LTC6906 Linear Technology, LTC6906 Datasheet - Page 10

• Manufacturer Part Number: LTC6906
• Description: Micropower
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC6906
Start-Up Time
When the LTC6906 is powered up, it holds the OUT pin
low. After the master oscillator has settled, the OUT pin is
enabled and the first output cycle is guaranteed to be
within specification. The time from power-up to the first
output transition is given approximately by:
The digital divider ratio, N, does not affect the start-up
time.
Power Supply Rejection
The LTC6906 has a very low supply voltage coefficient,
meaning that the output frequency is nearly insensitive to
the DC power supply voltage. In most cases, this error
term can be neglected.
High frequency noise on the power supply (V
potential to interfere with the LTC6906’s master oscillator.
10
t
START
Figure 12. Test Circuit with an Inductive Power Supply
≅ 64 • t
3.3V
3.5
2
1
0
3
4.65
Figure 13. Output Waveforms with
an Inductive Supply (See Figure 12)
330nH
OSC
4.75
L
S
U
+ 100µs
4.85
V
GND
DIV
TIME (µs)
+
LTC6906
U
C
C
C
C
LOAD
LOAD
LOAD
LOAD
GRD
OUT
SET
4.95
= 5pF
= 10pF
= 20pF
= 50pF
W
5.05
6906 F12
R
100k
SET
6906 F13
5.15
C
LOAD
+
) pin has the
U
1MHz
Periodic noise, such as that generated by a switching
power supply, can shift the output frequency or increase
jitter. The risk increases when the fundamental frequency
or harmonics of the noise fall near the master oscillator
frequency. It is relatively easy to filter the LTC6906 power
supply because of the very low supply current. For ex-
ample, an RC filter with R = 160Ω and C = 10µF provides
a 100Hz lowpass filter while dropping the supply voltage
only about 10mV.
Operating the LTC6906 with Supplies Higher
Than 3.6V
The LTC6906 may also be used with supply voltages
between 3.6V and 5.5V under very specific conditions. To
ensure proper functioning above 3.6V, a filter circuit must
be attached to the power supply and located within 1cm of
the device. A simple RC filter consisting of a 100Ω resistor
and 1µF capacitor (Figure 16) will ensure that supply
resonance at higher supply voltages does not induce
Figure 14. Test Circuit with a Resistive Power Supply
3.5
2
1
0
3
0.4
Figure 15. Output Waveforms with
a Resistive Supply (See Figure 14)
3.3V
0.5
100Ω
0.6
R
S
0.7
TIME (µs)
V
GND
DIV
+
LTC6906
C
C
C
C
LOAD
LOAD
LOAD
LOAD
0.8
GRD
OUT
SET
= 5pF
= 10pF
= 20pF
= 50pF
0.9
1.0
R
100k
6906 F14
SET
6906 F15
1.1
C
LOAD
1MHz
6906fa