LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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FEATURES
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
U
APPLICATIO S
Switching Power Supply Clock Reference
Portable and Battery-Powered Equipment
Precision Programmable Oscillator
Charge Pump Driver
LTC6908-1/LTC6908-2
Resistor Set SOT-23
Oscillator with Spread
Spectrum Modulation
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.
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.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
2.25MHz, 2.5V/8A Step-Down Regulator
VIN
2.8V TO 5.5V
CBYP
0.1µF
V+ OUT1
LTC6908-1
GND OUT2
44.2k
SET MOD
690812 TA01a
fOUT = 10MHz • 10k/RSET
2.2M
1000pF
41.2k
4.99k
SVIN TRACK PVIN
RT SW
LTC3418
PGOOD
RUN/SS PGND
ITH SGND
SYNC/MODE VFB
820pF
4.32k
CIN
100µF
0.2µH
2k
VOUT
2.5V
8A
COUT
100µF
×2
150kHz to 30MHz Output
Frequency Spectrum
(9kHz Res BW)
0
SSFM DISABLED
–10
–20
–30
–40
0
SSFM ENABLED
–10
–20
–30
–40
150kHz
FREQUENCY
30MHz
(FUNDAMENTAL AND HARMONICS SHOWN)
690812 TA01b
690812fa
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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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
UW U
PACKAGE/ORDER I FOR ATIO
TOP VIEW
654
7
V+ 1
GND 2
SET 3
TOP VIEW
6 OUT1
5 OUT2
4 MOD
123
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
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 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 specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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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
from the SET pin to the
is turned
V+ pin.
off
(MOD
is
connected
to
OUT2)
unless
otherwise
specified.
RSET
is
defined
as
the
resistor
connected
SYMBOL PARAMETER
CONDITIONS
ΔfOUT
Frequency Accuracy (Note 4)
V+ = 2.7V
250kHz ≤ fOUT ≤ 5MHz
250kHz ≤ fOUT ≤ 5MHz
50kHz ≤ fOUT < 250kHz
V+ = 5V
250kHz ≤ fOUT ≤ 5MHz
250kHz ≤ fOUT ≤ 5MHz
50kHz ≤ fOUT < 250kHz
5MHz < fOUT ≤ 10MHz
RSET Frequency Setting Resistor Range V+ = 2.7V
| ΔfOUT | ≤ 1.5%
| ΔfOUT | ≤ 2.5%
| ΔfOUT | ≤ 3.5%
V+ = 5V
| ΔfOUT | ≤ 2%
| ΔfOUT | ≤ 3%
| ΔfOUT | ≤ 4%
| ΔfOUT | ≤ 4.5%
ΔfOUT/ΔT Frequency Drift Over Temperature RSET = 100k
ΔfOUT/ΔV+ Frequency Drift Over Supply (Note 4) V+ = 2.7V to 3.6V, RSET = 100k
V+ = 4.5V to 5.5V, RSET = 100k
Period Variation
(Frequency Spreading)
RSET = 100k, MOD Pin = V+, GND or OPEN
Long-Term Stability of Output
Frequency (Note 8)
Duty Cycle (Note 5)
V+ Operating Supply Range
No Modulation, 250kHz ≤ fOUT ≤ 1MHz
IS Power Supply Current
VR+SE=T
= 2000k,
5V
RL
=
∞,
fOUT
=
50kHz,
MOD
Pin
=
V+
V+ = 2.7V
RSET = 20k, RL = ∞, fOUT = 5MHz, MOD Pin = GND
V+ = 5V
V+ = 2.7V
VIH_MOD High Level MOD Input Voltage
VIL_MOD
IMOD
VOH
VOL
tr
tf
Low Level MOD Input Voltage
MOD Pin Input Current (Note 6)
High Level Output Voltage (Note 6)
(OUT1, OUT2)
Low Level Output Voltage (Note 6)
Output Rise Time (Note 7)
Output Fall Time (Note 7)
MOD Pin = V+, V+ = 5V
MOD Pin = GND, V+ = 5V
V+ = 5V
V+ = 2.7V
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
V+ = 5V
V+ = 2.7V
MIN TYP MAX
±0.5 ±1.5
±2 ±2.5
±2.5 ±3.5
±1 ±2
±2.5 ±3
±3 ±4
±3.5 ±4.5
20
20
400
400
400
2000
20
20
400
10
400
400
2000
20
±0.004
0.04 0.25
0.4 0.9
±7.5 ±10 ±12.5
UNITS
%
%
%
%
%
%
%
k
k
k
k
k
k
k
%/°C
%/V
%/V
%
300 ppm/√kHr
45 50 55
2.7
5.5
%
V
0.4 0.65
mA
0.4 0.6
mA
V+ – 0.4
–4
4.75
4.4
2.35
1.85
1.25
0.9
2
–2
4.9
4.7
2.6
2.2
0.05
0.2
0.1
0.4
6
11
5
9
1.7
1.3
0.4
4
0.15
0.5
0.3
0.7
mA
mA
V
V
µA
µA
V
V
V
V
V
V
V
V
ns
ns
ns
ns
690812fa
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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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 –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
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.
TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Error vs RSET,
V+ = 3V
5
4 GUARANTEED MAX
3 OVER TEMPERATURE
TA = 25°C
2
1 TYPICAL MAX
0
–1 TYPICAL MIN
–2
–3 GUARANTEED MIN
–4 OVER TEMPERATURE
–5
10k
100k 1M
RSET ()
10M
690812 G01
Peak to Peak Jitter vs Output
Frequency
1.0
0.9
0.8
0.7 5V
0.6
0.5 3V
0.4
0.3
0.2
0.1
0
10k
100k
1M
FREQUENCY (Hz)
10M
690812 G04
4
Frequency Error vs RSET,
V+ = 5V
5
TA = 25°C
4
3
2
1 TYPICAL MAX
GUARANTEED MAX
OVER TEMPERATURE
0
–1
–2 TYPICAL MIN
–3
GUARANTEED MIN
OVER TEMPERATURE
–4
–5
10k
100k
1M
RSET ()
10M
690812 G02
Supply Current vs Output
Frequency
2.0
1.5
5V SSFM ENABLED
1.0
3V SSFM ENABLED
0.5
0
10k
3V SSFM DISABLED
5V SSFM DISABLED
100k
1M
FREQUENCY (Hz)
10M
690812 G05
Frequency Error vs Temperature
1.00
0.75
0.50
0.25
0 TYPICAL MAX
–0.25
–0.50
–0.75
TYPICAL MIN
–1.00
–40 –20 0
20 40 60
TEMPERATURE (°C)
80
690812 G03
Supply Current vs Temperature
800
CL 5pF ON BOTH OUTPUTS
750 FREQUENCY = 1MHz
SSFM DISABLED
700
650 V+ = 5V
600
550
500 V+ = 3V
450
400
–40 –20 0
20 40 60
TEMPERATURE (°C)
80
690812 G06
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Resistor Set SOT-23 Oscillator

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LTC6908-1/LTC6908-2
TYPICAL PERFOR A CE CHARACTERISTICS
Output Resistance vs Supply
Voltage
500
TA = 25°C
450
400
OUTPUT SOURCING CURRENT
350
300
250
200
150 OUTPUT SINKING CURRENT
100
50
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
SUPPLY VOLTAGE (V)
690812 G07
Output Operating at 5MHz,
V+ = 3V
80ns/DIV
690812 G08
Output Operating at 10MHz,
V+ = 5V
40ns/DIV
690812 G09
Output Frequency Spectrum with
SSFM Enabled and Disabled
20dBm
SSFM ENABLED
(N = 16)
RES BW = 220Hz
Output Frequency Spectrum with
SSFM Enabled and Disabled
20dBm
SSFM ENABLED
(N = 16)
RES BW = 9kHz
–80dBm
SSFM DISABLED
150kHz
FREQUENCY (7.5kHz/DIV)
690812 G10
–80dBm
SSFM DISABLED
5MHz
FREQUENCY (250kHz/DIV)
690812 G11
690812fa
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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PI FU CTIO S (DCB Package/S6 Package)
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 Out-
puts. 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 three-
state 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.
Exposed Pad (Pin 7/NA): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRA (S6 Package Pin Numbers)
V+
1
RSET
3
SET
VBIAS
fMASTER
=
20MHz
10k
IMASTER
V+ – V(SET)
=
20MHz
10k/RSET
fOUT = fMASTER/2
+
GAIN = 1
V+ – V(SET) 1.13V
V+ – V(SET)
ISET = RSET
1-POLE
LPF
MASTER
OSCILLATOR
V
OUT
IMASTER
COMPLEMENTARY
OR
QUADRATURE
OUTPUTS
0
90/180
6 OUT1
5 OUT2
V+
–+
2µA
MOD 4
–+
GND
2µA
IREF
MDAC
MUTE OUTPUT
UNTIL STABLE
POR
PSEUDO RANDOM
CODE GENERATOR
CLK
3-STATE
INPUT DECODER
DIVIDER SELECT
DIVIDE BY
16/32/64
DETECT
CLOCK INPUT
WHEN A CLOCK SIGNAL IS PRESENT AT THE
MOD INPUT, DISABLE THE MODULATION.
2 GND
6
690812 BD
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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U
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 modula-
tion (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 volt-
age. 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
TA = 25°C
1.3
1.2
V+ = 5V
1.1
V+ = 3V
1.0
0.9
0.8
0.1 1 10 100 1000
IRES (µA)
690812 F01
Figure 1. V+ – VSET Variation with IRES
LTC6908-1/LTC6908-2
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 fre-
quency. 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
10k
10k
100k 1M 10M
DESIRED OUTPUT FREQUENCY (Hz)
690812 F02
Figure 2. RSET vs Desired Output Frequency
LTC6908-1 (COMPLEMENTARY)
OUT1
OUT2
LTC6908-2 (QUADRATURE)
OUT1
OUT2
690812 F03
Figure 3. Output Waveforms for LTC6908-1, LTC6908-2
690812fa
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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U
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 fre-
quency band. This spreading decreases the peak electro-
magnetic 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.
To disable the SSFM, connect one of the outputs to the
MOD pin. An AC detector circuit shuts down the modula-
tion 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 pseu-
dorandom 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 set-
ting. 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 sec-
tion. Figure 4 illustrates how the output frequency varies
over time.
fOUT + 10%
128 STEPS
8
fOUT – 10%
tSTEP = N/fOUT
tSTEP
tREPEAT
TIME
Figure 4
tREPEAT = ((215 – 1) • N)/fOUT
690812 F04
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LTC6908-1 (Linear Technology)
Resistor Set SOT-23 Oscillator

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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
RSETMIN = 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.
CBYP
V+ V+ OUT1
OUT1
ICONTROL
GND OUT2
SET MOD
690812 F05
fOUT = 10k • (10MHz/1.13V) • ICONTROL(A)
V+
FLOAT
Figure 5. Current Controlled Oscillator
OUT2
Figure 6 shows the LTC6908 configured as a VCO. A voltage
source is connected in series with an external 10k resis-
tor. 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+
CBYP
V+ V+ OUT1
OUT1
VCONTROL –+
GND OUT2
RSET
SET
MOD
690812 F06
fOUT = 10k • 10MHz/RSET(1 – VCONTROL/1.13V)
V+
FLOAT
Figure 6. Voltage Controlled Oscillator
OUT2
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 gen-
eral 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 sys-
tems 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 modula-
tion rate divider setting, which is determined by the state
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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 no-
menclature 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 synchro-
nization 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 suf-
ficient 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
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 band-
width 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 suf-
ficiently 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 require-
ment in most cases, the regulator’s regulation, efficiency
10
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Resistor Set SOT-23 Oscillator

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LTC6908-1/LTC6908-2
APPLICATIO S I FOR ATIO
and load response are maintained while peak electromag-
netic 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
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 addi-
tional 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.
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
VTA+
=
=
25°C
3V
1000
START-UP TIME
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.
100
10
1k
10k 100k
RSET ()
1M 10M
690812 F07
Figure 7. Start-Up Time
690812fa
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Resistor Set SOT-23 Oscillator

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TYPICAL APPLICATIO S
VIN
3.3V CIN1
100µF
×4
RSVIN1
100
CSVIN1
1µF
X7R
VIN
2.8V TO 5.5V
CBYP
0.1µF
90.9k
V+ OUT1
LTC6908-1
GND OUT2
SET MOD
fOUT = 10MHz • 10k/RSET
RSVIN2
100
CSVIN2
1µF
X7R
RSS1
2.2M
CSS1
1000pF
X7R
C1A
47pF
X7R
RPG1
100k
3
4
9
10
22
23
28
29
24
35
5
7
26
ROSC1 69.8k 6
8
13
14
15
27
PVIN
PVIN
PVIN
PVIN
PVIN
PVIN
PVIN
PVIN
SVIN
LTC3418
SW
SW
SW
SW
SW
SW
SW
SW
VFB
TRACK
PGND
PGOOD
PGND
RUN/SS
PGND
ITH
RT
SGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
SYNC/MODE VREF
1
2
11
12
20
21
30
31
25
38
37
36
34
33
32
19
18
17
16
L1
0.2µH
C2
1000pF
X7R
CREF1
2.2µF
X7R
R1
2.55k
R2
2k
COUT
100µF
×4
VOUT
1.8V
16A
CIN2
100µF
×4
C1B
47pF
X7R
RITH
2k
CITH
2200pF
X7R
RPG2
100k
3
PVIN
4
PVIN
9
PVIN
10
PVIN
22
PVIN
23
PVIN
28
PVIN
29
PVIN
24
SVIN
1
SW
2
SW
11
SW
12
SW
20
SW
21
SW
30
SW
31
SW
25
VFB
35
5
7
26
ROSC2 69.8k 6
8
13
14
15
27
LTC3418
TRACK
PGND
PGOOD
PGND
RUN/SS
PGND
ITH
RT
SGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
PGND
SYNC/MODE VREF
38
37
36
34
33
32
19
18
17
16
L2
0.2µH
CREF2
2.2µF
X7R
CIN1, CIN2, COUT: TDK C3225X5R0J107M
L1, L2: VISHAY DALE IHLP-2525CZ-01
12
Figure 8a. 1.1MHz, 1.8V/16A Step-Down Regulator
690812 TA02
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TYPICAL APPLICATIO S
LTC6908-1/LTC6908-2
RES BW = 9kHz
150kHz
FREQUENCY (3MHz/DIV)
30MHz
690812 TA03
Figure 8b. Output Frequency Spectrum of Two-Phase
Regulator, Figure 8a, with SSFM Disabled
RES BW = 9kHz
150kHz
FREQUENCY (3MHz/DIV)
30MHz
690812 TA04
Figure 8c. Output Frequency Spectrum of Two-Phase
Regulator, Figure 8a, with SSFM Enabled
690812fa
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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
2.00 ±0.10
(2 SIDES)
R = 0.115
TYP
R = 0.05
TYP
4
0.40 ± 0.10
6
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
3.00 ±0.10 1.65 ± 0.10
(2 SIDES) (2 SIDES)
0.75 ±0.05
0.00 – 0.05
PIN 1 NOTCH
R0.20 OR 0.25
× 45° CHAMFER
31
(DCB6) DFN 0405
0.25 ± 0.05
0.50 BSC
1.35 ±0.10
(2 SIDES)
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
14
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PACKAGE DESCRIPTIO
0.62 0.95
MAX REF
LTC6908-1/LTC6908-2
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC
(NOTE 4)
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
0.20 BSC
DATUM ‘A’
0.95 BSC
0.80 – 0.90
1.00 MAX
0.30 – 0.50 REF
0.09 – 0.20
NOTE:
(NOTE 3)
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
1.90 BSC
0.30 – 0.45
6 PLCS (NOTE 3)
0.01 – 0.10
S6 TSOT-23 0302
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
690812fa
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TYPICAL APPLICATIO
Quick Evaluation Circuit for Effects Of Frequency Spreading
Modulation Rate. (DFN Package Demo Board DC814D-J/K)
V+
0.1µF
RSET
V+ OUT1
LTC6908-X
GND OUT2
OUT1
OUT2
V+
0.1µF
SET
MOD
690812 TA05a
N = 64
N = 32
N = 16
JUMPER
BLOCK
V+
1nF
RSET
Doubling the Output Frequency
V+ OUT1
LTC6908-2
GND OUT2
SET
MOD
690812 TA05b
NC7SZ86
OUT
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, 100µs Startup, 7ps RMS Jitter
No Trim Components Required
10kHz to 1MHz or 40kHz to 4MHz, 36µA at 400kHz
16 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
690812fa
LT 0506 REV A • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 2006




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