LTC6902
Multiphase Oscillator
with Spread Spectrum
Frequency Modulation
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FEATURES
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2-, 3- or 4-Phase Outputs
Optional Spread Spectrum Frequency Modulation
for Improved EMC Performance
5kHz to 20MHz Frequency Range
One External Resistor Sets the Frequency
One External Resistor Sets Percent Frequency
Spreading
400µA Typical Supply Current, VS = 3V, 1MHz
Frequency Error ≤1.5% Max, 5kHz to 10MHz
(TA = 25°C)
Frequency Error ≤ 2% Max, 5kHz to 10MHz
(TA = 0°C to 70°C)
±40ppm/°C Temperature Stability
Fast Start-Up Time: 50µs to 1.5ms
100Ω CMOS Output Driver
Operates from a Single 2.7V to 5.5V Supply
Available in 10-Lead MS Package
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APPLICATIO S
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Switching Power Supply Clock Reference
Portable and Battery-Powered Equipment
PDAs
Cell Phones
Clocking Switched Capacitor Filters
The LTC6902’s master oscillator is controlled by the RSET
resistor and has a range of 100kHz and 20MHz. In order
to accommodate a wider output frequency range, a programmable divider (divide by 1, 10 or 100) is included.
The integrated programmable multiphase circuit provides either 2-, 3- or 4-phase waveforms.
The LTC6902’s SSFM capability modulates the oscillator’s
frequency by a pseudorandom noise (PRN) signal to
spread the oscillator’s energy over a wide frequency band.
This spreading decreases the peak electromagnetic radiation level and improves electromagnetic compatibility
(EMC) performance. The amount of frequency spreading
is programmable by a single additional external resistor
(RMOD) and is disabled by grounding the MOD pin.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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The LTC®6902 is a precision, low power and easy-to-use
oscillator that provides multiphase outputs in a small
package. The oscillator frequency is set by a single external resistor (RSET). The LTC6902 also provides an optional
spread spectrum frequency modulation (SSFM) capability
that can be activated and controlled by an additional
external resistor (RMOD).
TYPICAL APPLICATIO
Output Frequency Spectrum With and Without SSFM
500kHz, 4-Phase Clock with 20% Frequency Spreading
5V
0.1µF
RMOD 10k
RSET 10k
LTC6902
OPEN
V+
SET
DIV
MOD
PH
GND
OUT1 OUT4
OUT4
OUT2 OUT3
OUT3
OUT2
6902 TA01
OUT1
0
RELATIVE AMPLITUDE (dBm)
■
DESCRIPTIO
OUTPUT
SPECTRUM
WITH SSFM
DISABLED
–20
OUTPUT
SPECTRUM
WITH 20%
SPREADING
–40
–60
–80
–100
400
550
450
500
FREQUENCY (kHz)
600
6902 TA02
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LTC6902
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
Supply Voltage (V +) to GND ........................– 0.3V to 6V
Voltage On Any Pin
(Referred to GND) ......................... – 0.3V to (V + + 0.3V)
Operating Temperature Range (Note 9)
LTC6902C .......................................... – 40°C to 85°C
LTC6902I ............................................ – 40°C to 85°C
Specified Temperature Range (Note 10)
LTC6902C .......................................... – 40°C to 85°C
LTC6902I ............................................ – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER
PART NUMBER
TOP VIEW
V+
DIV
PH
OUT1
OUT2
1
2
3
4
5
10
9
8
7
6
SET
MOD
GND
OUT4
OUT3
LTC6902CMS
LTC6902IMS
MS PACKAGE
10-LEAD PLASTIC MSOP
MS PART MARKING
TJMAX = 150°C, θJA = 250°C/W
LTK2
LTK3
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V+ = 2.7V to 5.5V, RL= 5k, CL = 5pF, Pin 3 (PH) = 0V (2-phase, M = 1)
unless otherwise specified. Pin 9 (MOD) is at 0V unless otherwise specified. RSET is defined as a resistor connected from the SET pin
to the V+ pin. RMOD is defined as a resistor connected from the MOD pin to the V+ pin.
SYMBOL
∆fOUT
PARAMETER
CONDITIONS
Frequency Accuracy (Notes 2, 3)
V+ = 5V
V+ = 2.7V
5kHz ≤ fOUT ≤ 10MHz
1kHz ≤ fOUT ≤ 5kHz
10MHz ≤ fOUT ≤ 20MHz
5kHz ≤ fOUT ≤ 10MHz, LTC6902C
5kHz < fOUT ≤ 10MHz, LTC6902I
5kHz ≤ fOUT ≤ 10MHz
1kHz ≤ fOUT ≤ 5kHz
5kHz ≤ fOUT ≤ 10MHz, LTC6902C
5kHz ≤ fOUT ≤ 10MHz, LTC6902I
MIN
●
●
●
TYP
MAX
UNITS
±0.5
±2.0
±3.0
±1.5
%
%
%
%
%
±0.5
±2.0
●
●
RSET
Frequency Setting Resistor Range
∆fOUT < 1.5%, V + = 5V
∆fOUT < 1.5%, V + = 2.7V
●
●
∆fOUT/∆T
Frequency Drift Over Temperature
(Note 3)
RSET = 63.2k
●
±0.004
∆fOUT/∆V
Frequency Drift Over Supply (Note 3) V+ = 2.7V to 5V, RSET = 63.2k
●
0.04
Timing Jitter (Note 4)
20
20
20k ≤ RSET ≤ 400k
Pin 2 = V + (N = 100)
Pin 2 = Open (N = 10)
Pin 2 = 0V (N = 1)
Long-Term Stability of
Output Frequency
Duty Cycle (Note 5)
±4.0
±2.0
±2.5
±1.5
±2.0
±2.5
%
%
%
%
400
400
kΩ
kΩ
%/°C
0.12
%/V
0.1
0.2
0.6
%
%
%
300
ppm/√kHr
Pin 2 = V + or Open (N = 100 or 10)
Pin 3 = 0V (2-Phase, M = 1)
Pin 3 = Open (3-Phase, M = 3)
Pin 3 = V + (4-Phase, M = 4)
●
●
●
49.0
32.3
49.0
50.0
33.3
50.0
51.0
34.3
51.0
%
%
%
Pin 2 = 0V (N = 1)
Pin 3 = 0V (2-Phase, M = 1)
Pin 3 = Open (3-Phase, M = 3)
Pin 3 = V + (4-Phase, M = 4)
●
●
●
45.0
32.3
49.0
50.0
33.3
50.0
55.0
34.3
51.0
%
%
%
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LTC6902
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V+ = 2.7V to 5.5V, RL= 5k, CL = 5pF, Pin 3 (PH) = 0V (2-phase, M = 1)
unless otherwise specified. Pin 9 (MOD) is at 0V unless otherwise specified. RSET is defined as a resistor connected from the SET pin
to the V+ pin. RMOD is defined as a resistor connected from the MOD pin to the V+ pin.
SYMBOL
PARAMETER
V+
Operating Supply Range
IS
Power Supply Current
CONDITIONS
MIN
●
RSET = 400k, RL = ∞, Pin 2 = V + (N = 100), fOUT = 5kHz
V + = 5V
2.7
●
●
2.35
1.40
3.50
1.80
mA
mA
RSET = 400k, RL = ∞, Pin 2 = V + (N = 100), RMOD = 800k
V + = 5V
●
●
V + = 2.7V
0.45
0.34
0.63
0.50
mA
mA
RSET = 20k, RL = ∞, Pin 2 = 0V (N = 1), RMOD = 40k
V + = 5V
V + = 2.7V
2.50
1.40
3.60
1.90
mA
mA
●
●
● V+ – 0.4
VIL_DIV
Low Level DIV Input Voltage
●
VIL_PH
Low Level PH Input Voltage
IPH
VOH
VOL
tr
tf
PH Input Currrent (Note 6)
Pin 2 = V +,
V+
= 5V
Pin 2 = 0V, V + = 5V
Low Level Output Voltage (Note 6)
(OUT1, OUT2, OUT3, OUT4)
Output Rise Time (Note 7)
(OUT1, OUT2, OUT3, OUT4)
Output Fall Time (Note 7)
(OUT1, OUT2, OUT3, OUT4)
Spread Spectrum Frequency
Modulation Spreading Percentage
(Downspread from Maximum
Frequency)
Percent = 100 • (fMAX – fMIN)/fMAX
(Note 8)
●
●
–4
V
2
–2
0.4
V
4
µA
µA
● V+ – 0.4
V
●
Pin 3 = V+, V+ = 5V
Pin 3 = 0V, V+ = 5V
High Level Output Voltage (Note 6)
(OUT1, OUT2, OUT3, OUT4)
V
mA
mA
High Level DIV Input Voltage
High Level PH Input Voltage
5.5
0.55
0.50
VIH_DIV
VIH_PH
UNITS
0.35
0.32
RSET = 20k, RL = ∞, Pin 2 = 0V (N = 1), fOUT = 10MHz
V + = 5V
V + = 2.7V
DIV Input Current (Note 6)
MAX
●
●
V + = 2.7V
IDIV
TYP
●
●
–4
2
–2
0.4
V
4
µA
µA
V + = 5V
IOH = – 1mA
IOH = – 4mA
●
●
4.75
4.40
4.90
4.70
V
V
V + = 2.7V
IOH = – 1mA
IOH = – 4mA
●
●
2.35
1.85
2.6
2.2
V
V
V + = 5V
IOL = 1mA
IOL = 4mA
●
●
0.05
0.20
0.15
0.40
V
V
V + = 2.7V
IOL = 1mA
IOL = 4mA
●
●
0.1
0.4
0.3
0.7
V
V
V + = 5V
Pin 2 = V+ or Open (N = 100 or N = 10)
Pin 2 = 0V (N = 1)
14
7
ns
ns
V + = 2.7V
Pin 2 = V+ or Open (N = 100 or N = 10)
Pin 2 = 0V (N = 1)
19
11
ns
ns
V + = 5V
Pin 2 = V+ or Open (N = 100 or N = 10)
Pin 2 = 0V (N = 1)
13
6
ns
ns
V + = 2.7V
Pin 2 = V+ or Open (N = 100 or N = 10)
Pin 2 = 0V (N = 1)
19
10
ns
ns
V + = 5V, N = 10, RSET = 20k, RMOD = 10k
V + = 5V, N = 10, RSET = 20k, RMOD = 40k
V + = 2.7V, N = 10, RSET = 20k, RMOD = 10k
V + = 2.7V, N = 10, RSET = 20k, RMOD = 40k
●
●
●
●
35
7.5
35
7.5
40
10
40
10
45.0
12.5
45.0
12.5
%
%
%
%
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LTC6902
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: Frequencies near 100kHz and 1MHz may be generated using two
different values of RSET (see Applications Information). For these
frequencies, the error is specified under the following assumption:
20k < RSET ≤ 400k for 5kHz ≤ fOUT ≤ 10MHz.
Note 3: Frequency accuracy is defined as the deviation from the fOUT
equation.
Note 4: Jitter is the ratio of the peak-to-peak distribution of the period to
the mean of the period. This specification is based on characterization and
is not 100% tested.
Note 5: Guaranteed by 5V test.
Note 6: To conform with the Logic IC Standard convention, current out of
a pin is arbitrarily given as 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: fMAX is defined as the highest frequency excursion and is equal to
the fOUT frequency set by the RSET resistor. fMIN is the lowest frequency
excursion.
Note 9: The LTC6902CMS and LTC6902IMS are guaranteed functional
over the operating temperature range of –40°C to 85°C.
Note 10: The LTC6902CMS is guaranteed to meet 0°C to 70°C specifications and are designed, characterized and expected to meet the specified
performance from –40°C to 85°C but is not tested or QA sampled at these
temperatures. The LTC6902IMS is guaranteed to meet specified performance from –40°C to 85°C.
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TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Variation Over
Temperature
Frequency Variation vs RSET
4
1.00
TA = 25°C
GUARANTEED LIMITS APPLY OVER
20kΩ ≤ RSET ≤ 400kΩ
2
0.75
VARIATION (%)
0
–1
0.9
0.8
0.50
TYPICAL HIGH
1
1.0
RSET = 63.4k
÷1 OR ÷10 OR ÷100
TYPICAL LOW
0.25
0
TYPICAL
LOW
–0.25
–2
÷1, VA = 5V
0.7
TYPICAL
HIGH
JITTER (%P-P)
3
VARIATION (%)
Peak-to-Peak Jitter vs Output
Frequency (M = 1, 2-Phase Mode)
0.6
÷1, VA = 3V
0.5
0.4
0.3
–0.50
÷10
0.2
–3
–0.75
–4
1k
10k
100k
RSET (Ω)
–1.00
–40
1M
÷100
0.1
–20
0
20
40
60
TEMPERATURE (°C)
6902 G01
0
80
1k
10k
100k
1M
OUTPUT FREQUENCY (Hz)
6902 G02
Supply Current vs Output
Frequency [SSFM Disabled,
2-Phase Mode (M = 1)]
10M
6902 G03
Output Resistance
vs Supply Voltage
3.5
140
TA = 25°C
OUTPUT RESISTANCE (Ω)
SUPPLY CURRENT (mA)
3.0
2.5
÷1, 5V
2.0
÷10, 5V
1.5
÷100, 5V
1.0
0.5
0
÷100, 3V ÷10, 3V
1k
120
OUTPUT SOURCING CURRENT
100
80
60
÷1, 3V
10k
100k
1M
OUTPUT FREQUENCY (Hz)
OUTPUT SINKING CURRENT
10M
6902 G04
40
2.5
3.0
3.5 4.0 4.5
5.0
SUPPLY VOLTAGE (V)
5.5
6.0
6902 G05
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LTC6902
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TYPICAL PERFOR A CE CHARACTERISTICS
Output Operating at 10MHz,
VS = 3V
Output Operating at 20MHz,
VS = 5V
V + = 5V, RSET = 10k, CL = 10pF
V + = 3V, RSET = 20k, CL = 10pF
1V/DIV
1V/DIV
0V
0V
69012 G06
12.5ns/DIV
25ns/DIV
69012 G07
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QUICK DESIG GUIDE
V+
RMOD
0.1µF
RSET
SET
DIV
DIV
MOD
PH
PH
GND
10MHz 20kΩ
•
; 5kHz ≤ fOUT ≤ 20MHz
N • M RSET
R
Spreading Percentage = 20 • SET
RMOD
fOUT =
LTC6902
V+
OUT1 OUT4
OUT4
OUT2 OUT3
OUT3
OUT2
6902 F01
OUT1
Figure 1. Typical Application with Design Equation
Step 1. Select Multiphase Mode, Setting M
DIVIDER SETTING
By selecting the multiphase mode, a division parameter M
is also chosen:
N=1
Connect DIV Pin to GND
2MHz to 20MHz
N = 10
Leave DIV Open
200kHz to 2MHz
N = 100
Connect DIV Pin to V+
2-Phase: Connect PH Pin to GND
3-Phase: Leave PH Open
4-Phase: Connect PH Pin to V+
M=1
M=3
M=4
Step 2. Choosing Programmable Divider Setting N
A. For applications using spread spectrum frequency
modulation (SSFM) or applications that are constant frequency where low clock jitter is the primary specification:
FREQUENCY RANGE (fOUT • M)
< 200kHz
Note: The frequency range numbers are for a 5V supply where a 20MHz
output is the maximum frequency supported. For low supply applications
(2.7V ≤ V+ ≤ 4V), the maximum rated output frequency is 10MHz and all
of the above numbers should be halved.
B. For constant frequency applications where frequency
accuracy is the primary specification:
DIVIDER SETTING
N=1
Connect DIV Pin to GND
N = 10
Leave DIV Open
N = 100
Connect DIV Pin to V+
FREQUENCY RANGE (fOUT • M)
> 500kHz*
50kHz to 500kHz
< 50kHz
*The maximum frequency (fOUT • M) is 20MHz for 5V applications and is
10MHz for low supply applications (2.7V ≤ V+ ≤ 4V).
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LTC6902
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QUICK DESIG GUIDE
Step 3. Calculating the RSET Resistor Value
Step 4. Calculating the RMOD Resistor Value
The RSET resistor, the multiphase mode and the divider
setting set the output frequency (fOUT) for constant frequency applications. For SSFM applications, the maximum frequency excursion (fMAX) is equal to fOUT.
(Note: For constant frequency applications RMOD is not
required. Disable SSFM by connecting the MOD pin to
GND)
10MHz
R SET = 20kΩ •
N • M • fOUT
100 DIV Pin = V +
N = 10 DIV Pin = Open
1 DIV Pin = 0 V
4
M = 3
1
(4 - Phase Output) PH Pin = V +
(3 - Phase Output) PH Pin = Open
(2 - Phase Output) PH Pin = 0 V
RMOD = 20 •
RSET
Spreading Percentage
where the Spreading Percentage is defined by the
following:
Spreading Percentage = 100 •
fMAX – fMIN
fMAX
where fMAX is the highest frequency excursion (set by the
RSET value calculated in Step 3) and fMIN is the lowest
frequency excursion.
Example
For a 4-phase, 250kHz clock with 40% spreading:
Connect PH Pin to V+ → Selects 4-Phase Mode, M = 4
Leave DIV Pin Open → N = 10
RSET = 20k → Sets fOUT = fMAX = 250kHz
RMOD = 10k → Sets Spreading to 40%
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PI FU CTIO S
V+ (Pin 1): Supply Voltage ( 2.7V ≤ V+ ≤ 5.5V). The supply
should be kept free from noise and ripple. It should be
bypassed directly to a ground plane with a 0.1µF capacitor
placed as close to the pin as possible.
DIV (Pin 2): Divider Setting Input. This three-state input
selects among three divider settings determining the value
of N in the frequency equation. Pin 2 should be tied to GND
for the ÷1 setting, the highest frequency range. Floating
Pin 2, leaving it open, divides the master oscillator by 10.
Tie Pin 2 to V+ for the ÷100 setting, the lowest frequency
range. To detect a floating DIV pin, the LTC6902 places the
pin at the midsupply point with active circuitry. Therefore,
driving the DIV pin high requires sourcing approximately
2µA. Similarly, driving the DIV pin low requires sinking
2µA. When the DIV pin is floated, it should be bypassed by
a 1nF capacitor to GND or it should be surrounded by a
ground shield to prevent excessive coupling from other
PCB traces.
PH (Pin 3): Phase Setting Input. This three-state input
selects among three multiphase options. This sets the
outputs to produce 2-phase, 3-phase or 4-phase signals.
It also sets the value of M in the frequency equation. Pin 3
should be tied to GND for the 2-phase setting. This is the
highest frequency range with M set to 1. Floating Pin 3,
leaving it open, selects the 3-phase setting. This also sets
M to 3. Tie Pin 3 to V+ for the 4-phase setting. This is the
lowest frequency range as M is set to 4. To detect a floating
PH pin, the LTC6902 places the pin at the midsupply point
with active circuitry. Therefore, driving the PH pin high
requires sourcing approximately 2µA. Similarly, driving
the PH pin low requires sinking 2µA. When the PH pin is
floated, it should be bypassed by a 1nF capacitor to GND
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LTC6902
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PI FU CTIO S
or it should be surrounded by a ground shield to prevent
excessive coupling from other PCB traces.
OUT1, OUT2, OUT3, OUT4 (Pins 4, 5, 6 and 7): Oscillator
Outputs. These pins can drive 5kΩ and/or 10pF loads.
Larger loads may cause inaccuracies due to supply bounce
at high frequencies.
GND (Pin 8): Ground. Should be tied to a ground plane for
best performance.
SET (Pin 10): Frequency Setting Resistor Input. The value
of the resistor (RSET) connected between Pin 10 and V+
determines the oscillator frequency. The voltage on this
pin is held at approximately 1.13V below the V+ voltage.
For best performance, use a precision metal film resistor
with a value between 10k and 2M and limit the capacitance
on this pin to less than 10pF.
MOD (Pin 9): Spread Spectrum Frequency Modulation
Setting Resistor Input. The value of the resistor (RMOD)
connected between Pin 9 and V+ determines the amount
of frequency modulation. The output frequency is always
modulated down from the frequency set by the RSET resistor. For best performance, use a precision metal film resistor with a value between 10k and 2M and limit the
capacitance on this pin to less than 10pF. The voltage on
this pin is not static. Limiting the capacitance on this pin
is important for the part to perform properly. To disable
the modulation, connect this pin to GND. Grounding the
MOD pin disables the modulation and shuts down the
modulation circuitry to save power. Leaving the pin open
to disable the modulation is not recommended. While
leaving the pin open, RMOD ≈ ∞, gives the mathematical
result of 0% modulation, the open pin is susceptible to
external noise coupling that can effect frequency accuracy.
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BLOCK DIAGRA
2
V + – VSET = 1.1V ±25%
V
1
+
MASTER OSCILLATOR
ISET
10
IMOD
SET
ISET
RMOD
–
+
–
G=1
IMASTER
fMASTER = 10MHz • 20kΩ • +
(V – VSET)
VBIAS
ISET
9
PH
+
PROGRAMMABLE
DIVIDER ÷N
(N = 1, 10 OR 100)
IMASTER
MULTIPHASE
CIRCUIT
SELECTS BETWEEN
MULTIPHASE
OPTIONS AND ÷M
2-PHASE (M = 1)
3-PHASE (M = 3)
4-PHASE (M = 4)
OUT1
OUT2
OUT3
OUT4
4
5
6
7
MOD
IMOD
–
IMOD
+
RSET
3
DIV
VSET
REF
MULTIPLYING
DAC
7
9-BIT PRBS
GENERATOR
÷ 3200
CURRENT MIRROR
8 GND
6902 BD
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LTC6902
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THEORY OF OPERATIO
As shown in the Block Diagram, the LTC6902’s master
oscillator is controlled by the ratio of the voltage between
the V+ and SET pins (V+ – VSET) and the current entering
the master oscillator, IMASTER. When the spread spectrum
frequency modulation (SSFM) is disabled, IMASTER is
strictly determined by the V+ – VSET voltage and the ISET
current. When SSFM is enabled, the current IMOD (modulation current) is subtracted from the ISET current to
determine the IMASTER current value. Here the IMASTER
current is maximally at ISET but more often than not it is
less than ISET by a value determined by the IMOD value. In
this way the frequency of the master oscillator is modulated to produce a frequency that is always less than or
equal to the frequency set by the ISET current.
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 ±8% at a particular input
current and supply voltage (see Figure 2). 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
LTC6902 is optimized for use with RSET resistors between
10k and 2M. This corresponds to master oscillator frequencies between 100kHz and 20MHz. Additionally, the
MOD pin’s voltage tracks the SET pin’s voltage. The RMOD
resistor connected between the V+ and MOD pins similarly
locks together the MOD pin voltage variation and the IMOD
current to once more yield excellent accuracy.
1.4
1.3
VRES = V + – VSET
V + = 5V
1.2
V + = 3V
1.1
1.0
0.9
0.8
0.1
1
10
IRES (µA)
100
The master oscillator’s output is connected to the programmable divider. The output of the programmable
divider is then connected to the multiphase circuit with its
four outputs directly connected to output drivers. The final
output frequency is determined by the RSET resistor value,
the programmable divider setting and the multiphase
mode selected. The formula for setting the output frequency, fOUT, is below:
fOUT =
10MHz 20kΩ
•
N • M RSET
where:
100 DIV Pin = V +
N = 10 DIV Pin = Open
1 DIV Pin = 0 V
4
M = 3
1
(4 - Phase Output) PH Pin = V +
(3 - Phase Output) PH Pin = Open
(2 - Phase Output) PH Pin = 0 V
When the spread spectrum frequency modulation (SSFM)
is disabled, the frequency fOUT is the final output frequency. When SSFM is enabled, fOUT is the maximum
output frequency with the RMOD resistor value determining the minimum output frequency.
The programmable divider divides the master oscillator
signal by 1, 10 or 100. The divide-by value is determined
by the state of the DIV input (Pin 2). Tie DIV to GND or drive
it below 0.5V to select ÷1. This is the highest frequency
range, with the master output frequency passed directly to
the multiphase circuit. The DIV pin may be floated or
driven to midsupply to select ÷10, the intermediate frequency range. The lowest frequency range, ÷100, is selected by tying DIV to V+ or driving it to within 0.4V of V+.
Figure 3 shows the relationship between RSET, divider
setting and output frequency, including the overlapping
frequency ranges near 100kHz and 1MHz.
1000
69012 F02
Figure 2. V + – VSET Variation with IRES
The multiphase circuit generates outputs that are either
2-, 3- or 4-phase waveforms. To generate the 3- and
4-phase output signals, the output from the programmable
6902f
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THEORY OF OPERATIO
divider goes through further division. In addition to further division, the duty cycle of the output depends on the
multiphase mode selected. Figure 4 shows the waveform
at each output for 2-, 3- and 4-phase modes.
2-Phase Mode
In 2-phase mode, all outputs are nominally 50% duty
cycle. OUT1 and OUT2 are 180 degrees out of phase.
Stated differently, OUT2 is OUT1 inverted. However, OUT2
10000
RSET (kΩ)
1000
÷100
÷10
3-Phase Mode
÷1
100
10
1
1k
is not simply OUT1 routed through a standard logic
inverter. This would lead to substantial delay for OUT2’s
transitions from OUT1’s transitions. OUT1 and OUT2 are
created by a delay matched inverting circuit. Apart from
the basic inversion, the delay matching is determined by
analog circuit parameters. With this type of design, OUT1
and OUT2 transitions are typically within 100ps. OUT3 and
OUT4 are replications of OUT1 and OUT2 respectively.
Since the two phases are generated via delay matched
inverters, there is not any further division and the parameter M in the frequency setting equation is 1 (M = 1).
10k
100k
1M
10M
DESIRED OUTPUT FREQUENCY (Hz)
100M
69012 F03
Figure 3. RSET vs Desired Output Frequency
(PH = GND, 2-Phase, M = 1)
In 3-phase mode, OUT1, OUT2 and OUT3 are active and all
three outputs have a 33.3% duty cycle. OUT4 is not active
and is at a logic low state. The three active outputs are all
120 degrees out of phase. OUT2 lags OUT1 by 120 degrees
and OUT3 lags OUT2 by 120 degrees. The signals are
generated by a shift register. The output frequency is the
programmable divider’s output further divided 3 (M = 3).
4-Phase Mode
In 4-phase mode, all outputs have a 50% duty cycle. The
outputs are all 90 degrees out of phase. OUT2 lags OUT1
2-PHASE, PH = GND
M=1
DUTY CYCLE = 50%
OUT1
OUT2
OUT3
OUT4
3-PHASE, PH = OPEN
M=3
DUTY CYCLE = 33%
(OUT4 = LOGIC LOW)
OUT1
OUT2
OUT3
OUT4
4-PHASE, PH = V +
M=4
DUTY CYCLE = 50%
OUT1
OUT2
OUT3
OUT4
69012 F04
Figure 4. Mulitphase Output Waveforms
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by 90 degrees, OUT3 lags OUT2 by 90 degrees and OUT4
lags OUT3 by 90 degrees. The signals are generated by
flip-flops. The output frequency is the programmable
divider’s output further divided 4 (M = 4).
The multiphase mode is determined by the state of the PH
input (Pin 3). Tie the PH pin to GND or drive it below 0.5V
to select the 2-phase mode. The PH pin may be floated or
driven to midsupply to select the 3-phase mode. The
4-phase mode is selected by tying the PH pin to V+ or
driving it to within 0.4V of V+.
The CMOS output drivers have an ON resistance that is
typically less than 100Ω. In the ÷1 (high frequency) mode,
the rise and fall times are typically 7ns with a 5V supply and
11ns with a 3V supply. These transition times maintain a
clean square wave at 10MHz (20MHz at 5V supply). In the
÷10 and ÷100 modes, where the output frequency is much
lower, slew rate control circuitry in the output driver increases the rise/fall times to typically 14ns for a 5V supply
and 19ns for a 3V supply. The reduced slew rate lowers EMI
(electromagnetic interference) and supply bounce.
Spread Spectrum Frequency Modulation
The LTC6902 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 determined by the
external resistor RMOD and the voltage between the V+ and
MOD pins (V+ – VMOD). Unlike the stationary SET pin
voltage (VSET), the MOD pin voltage (VMOD) is a dynamic
signal generated by a multiplying digital to analog converter (MDAC) referenced to VSET. Referencing to VSET
negates errors due to variations of the VSET voltage and
locks the two voltages together. The VMOD voltage is the
VSET voltage scaled by one fifth and multiplied by the
digital code sent to the MDAC from the pseudorandom
binary sequence (PRBS) generator. VMOD varies in a
pseudorandom noise-like manner. The (V+ – VMOD) voltage is 0V minimum and maximally one fifth (20%) of
(V+ – VSET).
Referencing VMOD to VSET allows the ratio of RSET to
RMOD to determine the amount of frequency spreading.
Consider the case when RSET is equal to RMOD. Here,
when the (V+ – VMOD) voltage is at its minimum of 0V,
IMOD = 0A, IMASTER = ISET and the master oscillator is at
its maximum frequency (fMAX) which is the fOUT frequency set by the RSET resistor. Furthermore, when the
(V+ – VMOD) voltage is at its maximum of 20% of (V+ –
VSET), IMOD = 0.2 • ISET, IMASTER = 0.8 • ISET and the
master oscillator is at its minimum frequency (fMIN)
which is 80% of the fOSC frequency set by the RSET
resistor. The general formula for the amount of frequency
spreading is below:
Frequency Spreading (in %) = 20 •
RSET
RMOD
where frequency spreading is defined as:
Frequency Spreading (in %) = 100 •
fMAX – fMIN
fMAX
The design procedure is to first choose the RSET resistor
value to set fMAX (fOUT) and then choose the RMOD resistor
value to set the amount of frequency spreading desired.
Note that the frequency is always modulated to a lower
value. This is often referred to as a down spread signal.
To disable the SSFM, connect the MOD pin to ground.
Grounding the MOD pin disables the modulation and shuts
down the modulation circuitry. While leaving the MOD pin
open, RMOD = ∞, gives a frequency spreading of 0%, this
is not a good method of disabling the modulation. The
open pin is susceptible to external noise coupling that can
affect the output frequency accuracy. Grounding the MOD
pin is the best way to disable the SSFM.
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 9 bits
long. The pseudorandom sequence will repeat every 512
(29) shift register clock cycles. The bottom seven bits of the
shift register are sent in parallel to the MDAC which produces the VMOD voltage. Being a digitally generated signal,
the output is not a perfectly smooth waveform but consists
of 128 (27) discrete steps that change every shift register
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clock cycle. Note that the shift register clock is the master
oscillator’s output divided by 3200. This results in a somewhat slow moving modulating signal where each step is
separated in time by 3200/fMASTER seconds and the pseudorandom sequence repeats every (512 • 3200)/fMASTER
seconds.
The servo loop in the LTC6902 cannot respond instantaneously to each step due to its limited bandwidth. The
VMOD voltage steps are converted to frequency steps by
the servo loop. The servo loop has a bandwidth of about
25kHz that limits the frequency change rate and softens
corners of the waveform. This is beneficial when the
LTC6902 is used to clock switching regulators as will be
discussed in the Applications Information section. Figure␣ 5 illustrates the how the output frequency varies over
time.
fMAX
FREQUENCY
128 STEPS
fMIN
τSTEP
TIME
τREPEAT
τSTEP = 3200 τREPEAT = 3200 • 512
fMASTER
fMASTER
69012 F05
Figure 5
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SELECTING THE DIVIDER SETTING AND RSET VALUE
The LTC6902’s master oscillator has a frequency range
spanning 0.1MHz to 20MHz. However, accuracy may
suffer if the master oscillator is operated at greater than
10MHz with a supply voltage lower than 4V. A programmable divider extends the frequency range to greater than
three decades. Additional frequency division may occur
depending on the multiphase mode selected. The
multiphase mode and the parameter M are generally
dependent on the application’s requirement and usually
do not offer any additional design flexibility.
The LTC6902’s master oscillator covers a 200:1 range
while the programmable divider has 10:1 steps (1, 10,
100). This wide frequency range coupled with the part’s
programmable divider yields at least two solutions for any
desired output frequency (the exception being the highest
output frequencies that cannot be divided down). Choosing the best divider setting and the correct RSET resistor
value depends on the application.
For spread spectrum frequency modulated (SSFM) applications, choose the highest divider setting. This forces the
master oscillator to run at its highest frequency. The
pseudorandom signal generator is clocked by the master
oscillator, not the output, and the faster the signal moves
the greater the improvement in EMC performance. For
most applications the multiphase mode is determined by
the specific application’s need. For these applications, the
parameter M is predetermined and fixed. Table 1 lists the
recommended output (fOUT) frequency range for each
divider setting when using SSFM.
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Table 1. Recommended Frequency Range vs Programmable
Divider Setting for SSFM Applications or for Low Jitter Constant
Frequency Applications
DIVIDER SETTING
N=1
FREQUENCY RANGE (fOUT • M)
DIV (Pin 2) = GND
2MHz to 20MHz
N = 10
DIV (Pin 2) = Open
200kHz to 2MHz
N = 100
DIV (Pin 2) = V+
< 200kHz
Note: The frequency range numbers are for a 5V supply where a 20MHz
output is the maximum frequency supported. For low supply applications
(2.7V ≤ V+ ≤ 4V), the maximum rated output frequency is 10MHz and all
of the above numbers should be halved.
For constant frequency applications, where SSFM is disabled, the best operating position depends on which
parameter is most important in the application. For the
lowest clock jitter it is best to set the divider to its highest
setting as done above. The divider reduces the master
oscillator’s jitter. The higher the division number the
greater the reduction in the master oscillator’s jitter. For
the best frequency accuracy it is best to run the programmable divider at its lowest setting, and thus, the master
oscillator runs at a lower frequency. The lower master
oscillator frequencies are more accurate and use less
power. To determine a tradeoff between frequency accuracy and jitter consult the Typical Performance Characteristics curves. Table 2 lists the recommended output frequency range for each divider setting for continuous
frequency applications where frequency accuracy is the
primary specification.
Table 2. Recommended Frequency Range vs Programmable
Divider Setting for Best Frequency Accuracy, Constant Frequency
Applications (SSFM disabled)
DIVIDER SETTING
FREQUENCY RANGE (fOUT • M)
N=1
DIV (Pin 2) = GND
> 500kHz*
N = 10
DIV (Pin 2) = Open
50kHz to 500kHz
N = 100
DIV (Pin 2) = V+
< 50kHz
*The maximum frequency (fOUT • M) is 20MHz for 5V applications and is
10MHz for low supply applications (2.7V ≤ V+ ≤ 4V).
For some applications, the multiphase circuit is also useful
in forcing the master oscillator to run at a higher or lower
frequency. If the application requires a single clock source,
the multiphase circuit can be set in whatever mode gives
the highest or lowest divider number (M) and thus the
highest or lowest master oscillator frequency. Additionally, if the application requires just two phases, the 4-phase
mode can be selected with only the OUT1 and OUT3
outputs are used (or alternatively the OUT2 and OUT4
outputs).
For instance, a 500kHz, 2-phase clock can be obtained in
four different ways. Table 3 lists the possible solutions.
For an SSFM application, the preferred solution for best
EMC performance is the last alternative where the master
oscillator is at 20MHz. For a constant frequency application, the preferred solution is the first alternative with the
master oscillator at 500kHz.
Table 3. Four Possible Ways to Obtain a 500kHz, 2-Phase Clock
RSET
N
MULTIPHASE MODE
M
fMASTER
OUTPUTS
400k
1
2
1
500kHz
OUT1, OUT2
100k
1
4
4
2MHz
OUT1, OUT3
40k
10
2
1
5MHz
OUT1, OUT2
10k
10
4
4
20MHz
OUT1, OUT3
After choosing the proper divider setting, determine the
correct frequency-setting resistor. Because of the linear
correspondence between oscillation period and resistance, a simple equation relates resistance with frequency.
4
100
10MHz
RSET = 20k •
, N = 10 M = 3
M • N • fOUT
1
1
(RSETMIN = 10k, RSETMAX = 2M)
Any resistor, RSET, tolerance adds to the inaccuracy of the
oscillator, fOUT.
SETTING THE SPREAD SPECTRUM MODULATION
SPREADING PERCENTAGE WITH THE LTC6902
Setting the spread spectrum modulation percentage on
the LTC6902 is very simple and straightforward. Since the
spreading is ratiometric, in percentage, the programmable divider and multiphase mode selection have no
influence on the spreading percentage. In general, for
greatest EMC improvement, each application should apply
as much spreading as possible. The amount of spreading
that any particular application can tolerate is dependent on
the specific nature of that application. Once the RSET
resistor value is calculated to set fMAX and the desired
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spreading is determined, the RMOD value is calculated
using the simple equation below:
RMOD = 20 •
RSET
Spreading Percentage
The only limitations for this formula are in the RMOD value
range and the spreading percentage range. The range of
the RMOD resistor value is the same as that for RSET
ranging from 10k to 400k. The LTC6902 is tested and
specified for spreading of 10% and 40%. These are
practical limits that would apply to many systems but they
are not the actual limits of the part. The lower end limit is
set by internal offsets and mismatches. At lower spreading
percentages, these mismatches become more significant
and the error from the calculated, desired spreading
increases. A practical lower end limit would be about 5%
spreading. At the higher end internal mismatching becomes less significant, however other factors come into
play and a theoretical limit approaching 100% (fMIN approaching zero) cannot be reliably achieved. A practical
upper limit would be about 80% spreading.
To disable the SSFM, connect the MOD pin to ground.
Grounding the MOD pin disables the modulation and shuts
down the modulation circuitry. While leaving the MOD pin
open, RMOD = ∞, gives a frequency spreading of 0%, this
is not a good method of disabling the modulation. The
open pin is susceptible to external noise coupling that can
affect the output frequency accuracy. Grounding the MOD
pin is the best way to disable the SSFM.
DRIVING LOGIC CIRCUITS
The outputs of the LTC6902 are suitable for driving
general digital logic circuits. The CMOS output drivers
have an ON resistance that is typically less than 100Ω and
are very similar in performance to HCMOS logic outputs.
However, the form of frequency spreading used in the
LTC6902 may not be suitable for many logic designs.
Many logic designs have fairly tight timing and cycle-tocycle 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 LTC6902 uses a pseudorandom
modulating signal where the frequency transitions have
been slowed and the corners rounded by a 25kHz lowpass
filter. 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 LTC6902 is designed primarily to provide an accurate
and stable clock for switching regulator systems, especially those systems with multiple switching regulators
where all of the regulators are interleaved and are run at the
same frequency. This lowers the input capacitor requirements and prevents beat notes formed by mixing numerous clock frequencies and their harmonics. The multiphase
outputs have CMOS drivers with an ON resistance that is
typically less than 100Ω and are very similar in performance to HCMOS logic outputs. This is 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 exact operating details depend on the
switching regulator in use, but generally switching is
synchronized to the rising edge of the clock. Since the
LTC6902’s master oscillator is passed through inverters
or flip-flops to generate its multiphase outputs, coincident
rising edges (or falling edges) cannot occur. This is true
even when the LTC6902 is used with a high percentage of
spreading.
For the best EMC performance, the LTC6902 should be
run with SSFM enabled and the master oscillator at its
highest frequency. The pseudorandom modulation signal
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generator is driven by the master oscillator frequency, not
the output frequency. This gives some design flexibility in
the choice of the RSET and the programmable divider
setting. When making the choice, usually the faster master oscillator is the better choice. This is especially true
when the main goal is to lower peak radiated or conducted
signal levels measured during regulatory agency testing.
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 switching 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.
Even when running the LTC6902 at the maximum modulation rate, the frequency hopping transitions are slowed
by the part’s servo loop. The frequency transitions are
slowed by a 25kHz lowpass. 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 input 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 switching regulator is sufficiently high, beyond
25kHz, 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 MOSFETs, fixed inductor,
fixed capacitors, the amount of ripple will vary some with
the regulator’s operating frequency (the main exception
being hysteresis 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 and giving a distorted output. Depending on the
inductor design and triangle wave frequency, it may even
result in an audible noise. The LTC6902 uses a pseudorandom noise-like modulating signal. This results in the
regulator’s output ripple being modulated by the wideband
pseudorandom noise-like signal. On an oscilloscope, it
looks essentially noise-like of even amplitude. The signal
is broadband and any mixing issues are minimized. Additionally, the pseudorandom signal repeats at such a low
rate that it is well below the audible range.
The LTC6902 directly drives many switching regulators.
The LTC6902 with the spread spectrum frequency modulation results in improved EMC performance. If 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.
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POWER SUPPLY SENSITIVITY
Figure 6 shows the output frequency sensitivity to power
supply voltage at several different temperatures. The
LTC6902 has a guaranteed voltage coefficient of 0.1%/V
but, as Figure 6 shows, the typical supply sensitivity is
twice as low.
START-UP TIME
The start-up time and settling time to within 1% of the final
value can be estimated by tSTART ≅ RSET(3.7µs/kΩ) +
10µs. Note the start-up time depends on RSET and is
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. The capacitance from the SET pin,
(Pin 3), to ground must be less than 10pF. If this requirement is not met, the jitter will increase.
70
RSET = 63.2k
PIN 4 = FLOATING (÷10)
0.10
25°C
–40°C
0.05
TA = 25°C
V+ = 5V
60
FREQUENCY ERROR (%)
FREQUENCY DEVIATION (%)
0.15
independent from the setting of the divider pin. For instance with RSET = 100k, the LTC6902 will settle with 1%
of its 200kHz final value (N = 10) in approximately 380µs.
Figure 7 shows start-up times for various RSET resistors.
85°C
0
50
40
30
20
400k
10
63.2k
0
–0.05
–10
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
20k
0
200
400
800
600
TIME AFTER POWER APPLIED (µs)
69012 F07
69012 F06
Figure 6. Supply Sensitivity
1000
Figure 7. Start-Up Time
6902f
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.
15
LTC6902
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PACKAGE DESCRIPTIO
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.2 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.50
0.305 ± 0.038
(.0197)
(.0120 ± .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
10 9 8 7 6
3.00 ± 0.102
(.118 ± .004)
NOTE 4
4.90 ± 0.15
(1.93 ± .006)
DETAIL “A”
0.497 ± 0.076
(.0196 ± .003)
REF
0° – 6° TYP
GAUGE PLANE
1 2 3 4 5
0.53 ± 0.01
(.021 ± .006)
1.10
(.043)
MAX
DETAIL “A”
0.86
(.034)
REF
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
NOTE:
BSC
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.13 ± 0.076
(.005 ± .003)
MSOP (MS) 0802
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
TM
LTC1799
1kHz to 30MHz ThinSOT Oscillator
Single Output, Higher Frequency Operation
LTC6900
1kHz to 20MHz ThinSOT Oscillator
Single Output, Lower Power
ThinSOT is a trademark of Linear Technology Corporation.
6902f
16
Linear Technology Corporation
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