LTC6908-1/LTC6908-2
Resistor Set SOT-23 Oscillator with
Spread Spectrum Modulation
FEATURES
DESCRIPTION
LTC6908-1: Complementary Outputs (0°/180°)
n LTC6908-2: Quadrature Outputs (0°/90°)
n 50kHz to 10MHz Frequency Range
n One External Resistor Sets the Frequency
n Optional Spread Spectrum Frequency Modulation
for Improved EMC Performance
n ±10% Frequency Spreading
n 400µA Supply Current Typical (V+ = 5V, 50kHz)
n Frequency Error ≤1.5% Max (T = 25°C, V+ = 3V)
A
n ±40ppm/°C Temperature Stability
n Fast Start-Up Time: 260µs Typical (1MHz)
n Outputs Muted Until Stable
n Operates from a Single 2.7V to 5.5V Supply
n Available in Low Profile (1mm) ThinSOT and DFN
(2mm × 3mm) Packages
n AEC-Q100 Qualified for Automotive Applications
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.
n
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.
APPLICATIONS
Switching Power Supply Clock Reference
Portable and Battery-Powered Equipment
n Precision Programmable Oscillator
n Charge Pump Driver
n
n
All registered trademarks and trademarks are the property of their respective owners.
TYPICAL APPLICATION
150kHz to 30MHz Output
Frequency Spectrum
(9kHz Res BW)
2.25MHz, 2.5V/8A Step-Down Regulator
0.1μF
SVIN TRACK PVIN
V+
RT
OUT1
LTC6908-1
GND
OUT2
SET
MOD
44.2k
2.2M
41.2k
690812 TA01a
fOUT = 10MHz • 10k/RSET
SW
LTC3418
RUN/SS
ITH
1000pF
4.99k
820pF
0.2μH
PGOOD
PGND
SGND
SYNC/MODE VFB
4.32k
2k
VOUT
2.5V
8A
OUTPUT (dBc)
CBYP
0
CIN
100μF
COUT
100μF
×2
SSFM DISABLED
–10
–20
–30
–40
0
OUTPUT (dBc)
VIN
2.8V TO 5.5V
SSFM ENABLED
–10
–20
–30
–40
150kHz
30MHz
FREQUENCY
(FUNDAMENTAL AND HARMONICS SHOWN)
690812 TA01b
Rev C
Document Feedback
For more information www.analog.com
1
LTC6908-1/LTC6908-2
ABSOLUTE MAXIMUM RATINGS
(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
PIN CONFIGURATION
MOD
OUT2
OUT1
TOP VIEW
6
5
4
TOP VIEW
1
2
3
SET
V+
GND
7
V+ 1
6 OUT1
GND 2
5 OUT2
SET 3
4 MOD
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 150°C, θJA = 230°C/W
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
ORDER INFORMATION
Lead Free Finish
TAPE AND REEL (MINI)
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC6908CDCB-1#TRMPBF
LTC6908CDCB-1#TRPBF
LBXZ
6-Lead (2mm × 3mm) Plastic DFN
0°C to 70°C
LTC6908IDCB-1#TRMPBF
LTC6908IDCB-1#TRPBF
LBXZ
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 85°C
LTC6908CDCB-2#TRMPBF
LTC6908CDCB-2#TRPBF
LBYB
6-Lead (2mm × 3mm) Plastic DFN
0°C to 70°C
LTC6908IDCB-2#TRMPBF
LTC6908IDCB-2#TRPBF
LBYB
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 85°C
LTC6908CS6-1#TRMPBF
LTC6908CS6-1#TRPBF
LTBYC
6-Lead Plastic TSOT-23
0°C to 70°C
LTC6908IS6-1#TRMPBF
LTC6908IS6-1#TRPBF
LTBYC
6-Lead Plastic TSOT-23
–40°C to 85°C
LTC6908HS6-1#TRMPBF
LTC6908HS6-1#TRPBF
LTBYC
6-Lead Plastic TSOT-23
–40°C to 125°C
LTC6908CS6-2#TRMPBF
LTC6908CS6-2#TRPBF
LTBYD
6-Lead Plastic TSOT-23
0°C to 70°C
LTC6908IS6-2#TRMPBF
LTC6908IS6-2#TRPBF
LTBYD
6-Lead Plastic TSOT-23
–40°C to 85°C
LTC6908HS6-2#TRMPBF
LTC6908HS6-2#TRPBF
LTBYD
6-Lead Plastic TSOT-23
–40°C to 125°C
Rev C
2
For more information www.analog.com
LTC6908-1/LTC6908-2
ORDER INFORMATION
Lead Free Finish
TAPE AND REEL (MINI)
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
AUTOMOTIVE PRODUCTS**
LTC6908IS6-1#WTRMPBF
LTC6908IS6-1#WTRPBF
LTBYC
6-Lead Plastic TSOT-23
–40°C to 85°C
LTC6908HS6-1#WTRMPBF
LTC6908HS6-1#WTRPBF
LTBYC
6-Lead Plastic TSOT-23
–40°C to 125°C
LTC6908IS6-2#WTRMPBF
LTC6908IS6-2#WTRPBF
LTBYD
6-Lead Plastic TSOT-23
–40°C to 85°C
LTC6908HS6-2#WTRMPBF
LTC6908HS6-2#WTRPBF
LTBYD
6-Lead Plastic TSOT-23
–40°C to 125°C
TRM = 500 pieces. *Temperature grades are identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
**Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. These
models are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your
local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for
these models.
ELECTRICAL CHARACTERISTICS
The l 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
ΔfOUT/ΔT
PARAMETER
CONDITIONS
Frequency Accuracy (Note 4)
V+ = 2.7V
Frequency Setting Resistor Range
Frequency Drift Over Temperature
MIN
Duty Cycle (Note 5)
IS
l
l
±0.5
±2
±2.5
±1.5
±2.5
±3.5
%
%
%
V+ = 5V
250kHz ≤ fOUT ≤ 5MHz
250kHz ≤ fOUT ≤ 5MHz
50kHz ≤ fOUT < 250kHz
5MHz < fOUT ≤ 10MHz
l
l
l
±1
±2.5
±3
±3.5
±2
±3
±4
±4.5
%
%
%
%
V+ = 2.7V
| ΔfOUT | ≤ 1.5%
| ΔfOUT | ≤ 2.5%
| ΔfOUT | ≤ 3.5%
l
l
20
20
400
400
400
2000
k
k
k
V+ = 5V
| ΔfOUT | ≤ 2%
| ΔfOUT | ≤ 3%
| ΔfOUT | ≤ 4%
| ΔfOUT | ≤ 4.5%
l
l
l
20
20
400
10
400
400
2000
20
k
k
k
k
RSET = 100k
RSET = 100k, MOD Pin = V+, GND or OPEN
l
±0.004
l
l
0.04
0.4
0.25
0.9
%/V
%/V
±10
±12.5
%
l
±7.5
No Modulation, 250kHz ≤ fOUT ≤ 1MHz
RSET = 2000k, RL = ∞, fOUT
V+ = 5V
V+ = 2.7V
l
45
l
2.7
= 50kHz, MOD Pin = V+
RSET = 20k, RL = ∞, fOUT = 5MHz, MOD Pin = GND
V+ = 5V
V+ = 2.7V
VIH_MOD
%/°C
300
Operating Supply Range
Power Supply Current
UNITS
250kHz ≤ fOUT ≤ 5MHz
250kHz ≤ fOUT ≤ 5MHz
50kHz ≤ fOUT < 250kHz
Long-Term Stability of Output
Frequency (Note 8)
V+
MAX
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)
TYP
High Level MOD Input Voltage
50
ppm/√kHr
55
%
5.5
V
l
l
0.4
0.4
0.65
0.6
mA
mA
l
l
1.25
0.9
1.7
1.3
mA
mA
l
V+ – 0.4
V
Rev C
For more information www.analog.com
3
LTC6908-1/LTC6908-2
ELECTRICAL CHARACTERISTICS
The l 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.
VIL_MOD
Low Level MOD Input Voltage
l
0.4
V
4
µA
µA
IMOD
MOD Pin Input Current (Note 6)
MOD Pin = V+, V+ = 5V
MOD Pin = GND, V+ = 5V
VOH
High Level Output Voltage (Note 6)
(OUT1, OUT2)
V+ = 5V IOH = –0.3mA
IOH = –1.2mA
l
l
4.75
4.4
4.9
4.7
V
V
V+ = 2.7V IOH = –0.3mA
IOH = –0.8mA
l
l
2.35
1.85
2.6
2.2
V
V
V+ = 5V IOL = 0.3mA
IOL = 1.2mA
l
l
0.05
0.2
0.15
0.5
V
V
V+ = 2.7V IOL = 0.3mA
IOL = 0.8mA
l
l
0.1
0.4
0.3
0.7
V
V
VOL
Low Level Output Voltage (Note 6)
l
l
–4
2
–2
tr
Output Rise Time (Note 7)
V+ = 5V
V+ = 2.7V
6
11
ns
ns
tf
Output Fall Time (Note 7)
V+ = 5V
V+ = 2.7V
5
9
ns
ns
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 PERFORMANCE CHARACTERISTICS
5
FREQUENCY ERROR (%)
3
TA = 25°C
0
–1
TYPICAL MIN
–2
–4
GUARANTEED MIN
OVER TEMPERATURE
–5
10k
100k
0.75
3
TYPICAL MAX
1
Frequency Error vs Temperature
GUARANTEED MAX
OVER TEMPERATURE
2
1 TYPICAL MAX
0
–1
–2 TYPICAL MIN
–3
GUARANTEED MIN
OVER TEMPERATURE
10M
RSET (Ω)
–5
10k
100k
10M
1M
RSET (Ω)
690812 G01
0.50
0.25
TYPICAL MAX
0
–0.25
–0.50
TYPICAL MIN
–0.75
–4
1M
1.00
TA = 25°C
4
GUARANTEED MAX
OVER TEMPERATURE
2
–3
5
FREQUENCY ERROR (%)
4
Frequency Error vs RSET,
V+ = 5V
FREQUENCY ERROR (%)
Frequency Error vs RSET,
V+ = 3V
690812 G02
–1.00
–40
–20
0
40
20
TEMPERATURE (°C)
60
80
690812 G03
Rev C
4
For more information www.analog.com
LTC6908-1/LTC6908-2
TYPICAL PERFORMANCE CHARACTERISTICS
Peak to Peak Jitter vs Output
Frequency
Supply Current vs Output
Frequency
Supply Current vs Temperature
800
1.0
CL 5pF ON BOTH OUTPUTS
750 FREQUENCY = 1MHz
SSFM DISABLED
2.0
0.9
0.6
3V
0.5
0.4
0.3
0.2
1.5
5V SSFM ENABLED
1.0
3V SSFM ENABLED
0.5
3V SSFM DISABLED
0.1
0
10k
100k
1M
FREQUENCY (Hz)
0
10k
10M
V+ = 5V
600
550
V+ = 3V
500
–20
0
40
20
TEMPERATURE (°C)
60
690812 G05
80
690812 G06
Output Operating at 5MHz,
V+ = 3V
Output Resistance vs Supply
Voltage
500
650
400
–40
10M
100k
1M
FREQUENCY (Hz)
700
450
5V SSFM DISABLED
690812 G04
Output Operating at 10MHz,
V+ = 5V
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
100
40ns/DIV
690812 G09
50
3.0
3.5
4.0 4.5 5.0
SUPPLY VOLTAGE (V)
5.5
6.0
690812 G07
Output Frequency Spectrum with
SSFM Enabled and Disabled
20dBm
RES BW = 220Hz
Output Frequency Spectrum with
SSFM Enabled and Disabled
20dBm
SSFM ENABLED
(N = 16)
RES BW = 9kHz
SSFM ENABLED
(N = 16)
10dB/DIV
0
2.5
10dB/DIV
OUTPUT RESISTANCE (Ω)
SUPPLY CURRENT (μA)
5V
0.7
JITTER (% P-P)
SUPPLY CURRENT (mA)
0.8
SSFM DISABLED
–80dBm
SSFM DISABLED
690812 G10
150kHz
–80dBm
FREQUENCY (7.5kHz/DIV)
5MHz
690812 G11
FREQUENCY (250kHz/DIV)
Rev C
For more information www.analog.com
5
LTC6908-1/LTC6908-2
PIN FUNCTIONS
(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 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.
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
Outputs. These pins can drive 5k and/or 10pF loads.
Larger loads may cause inaccuracies due to supply
bounce at high frequencies.
BLOCK DIAGRAM
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)
V+
+
1
GAIN = 1
RSET
SET
MASTER
OSCILLATOR
V+ – V(SET) 1.13V
OUT
ISET =
VBIAS
0
V
–
3
V+ – V(SET)
RSET
fOUT = fMASTER/2
1-POLE
LPF
COMPLEMENTARY
OR
QUADRATURE
OUTPUTS
IMASTER
6 OUT1
90/180
5 OUT2
MUTE OUTPUT
UNTIL STABLE
IREF
2 GND
MDAC
POR
V+
+
–
2μA
MOD 4
PSEUDO RANDOM
CODE GENERATOR
3-STATE
INPUT DECODER
+
–
CLK
DIVIDE BY
16/32/64
DIVIDER SELECT
2μA
GND
DETECT
CLOCK INPUT
WHEN A CLOCK SIGNAL IS PRESENT AT THE
MOD INPUT, DISABLE THE MODULATION.
690812 BD
Rev C
6
For more information www.analog.com
LTC6908-1/LTC6908-2
OPERATION
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
1.4
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
10M
1M
100k
10k
10k
TA = 25C
100k
1M
DESIRED OUTPUT FREQUENCY (Hz)
10M
690812 F02
1.3
VRES = V+ – VSET
The master oscillator signal is divided by 2 before driving
the output pins, resulting in the simple formula for the
output frequency, fOUT, below (see Figure 2):
RSET (Ω)
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.
Figure 2. RSET vs Desired Output Frequency
1.2
LTC6908-1 (COMPLEMENTARY)
V+ = 5V
OUT1
1.1
V+ = 3V
OUT2
1.0
LTC6908-2 (QUADRATURE)
0.9
OUT1
0.8
0.1
1
10
IRES (μA)
100
1000
OUT2
690812 F03
690812 F01
Figure 1. V+ – V
SET Variation with IRES
Figure 3. Output Waveforms for LTC6908-1, LTC6908-2
Rev C
For more information www.analog.com
7
LTC6908-1/LTC6908-2
OPERATION
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.
very high current applications, a significant improvement
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.
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 rise and fall times are typically 6ns with a 5V supply
and 11ns with a 3V supply. An internal counter mutes the
outputs (OUT1 = Low, OUT2 = High) 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 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.
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
fOUT + 10%
128 STEPS
fOUT – 10%
tSTEP = N/fOUT
tSTEP
tREPEAT
tREPEAT = ((215 – 1) • N)/fOUT
TIME
690812 F04
Figure 4.
Rev C
8
For more information www.analog.com
LTC6908-1/LTC6908-2
APPLICATIONS INFORMATION
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%,
V+
ICONTROL
CBYP
V+
OUT1
GND
OUT2
OUT1
V+
SET
MOD
OUT2
FLOAT
690812 F05
fOUT = 10k • (10MHz/1.13V) • ICONTROL(A)
Figure 5. Current Controlled Oscillator
therefore, the Figure 5 circuit is less accurate than if a
resistor controls the output frequency.
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
CBYP
V+
VCONTROL
+
–
V+
OUT1
GND
OUT2
OUT1
V+
RSET
SET
MOD
OUT2
FLOAT
690812 F06
fOUT = 10k • 10MHz/RSET(1 – VCONTROL/1.13V)
Figure 6. Voltage Controlled Oscillator
V+ and the SET pin. Again, this circuit decouples the relationship between the input current and the voltage between
V+ 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
Rev C
For more information www.analog.com
9
LTC6908-1/LTC6908-2
APPLICATIONS INFORMATION
to the modulation rate (fOUT/N), where N is the modulation rate divider setting, which is determined by the state
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 (LTC69082) 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.
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 sufficiently high, beyond fOUT/N, then there will not be any
regulation issues.
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.
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 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 LTC6908 directly drives many switching regulators. The LTC6908 with the spread spectrum frequency
modulation results in improved EMC performance. If
Rev C
10
For more information www.analog.com
LTC6908-1/LTC6908-2
APPLICATIONS INFORMATION
the bandwidth of the switching regulator is sufficient,
not a difficult requirement in most cases, the regulator’s
regulation, efficiency 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
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. Figure 7 shows start-up times for various
RSET resistors. An internal counter mutes the outputs
(OUT1 = Low, OUT2 = High) 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
STARTUP DELAY (μs)
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
TA = 25°C
V+ = 3V
1000
100
10
1k
10k
100k
RSET (Ω)
1M
10M
690812 F07
Figure 7. Start-Up Time
Rev C
For more information www.analog.com
11
LTC6908-1/LTC6908-2
TYPICAL APPLICATIONS
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
35
CSS1
1000pF
X7R
5
7
C1A
47pF
X7R
VIN
2.8V TO 5.5V
CBYP
0.1μF
V+
OUT1
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
SVIN
VFB
LTC3418
TRACK
PGND
PGOOD
PGND
RUN/SS
PGND
26
I
ROSC1 69.8k 6 TH
RT
8
SGND
13
PGND
14
PGND
15
PGND
27
SYNC/MODE
PGND
PGND
PGND
PGND
PGND
PGND
VREF
1
2
11
12
OUT2
SET
MOD
C2
1000pF
X7R
20
21
R1
2.55k
30
31
25
38
37
36
COUT
100μF
×4
34
33
VOUT
1.8V
16A
R2
2k
32
19
18
17
16
LTC6908-1
GND
L1
0.2μH
CREF1
2.2μF
X7R
90.9k
RITH
2k
fOUT = 10MHz • 10k/RSET
3
CIN2
100μF
×4
4
9
10
22
RPG2
100k
23
28
29
24
RSVIN2
100Ω
CSVIN2
1μF
X7R
35
5
7
C1B
47pF
X7R
CITH
2200pF
X7R
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
PVIN
SW
SVIN
VFB
LTC3418
TRACK
PGND
PGOOD
PGND
RUN/SS
26
I
ROSC2 69.8k 6 TH
RT
8
SGND
13
PGND
14
PGND
15
PGND
27
SYNC/MODE
PGND
PGND
PGND
PGND
PGND
PGND
PGND
VREF
1
L2
0.2μH
2
11
12
20
21
30
31
25
38
37
36
34
33
32
19
18
17
16
CREF2
2.2μF
X7R
CIN1, CIN2, COUT: TDK C3225X5R0J107M
L1, L2: VISHAY DALE IHLP-2525CZ-01
690812 F08a
Figure 8. (a) 1.1MHz, 1.8V/16A Step-Down Regulator
Rev C
12
For more information www.analog.com
LTC6908-1/LTC6908-2
TYPICAL APPLICATIONS
10dB/DIV
RES BW = 9kHz
30MHz
150kHz
FREQUENCY (3MHz/DIV)
690812 F08b
Figure 8. (b) Output Frequency Spectrum of TwoPhase Regulator, Figure 8a, with SSFM Disabled
10dB/DIV
RES BW = 9kHz
30MHz
150kHz
FREQUENCY (3MHz/DIV)
690812 F08c
Figure 8. (c) Output Frequency Spectrum of TwoPhase Regulator, Figure 8a, with SSFM Enabled
Rev C
For more information www.analog.com
13
LTC6908-1/LTC6908-2
PACKAGE DESCRIPTION
DCB Package
6-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1715 Rev A)
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
0.25 ±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
Rev C
14
For more information www.analog.com
LTC6908-1/LTC6908-2
PACKAGE DESCRIPTION
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
2.80 BSC
1.4 MIN
3.85 MAX 2.62 REF
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
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
REVISION HISTORY
S6 TSOT-23 0302
(Revision history begins at Rev B)
REV
DATE
DESCRIPTION
B
05/18
Clarified state of output pins when muted on power-up.
PAGE NUMBER
C
1/20
Added AEC-Q100 Qualified note to front page.
1
Added W-Grade order information.
2
8, 11
Rev C
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license For
is granted
implication or
otherwise under any patent or patent rights of Analog Devices.
more by
information
www.analog.com
15
LTC6908-1/LTC6908-2
TYPICAL APPLICATION
Quick Evaluation Circuit for Effects Of Frequency Spreading
Modulation Rate. (DFN Package Demo Board DC814D-J/K)
V+
V+
OUT1
OUT1
LTC6908-X
0.1μF
RSET
GND
OUT2
SET
OUT2
Doubling the Output Frequency
V+
V+
RSET
GND
MOD
690812 TA05a
N = 64
N = 32
OUT1
LTC6908-2
0.1μF
SET
V+
OUT
OUT2
NC7SZ86
MOD
690812 TA05b
1nF
N = 16
JUMPER
BLOCK
RELATED PARTS
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, I2C 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
LTC6909
Multiphase Oscillator with Spread Spectrum Modulation
3- to 8-Phase Outputs
Rev C
16
01/20
www.analog.com
For more information www.analog.com
ANALOG DEVICES, INC. 2006-2020
工商网监
湘ICP备2023018690号
