U QUICK DESIG GUIDE V+ RMOD 0.1µF RSET SET DIV DIV MOD PH PH GND 10MHz  20kΩ  •  ; 5kHz ≤ fOUT ≤ 20MHz N • M  RSET  R Spreading Percentage = 20 • SET RMOD fOUT = LTC6902 V+ OUT1 OUT4 OUT4 OUT2 OUT3 OUT3 OUT2 6902 F01 OUT1 Figure 1.
PAGE 6
LTC6902 U QUICK DESIG GUIDE Step 3. Calculating the RSET Resistor Value Step 4. Calculating the RMOD Resistor Value The RSET resistor, the multiphase mode and the divider setting set the output frequency (fOUT) for constant frequency applications. For SSFM applications, the maximum frequency excursion (fMAX) is equal to fOUT. (Note: For constant frequency applications RMOD is not required.
PAGE 7
LTC6902 U U U PI FU CTIO S or it should be surrounded by a ground shield to prevent excessive coupling from other PCB traces. OUT1, OUT2, OUT3, OUT4 (Pins 4, 5, 6 and 7): Oscillator Outputs. These pins can drive 5kΩ and/or 10pF loads. Larger loads may cause inaccuracies due to supply bounce at high frequencies. GND (Pin 8): Ground. Should be tied to a ground plane for best performance. SET (Pin 10): Frequency Setting Resistor Input.
PAGE 8
LTC6902 U THEORY OF OPERATIO As shown in the Block Diagram, the LTC6902’s master oscillator is controlled by the ratio of the voltage between the V+ and SET pins (V+ – VSET) and the current entering the master oscillator, IMASTER. When the spread spectrum frequency modulation (SSFM) is disabled, IMASTER is strictly determined by the V+ – VSET voltage and the ISET current. When SSFM is enabled, the current IMOD (modulation current) is subtracted from the ISET current to determine the IMASTER current value.
PAGE 9
LTC6902 U THEORY OF OPERATIO divider goes through further division. In addition to further division, the duty cycle of the output depends on the multiphase mode selected. Figure 4 shows the waveform at each output for 2-, 3- and 4-phase modes. 2-Phase Mode In 2-phase mode, all outputs are nominally 50% duty cycle. OUT1 and OUT2 are 180 degrees out of phase. Stated differently, OUT2 is OUT1 inverted.
PAGE 10
LTC6902 U THEORY OF OPERATIO by 90 degrees, OUT3 lags OUT2 by 90 degrees and OUT4 lags OUT3 by 90 degrees. The signals are generated by flip-flops. The output frequency is the programmable divider’s output further divided 4 (M = 4). The multiphase mode is determined by the state of the PH input (Pin 3). Tie the PH pin to GND or drive it below 0.5V to select the 2-phase mode. The PH pin may be floated or driven to midsupply to select the 3-phase mode.
PAGE 11
LTC6902 U THEORY OF OPERATIO clock cycle. Note that the shift register clock is the master oscillator’s output divided by 3200. This results in a somewhat slow moving modulating signal where each step is separated in time by 3200/fMASTER seconds and the pseudorandom sequence repeats every (512 • 3200)/fMASTER seconds. The servo loop in the LTC6902 cannot respond instantaneously to each step due to its limited bandwidth. The VMOD voltage steps are converted to frequency steps by the servo loop.
PAGE 12
LTC6902 U W U U APPLICATIO S I FOR ATIO Table 1. Recommended Frequency Range vs Programmable Divider Setting for SSFM Applications or for Low Jitter Constant Frequency Applications DIVIDER SETTING N=1 FREQUENCY RANGE (fOUT • M) DIV (Pin 2) = GND 2MHz to 20MHz N = 10 DIV (Pin 2) = Open 200kHz to 2MHz N = 100 DIV (Pin 2) = V+ < 200kHz Note: The frequency range numbers are for a 5V supply where a 20MHz output is the maximum frequency supported. For low supply applications (2.
PAGE 13
LTC6902 U W U U APPLICATIO S I FOR ATIO spreading is determined, the RMOD value is calculated using the simple equation below: RMOD = 20 • RSET Spreading Percentage The only limitations for this formula are in the RMOD value range and the spreading percentage range. The range of the RMOD resistor value is the same as that for RSET ranging from 10k to 400k. The LTC6902 is tested and specified for spreading of 10% and 40%.
PAGE 14
LTC6902 U W U U APPLICATIO S I FOR ATIO generator is driven by the master oscillator frequency, not the output frequency. This gives some design flexibility in the choice of the RSET and the programmable divider setting. When making the choice, usually the faster master oscillator is the better choice. This is especially true when the main goal is to lower peak radiated or conducted signal levels measured during regulatory agency testing.
PAGE 15
LTC6902 U W U U APPLICATIO S I FOR ATIO POWER SUPPLY SENSITIVITY Figure 6 shows the output frequency sensitivity to power supply voltage at several different temperatures. The LTC6902 has a guaranteed voltage coefficient of 0.1%/V but, as Figure 6 shows, the typical supply sensitivity is twice as low. START-UP TIME The start-up time and settling time to within 1% of the final value can be estimated by tSTART ≅ RSET(3.7µs/kΩ) + 10µs.
PAGE 16
LTC6902 U PACKAGE DESCRIPTIO MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.2 – 3.45 (.126 – .136) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.50 0.305 ± 0.038 (.0197) (.0120 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 10 9 8 7 6 3.00 ± 0.102 (.118 ± .004) NOTE 4 4.90 ± 0.15 (1.93 ± .006) DETAIL “A” 0.497 ± 0.076 (.0196 ± .003) REF 0° – 6° TYP GAUGE PLANE 1 2 3 4 5 0.53 ± 0.01 (.021 ± .006) 1.10 (.