Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsLinear ICsLTC6908CS6-1#TRMPBF

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 (T

= 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

TRACK

LTC3418

100µF

0.2µH

LTC6908-1

RUN/SS

PGOOD

PGND

SGND

SYNC/MODE V

820pF

1000pF

0.1µF

OUT

100µF

×2

OUT

2.5V

4.32k

41.2k

2.2M

2.8V TO 5.5V

4.99k

150kHz to 30MHz Output

Frequency Spectrum

(9kHz Res BW)

Summary of content (16 pages)

PAGE 1
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.
PAGE 2
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 ......
PAGE 3
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.
PAGE 4
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.
PAGE 5
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.
PAGE 6
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.
PAGE 7
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.
PAGE 8
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.
PAGE 9
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.
PAGE 10
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.
PAGE 11
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.
PAGE 12
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.
PAGE 13
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.
PAGE 14
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.
PAGE 15
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.
PAGE 16
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.