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PDF LTC6902 Data sheet ( Hoja de datos )
| Número de pieza | LTC6902 | |
| Descripción | Multiphase Oscillator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s 2-, 3- or 4-Phase Outputs
s Optional Spread Spectrum Frequency Modulation
for Improved EMC Performance
www.DataShseet54UkH.cozmto 20MHz Frequency Range
s One External Resistor Sets the Frequency
s One External Resistor Sets Percent Frequency
Spreading
s 400µA Typical Supply Current, VS = 3V, 1MHz
s Frequency Error ≤1.5% Max, 5kHz to 10MHz
(TA = 25°C)
s Frequency Error ≤ 2% Max, 5kHz to 10MHz
(TA = 0°C to 70°C)
s ±40ppm/°C Temperature Stability
s Fast Start-Up Time: 50µs to 1.5ms
s 100Ω CMOS Output Driver
s Operates from a Single 2.7V to 5.5V Supply
s Available in 10-Lead MS Package
U
APPLICATIO S
s Switching Power Supply Clock Reference
s Portable and Battery-Powered Equipment
s PDAs
s Cell Phones
s Clocking Switched Capacitor Filters
LTC6902
Multiphase Oscillator
with Spread Spectrum
Frequency Modulation
DESCRIPTIO
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 exter-
nal 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).
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 pro-
grammable divider (divide by 1, 10 or 100) is included.
The integrated programmable multiphase circuit pro-
vides 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 radia-
tion 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.
TYPICAL APPLICATIO
500kHz, 4-Phase Clock with 20% Frequency Spreading
5V
0.1µF
RMOD 10k
RSET 10k
OPEN
LTC6902
V+ SET
DIV MOD
PH GND
OUT1 OUT4
OUT2 OUT3
OUT4
OUT3
OUT2
OUT1
6902 TA01
Output Frequency Spectrum With and Without SSFM
0
–20
OUTPUT
–40
SPECTRUM
WITH 20%
SPREADING
–60
OUTPUT
SPECTRUM
WITH SSFM
DISABLED
–80
–100
400
450 500 550
FREQUENCY (kHz)
600
6902 TA02
6902f
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1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
Output Operating at 20MHz,
VS = 5V
V+ = 5V, RSET = 10k, CL = 10pF
Output Operating at 10MHz,
VS = 3V
V+ = 3V, RSET = 20k, CL = 10pF
LTC6902
www.DataSheet4U.com
1V/DIV
0V
12.5ns/DIV
69012 G06
1V/DIV
0V
25ns/DIV
69012 G07
QUICK DESIG GUIDE
V+
0.1µF
RMOD
RSET
LTC6902
V+ SET
DIV DIV MOD
PH PH GND
OUT1 OUT4
OUT2 OUT3
OUT4
OUT3
OUT2
OUT1
6902 F01
fOUT
=
10MHz
N•M
•
20kΩ
RSET
;
5kHz
≤
fOUT
≤
20MHz
Spreading Percentage = 20 • RSET
RMOD
Figure 1. Typical Application with Design Equation
Step 1. Select Multiphase Mode, Setting M
By selecting the multiphase mode, a division parameter M
is also chosen:
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 fre-
quency where low clock jitter is the primary specification:
DIVIDER SETTING
FREQUENCY RANGE (fOUT • M)
N=1
Connect DIV Pin to GND
2MHz to 20MHz
N = 10 Leave DIV Open
N = 100 Connect DIV Pin to V+
200kHz to 2MHz
< 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
FREQUENCY RANGE (fOUT • M)
N=1
Connect DIV Pin to GND
> 500kHz*
N = 10 Leave DIV Open
N = 100 Connect DIV Pin to V+
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).
6902f
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5 Page 
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THEORY OF OPERATIO
clock cycle. Note that the shift register clock is the master
oscillator’s output divided by 3200. This results in a some-
what slow moving modulating signal where each step is
separated in time by 3200/fMASTER seconds and the pseu-
dorandom sequence repeats every (512 • 3200)/fMASTER
seconds.
The servo loop in the LTC6902 cannot respond instanta-
neously to each step due to its limited bandwidth. The
www.DataSheet4U.com
LTC6902
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. Fig-
ure␣ 5 illustrates the how the output frequency varies over
time.
fMAX
128 STEPS
fMIN τSTEP
τREPEAT
TIME
τSTEP
=
3200
fMASTER
τREPEAT
=
3200 • 512
fMASTER
69012 F05
Figure 5
APPLICATIO S I FOR ATIO
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 program-
mable 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). Choos-
ing the best divider setting and the correct RSET resistor
value depends on the application.
For spread spectrum frequency modulated (SSFM) appli-
cations, 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.
6902f
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC6902.PDF ] | |
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