Datasheet LTC6908CS6-2, LTC6908 Datasheet (Linear Technology)

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LTC6908-1/LTC6908-2

690812fa

690812 TA01b

FREQUENCY

(FUNDAMENTAL AND HARMONICS SHOWN)

150kHz

–10

–20

–30

–40

–10

–20

–30

–40

OUTPUT (dBc) OUTPUT (dBc)

30MHz

SSFM DISABLED

SSFM ENABLED

Resistor Set SOT-23 Oscillator with Spread Spectrum Modulation

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 (R

SET

) 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:

OUT

=10MHz • 10k/R

SET

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 ﬁ xed 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 f

OUT

/16,

OUT

/32 and f

OUT

/64.

■

Switching Power Supply Clock Reference

■

Portable and Battery-Powered Equipment

■

Precision Programmable Oscillator

■

Charge Pump Driver

■

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 Proﬁ le (1mm) ThinSOT and DFN

(2mm × 3mm) Packages

APPLICATIO S

FEATURES

DESCRIPTIO

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

All other trademarks are the property of their respective owners.

2.25MHz, 2.5V/8A Step-Down Regulator

TYPICAL APPLICATIO

BYP

44.2k

OUT1

OUT2

OUT

= 10MHz • 10k/R

SET

MOD

GND

SET

690812 TA01a

SVINTRACK

LTC3418

100µF

0.2µH

LTC6908-1

RUN/SS I

PGOOD

PGND SGND

SYNC/MODE V

820pF

1000pF

0.1µF C

OUT

100µF ×2

OUT

2.5V 8A

4.32k

41.2k

2.2M

2.8V TO 5.5V

4.99k

150kHz to 30MHz Output

Frequency Spectrum

(9kHz Res BW)

LTC6908-1/LTC6908-2

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Total Supply Voltage (V+ to GND) ...............................6V

Maximum Voltage on any Pin (GND – 0.3V) ≤ V

≤ (V+ + 0.3V)

Output Short Circuit Duration .......................... Indeﬁ nite

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

(Note 1)

ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

UUW

TOP VIEW

MOD

OUT2

OUT1

SET

GND

DCB PACKAGE

6-LEAD (2mm × 3mm) PLASTIC DFN

JMAX

= 125°C, θJA = 64°C/W

EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB

V+ 1

GND 2

SET 3

6 OUT1

5 OUT2

4 MOD

TOP VIEW

S6 PACKAGE

6-LEAD PLASTIC TSOT-23

JMAX

= 150°C, θJA = 230°C/W

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 speciﬁ ed with wider operating temperature ranges. *The temperature grade is identiﬁ ed by a label on the shipping container.

Speciﬁ ed 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

LTC6908-1/LTC6908-2

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The

●

denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at T

= 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 speciﬁ ed. R

SET

is deﬁ ned as the resistor connected

from the SET pin to the V

pin.

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Δf

OUT

Frequency Accuracy (Note 4) V+ = 2.7V 250kHz ≤ f

OUT

≤ 5MHz

250kHz ≤ f

OUT

≤ 5MHz

50kHz ≤ f

OUT

< 250kHz

●

±0.5

±2

±2.5

±1.5 ±2.5 ±3.5

% % %

= 5V 250kHz ≤ f

OUT

≤ 5MHz

250kHz ≤ f

OUT

≤ 5MHz

50kHz ≤ f

OUT

< 250kHz

5MHz < f

OUT

≤ 10MHz

●

±1

±2.5

±3

±3.5

±2 ±3 ±4

±4.5

% % % %

SET

Frequency Setting Resistor Range V+ = 2.7V | Δf

OUT

| ≤ 1.5%

| Δf

OUT

| ≤ 2.5%

| Δf

OUT

| ≤ 3.5%

●

20 20

400

400 400

2000

k k k

= 5V | Δf

OUT

| ≤ 2%

| Δf

OUT

| ≤ 3%

| Δf

OUT

| ≤ 4%

| Δf

OUT

| ≤ 4.5%

●

20 20

400

400 400

2000

k k k k

Δf

OUT

/ΔT Frequency Drift Over Temperature R

SET

= 100k

●

±0.004 %/°C

Δf

OUT

/ΔV+Frequency Drift Over Supply (Note 4) V+ = 2.7V to 3.6V, R

SET

= 100k

= 4.5V to 5.5V, R

SET

= 100k

●

0.04

0.4

0.25

0.9

%/V %/V

Period Variation (Frequency Spreading)

SET

= 100k, MOD Pin = V+, GND or OPEN

●

±7.5 ±10 ±12.5 %

Long-Term Stability of Output Frequency (Note 8)

300 ppm/√kHr

Duty Cycle (Note 5) No Modulation, 250kHz ≤ f

OUT

≤ 1MHz

●

45 50 55 %

Operating Supply Range

●

2.7 5.5 V

Power Supply Current R

SET

= 2000k, RL = ∞, f

OUT

= 50kHz, MOD Pin = V

V+ = 5V V

= 2.7V

●

0.4

0.65

0.6

mA mA

SET

= 20k, RL = ∞, f

OUT

= 5MHz, MOD Pin = GND

= 5V

= 2.7V

●

1.25

0.9

1.7

1.3

mA mA

IH_MOD

High Level MOD Input Voltage

●

V+ – 0.4 V

IL_MOD

Low Level MOD Input Voltage

●

0.4 V

MOD

MOD Pin Input Current (Note 6) MOD Pin = V+, V+ = 5V

MOD Pin = GND, V

= 5V

●

–4

–2

4µA

µA

High Level Output Voltage (Note 6) (OUT1, OUT2)

V+ = 5V IOH = –0.3mA I

= –1.2mA

●

4.75

4.4

4.9

4.7

V V

= 2.7V IOH = –0.3mA

= –0.8mA

●

2.35

1.85

2.6

2.2

V V

Low Level Output Voltage (Note 6) V+ = 5V IOL = 0.3mA

= 1.2mA

●

0.05

0.2

0.15

0.5

V V

V+ = 2.7V IOL = 0.3mA I

= 0.8mA

●

0.1

0.4

0.3

0.7

V V

Output Rise Time (Note 7) V+ = 5V

= 2.7V

ns ns

Output Fall Time (Note 7) V+ = 5V

= 2.7V

5 9

ns ns

LTC6908-1/LTC6908-2

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TEMPERATURE (°C)

–

SUPPLY CURRENT (µA)

690812 G06

–

20 0 40

800

V+ = 5V

V+ = 3V

CL 5pF ON BOTH OUTPUTS FREQUENCY = 1MHz SSFM DISABLED

SET

(Ω)

10k

FREQUENCY ERROR (%)

–1

–2

–3

–4

–5

100k 1M 10M

690812 G01

TA = 25°C

TYPICAL MAX

GUARANTEED MAX OVER TEMPERATURE

GUARANTEED MIN OVER TEMPERATURE

TYPICAL MIN

SET

(Ω)

10k

FREQUENCY ERROR (%)

–1

–2

–3

–4

–5

100k 1M 10M

690812 G02

TA = 25°C

TYPICAL MAX

GUARANTEED MAX OVER TEMPERATURE

GUARANTEED MIN OVER TEMPERATURE

TYPICAL MIN

TEMPERATURE (°C)

–

FREQUENCY ERROR (%)

690812 G03

–

20 0 40

1.00

0.75

0.50

0.25

–

0.25

–

0.50

–

0.75

–

1.00 80

TYPICAL MAX

TYPICAL MIN

FREQUENCY (Hz)

10k

JITTER (%

P-P

)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

100k 1M 10M

690812 G04

FREQUENCY (Hz)

10k

SUPPLY CURRENT (mA)

2.0

1.5

1.0

0.5

100k 1M 10M

690812 G05

5V SSFM DISABLED

3V SSFM DISABLED

5V SSFM ENABLED

3V SSFM ENABLED

Frequency Error vs R

SET

= 3V

Frequency Error vs R

SET

= 5V Frequency Error vs Temperature

Peak to Peak Jitter vs Output Frequency

Supply Current vs Output Frequency Supply Current vs Temperature

TYPICAL PERFOR A CE 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 –40°C to 85°C.

Note 3: LTC6908C is guaranteed to meet speciﬁ ed performance from 0°C to 70°C. The LTC6908C is designed, characterized and expected to meet speciﬁ ed performance from –40°C to 85°C but is not tested or QA sampled at these temperatures. The LTC6908I is guaranteed to meet the speciﬁ ed performance limits from –40°C to 85°C. The LTC6908H is guaranteed to meet the speciﬁ ed performance limits from –40°C to 125°C.

Note 4: Frequency accuracy is deﬁ ned as the deviation from the f

OUT

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 speciﬁ cations 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 speciﬁ ed 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.

ELECTRICAL CHARACTERISTICS

LTC6908-1/LTC6908-2

690812fa

690812 G10

150kHz

FREQUENCY (7.5kHz/DIV)

SSFM DISABLED

RES BW = 220Hz

SSFM ENABLED

(N = 16)

20dBm

–

80dBm

10dB/DIV

690812 G11

5MHz

20dBm

–80dBm

10dB/DIV

FREQUENCY (250kHz/DIV)

SSFM DISABLED

RES BW = 9kHz

SSFM ENABLED

(N = 16)

690812 G09

40ns/DIV

OUT

(1V/DIV)

SUPPLY VOLTAGE (V)

2.5

OUTPUT RESISTANCE (Ω)

4.5

5.5

690812 G07

3.0 3.5

4.0 5.0

6.0

OUTPUT SINKING CURRENT

OUTPUT SOURCING CURRENT

TA = 25°C

Output Resistance vs Supply Voltage

TYPICAL PERFOR A CE CHARACTERISTICS

Output Operating at 5MHz, V

= 3V

Output Operating at 10MHz, V+ = 5V

Output Frequency Spectrum with SSFM Enabled and Disabled

690812 G08

80ns/DIV

OUT

(1V/DIV)

LTC6908-1/LTC6908-2

690812fa

BLOCK DIAGRA

OUT1

690812 BD

OUT2

GND

SET

MOD 4

GND

COMPLEMENTARY

QUADRATURE

OUTPUTS

MUTE OUTPUT UNTIL STABLE

POR

1-POLE

LPF

90/180

DIVIDE BY

16/32/64

3-STATE

INPUT DECODER

DETECT

CLOCK INPUT

WHEN A CLOCK SIGNAL IS PRESENT AT THE MOD INPUT, DISABLE THE MODULATION.

DIVIDER SELECT

PSEUDO RANDOM

CODE GENERATOR

–

GAIN = 1

– V(SET) ≈ 1.13V

MASTER

= 20MHz • 10k •

OUT

= f

MASTER

= 20MHz • 10k/R

SET

V+ – V(SET)

MASTER

OSCILLATOR

MASTER

REF

MDAC

CLK

2µA

BIAS

SET

OUT

+ –

SET

V+ – V(SET)

PI FU CTIO S

UUU

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 ﬁ lm resistor with a value between 20k and 400k and limit the capacitance on this pin to less than 10pF.

(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.

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 f

OUT

/16

modulation rate. Floating the MOD pin selects the f

OUT

/32

modulation rate. The MOD pin should be tied to V

for the

OUT

/64 modulation rate. Tying one of the outputs to the MOD pin turns the modulation off. To detect a ﬂ oating 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 ﬂ oated, 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.

(DCB Package/S6 Package)

(S6 Package Pin Numbers)

LTC6908-1/LTC6908-2

690812fa

RES

(µA)

RES

= V

– V

SET

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.1 10 100 1000

690812 F01

V+ = 5V

V+ = 3V

TA = 25°C

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

MASTER

). When the spread spectrum frequency modula-

tion (SSFM) is disabled, I

MASTER

is strictly determined by

the (V

– V

SET

) voltage and the R

SET

resistor. When SSFM is

enabled, I

MASTER

is modulated by a ﬁ ltered pseudorandom

noise (PRN) signal. Here the I

MASTER

current is a random

value uniformly distributed between (I

SET

– 10%) and (I

SET

+ 10%). In this way the frequency of the master oscillator is modulated to produce an approximately ﬂ at frequency spectrum that is centered at the frequency set by the I

SET

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 (R

SET

= 10k) are attainable if the supply voltage is greater than 4V. The R

SET

resistor, connected between the V+ and SET pins,

locks together the (V

– V

SET

) voltage and the current I

SET

. This allows the parts to attain excellent frequency accuracy regardless of the precision of the SET pin voltage. The master oscillation frequency is:

MASTER

= 20MHz • 10k/R

SET

The master oscillator signal is divided by 2 before driving the output pins, resulting in the simple formula for the

Figure 3. Output Waveforms for LTC6908-1, LTC6908-2

Figure 2. R

SET

vs Desired Output Frequency

Figure 1. V

– V

SET

Variation with I

RES

OUT1

OUT2

OUT1

LTC6908-1 (COMPLEMENTARY)

LTC6908-2 (QUADRATURE)

OUT2

690812 F03

output frequency, f

OUT

, below (see Figure 2):

OUT

= 10MHz • 10k/R

SET

When the spread spectrum frequency modulation (SSFM) is disabled, the frequency f

OUT

is the ﬁ nal output fre-

quency. When SSFM is enabled, 0.9 • f

OUT

is the minimum

output frequency and 1.1 • f

OUT

is the maximum output

frequency. Both outputs are nominally 50% duty cycle. There are 2

possible output conﬁ gurations for the LTC6908, shown in Figure 3.

Output Conﬁ gurations

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 signiﬁ cant improvement

DESIRED OUTPUT FREQUENCY (Hz)

10k

SET

(Ω)

100k

10M

100k 1M 10M

690812 F02

LTC6908-1/LTC6908-2

690812fa

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 ﬁ rst 64 clock cycles after power-up, ensuring that the ﬁ rst 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 ﬁ xed at 20% (±10%), where frequency spreading is deﬁ ned as:

Frequency Spreading (in %) = 100 • ( f

MAX

– f

MIN

)/f

OUT

The I

MASTER

current is a dynamic signal generated by a multiplying digital to analog converter (MDAC) referenced to I

SET

and lowpass ﬁ ltered. I

MASTER

varies in a pseudorandom

noise-like manner between 0.9 • I

SET

and 1.1 • I

SET

. This causes the output frequency to vary in a pseudorandom noise-like manner between 0.9 • f

OUT

and 1.1 • f

OUT

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 (2

– 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 (2

) discrete steps that change every shift register clock cycle. Note that the shift register clock is the output frequency, f

OUT

, 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 ﬁ ltered by a lowpass ﬁ lter with a corner frequency set to the modulation rate (f

OUT

/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 ﬁ lter are approximately 0.35/f

CORNER

. This is beneﬁ cial 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.

Figure 4

REPEAT

= ((215 – 1) • N)/f

OUT

STEP

= N/f

OUT

+ 10%

128 STEPS

REPEAT

STEP

OUT

– 10%

TIME

FREQUENCY

690812 F04

LTC6908-1/LTC6908-2

690812fa

APPLICATIO S I FOR ATIO

WUU

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.

SET

=10k • 10MHz/f

OUT

SETMIN

= 10k (5V supply), 20k (3V supply),

SETMAX

= 2M

Any resistor, R

SET

, tolerance will shift the output frequency,

OUT

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 f

OUT

, the resistor R

SET

replaced by the ratio of 1.1V/I

CONTROL

. 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.

Figure 5. Current Controlled Oscillator

Figure 6 shows the LTC6908 conﬁ gured as a VCO. A voltage source is connected in series with an external 10k resistor. The output frequency, f

OUT

, will vary with V

CONTROL

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

Figure 6. Voltage Controlled Oscillator

and SET; the frequency accuracy will be degraded. The oscillator frequency, however, will increase monotonically with decreasing V

CONTROL

SETTING THE MODULATION RATE OF THE LTC6908

The modulation rate of the LTC6908 is equal to f

OUT

/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 beneﬁ t 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 ﬁ rst order lowpass ﬁ lter with a corner frequency set to the modulation rate (f

OUT

/N), where N is the modula-

tion rate divider setting, which is determined by the state

BYP

CONTROL

OUT1

OUT2

OUT1

OUT

= 10k • (10MHz/1.13V) • I

CONTROL

(A)

OUT2

MOD

FLOAT

GND

SET

690812 F05

BYP

CONTROL

OUT1

OUT2

OUT1

OUT

= 10k • 10MHz/R

SET

(1 – V

CONTROL

/1.13V)

OUT2

MOD

FLOAT

GND

SET

690812 F06

SET

+ –

LTC6908-1/LTC6908-2

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APPLICATIO S I FOR ATIO

WUU

of the MOD pin. This ﬁ ltered 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 f

OUT

/16). Regulatory testing is done with strictly speciﬁ ed 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 speciﬁ c system conditions to determine the optimal rate. Depending on the speciﬁ c 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 ﬁ lter should be designed to have sufﬁ cient capture range and bandwidth.

The frequency hopping transitions of the LTC6908 are slowed by a lowpass ﬁ lter. The corner frequency of this ﬁ lter is set to the modulation rate (f

OUT

/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. 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 sufﬁ ciently high, beyond f

OUT

/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 ﬁ xed MOSFET’s, ﬁ xed inductor, ﬁ xed 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 sufﬁ cient, not a difﬁ cult requirement in most cases, the regulator’s regulation, efﬁ ciency

LTC6908-1/LTC6908-2

690812fa

SET

(Ω)

100

STARTUP DELAY (µs)

1000

1k 100k 1M 10M

690812 F07

10k

10000

TA = 25°C V

= 3V

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 ﬁ xed 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 30mV

P-P

could create an additional 0.2% of frequency error. In applications where a ﬁ xed 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 30mV

P-P

START-UP TIME

The start-up time and settling time to within 1% of the ﬁ nal value can be estimated by t

START

≈ R

SET

• (2.5µs/k) + 10µs.

For instance, with R

SET

= 100k, the LTC6908 will settle to

within 1% of its 1MHz ﬁ nal value in approximately 260µs.

APPLICATIO S I FOR ATIO

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Figure 7 shows start-up times for various R

SET

resistors. An internal counter mutes the outputs for the ﬁ rst 64 clock cycles after power-up, ensuring that the ﬁ rst 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 speciﬁ cations 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.

Figure 7. Start-Up Time

LTC6908-1/LTC6908-2

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TYPICAL APPLICATIO S

Figure 8a. 1.1MHz, 1.8V/16A Step-Down Regulator

TRACK

PGOOD

RUN/SS

SGND

PGND

SYNC/MODE

PGND

REF

LTC3418

0.2µH

C2 1000pF X7R

OUT

100µF ×4

IN1

100µF ×4

SVIN1

1µF X7R

3.3V

OUT

1.8V 16A

2.55k

R2 2k

PG1

100k

SS1

2.2M

SS1

1000pF

X7R

C1A

47pF

X7R

ITH

2200pF

X7R

REF1

2.2µF X7R

ITH

OSC1

69.8k

SVIN1

100Ω

SVIN2

1µF X7R

SVIN2

100Ω

REF2

2.2µF X7R

C1B

47pF

X7R

690812 TA02

TRACK

PGOOD

RUN/SS

SGND

PGND

SYNC/MODE

PGND

REF

LTC3418

0.2µH

IN2

100µF ×4

IN1

, C

IN2

, C

OUT

: TDK C3225X5R0J107M

L1, L2: VISHAY DALE IHLP-2525CZ-01

PG2

100k

OSC2

69.8k

BYP

90.9k

OUT1

OUT2

OUT

= 10MHz • 10k/R

SET

MOD

GND

SET

LTC6908-1

0.1µF

2.8V TO 5.5V

LTC6908-1/LTC6908-2

690812fa

TYPICAL APPLICATIO S

Figure 8b. Output Frequency Spectrum of Two-Phase Regulator, Figure 8a, with SSFM Disabled

FREQUENCY (3MHz/DIV)

150kHz

10dB/DIV

690812 TA03

30MHz

RES BW = 9kHz

FREQUENCY (3MHz/DIV)

150kHz

10dB/DIV

690812 TA04

30MHz

RES BW = 9kHz

Figure 8c. Output Frequency Spectrum of Two-Phase Regulator, Figure 8a, with SSFM Enabled

LTC6908-1/LTC6908-2

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PACKAGE DESCRIPTIO

DCB Package

6-Lead Plastic DFN (2mm × 3mm)

(Reference LTC DWG # 05-08-1715)

3.00 ±0.10 (2 SIDES)

2.00 ±0.10 (2 SIDES)

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

0.40 ± 0.10

BOTTOM VIEW—EXPOSED PAD

1.65 ± 0.10 (2 SIDES)

0.75 ±0.05

R = 0.115

TYP

R = 0.05

TYP

1.35 ±0.10 (2 SIDES)

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

0.200 REF

0.00 – 0.05

(DCB6) DFN 0405

0.25 ± 0.05

0.50 BSC

PIN 1 NOTCH R0.20 OR 0.25 × 45° CHAMFER

0.25 ± 0.05

1.35 ±0.05 (2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

1.65 ±0.05 (2 SIDES)

2.15 ±0.05

0.70 ±0.05

3.55 ±0.05

PACKAGE OUTLINE

0.50 BSC

LTC6908-1/LTC6908-2

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.

PACKAGE DESCRIPTIO

S6 Package

6-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1636)

1.50 – 1.75 (NOTE 4)

2.80 BSC

0.30 – 0.45 6 PLCS (NOTE 3)

DATUM ‘A’

0.09 – 0.20 (NOTE 3)

S6 TSOT-23 0302

2.90 BSC (NOTE 4)

0.95 BSC

1.90 BSC

0.80 – 0.90

1.00 MAX

0.01 – 0.10

0.20 BSC

0.30 – 0.50 REF

PIN ONE ID

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

3.85 MAX

0.62

MAX

0.95 REF

RECOMMENDED SOLDER PAD LAYOUT

PER IPC CALCULATOR

1.4 MIN

2.62 REF

1.22 REF

LTC6908-1/LTC6908-2

690812fa

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear.com

LT 0506 REV A • PRINTED IN USA

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, I

C 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

TYPICAL APPLICATIO

RELATED PARTS

Doubling the Output Frequency

Quick Evaluation Circuit for Effects Of Frequency Spreading

Modulation Rate. (DFN Package Demo Board DC814D-J/K)

SET

OUT1

OUT2

MOD

OUT2

OUT1

GND

SET

690812 TA05a

LTC6908-X

0.1µF

1nF

JUMPER

BLOCK

N = 16

N = 32

N = 64

SET

OUT1

OUT2

MOD

OUT

NC7SZ86

GND

SET

690812 TA05b

LTC6908-2

0.1µF

Datasheet LTC6908CS6-2, LTC6908 Datasheet (Linear Technology)

Specifications and Main Features

Frequently Asked Questions

User Manual