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PDF LTC6908-2 Data sheet ( Hoja de datos )
| Número de pieza | LTC6908-2 | |
| Descripción | Resistor Set SOT-23 Oscillator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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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/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/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 (Ω)
1M 10M
690812 F07
Figure 7. Start-Up Time
690812fa
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11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC6908-2.PDF ] | |
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